# Lecture 4: Gram-Positive Rods ## Unit 2.8: Microbiology --- ## Learning Objectives By the end of this lecture, students will be able to: 1. Describe Bacillus species and anthrax 2. Explain Clostridium species and their toxin-mediated diseases 3. Describe Listeria monocytogenes and listeriosis 4. Explain Corynebacterium diphtheriae and diphtheria 5. Describe other clinically significant gram-positive rods 6. Explain the laboratory identification of gram-positive rods --- ## Lecture Outline ### I. Overview of Gram-Positive Rods The gram-positive rods comprise a diverse group of bacteria with widely varying clinical significance, from ubiquitous environmental organisms that rarely cause disease to highly virulent pathogens capable of producing rapidly fatal illness. This group includes organisms that cause some of the most dramatic clinical syndromes in medicine, including anthrax, tetanus, botulism, and gas gangrene. Many of these organisms exert their pathogenic effects through potent toxins rather than direct tissue invasion, and several are spore-formers capable of surviving in the environment for extended periods. Understanding the characteristics, disease manifestations, and treatment of gram-positive rods is essential for clinical practice. The most fundamental distinction among gram-positive rods is between spore-forming and non-spore-forming organisms. The spore-forming gram-positive rods are divided into aerobic species (the genus Bacillus) and anaerobic species (the genus Clostridium). Spore formation allows these organisms to survive harsh environmental conditions, resist standard disinfection methods, and persist for years in soil, dust, and other environments. When conditions become favorable, spores germinate into metabolically active vegetative cells capable of causing disease. This property has profound clinical implications: Clostridium tetani spores in soil can contaminate wounds and cause tetanus; Clostridium botulinum spores can survive in improperly canned foods and germinate under anaerobic conditions to produce botulinum toxin; and Bacillus anthracis spores can persist in soil for decades and have been weaponized for bioterrorism. The non-spore-forming gram-positive rods include Listeria monocytogenes, an important cause of meningitis and sepsis in vulnerable populations; Corynebacterium diphtheriae, the causative agent of diphtheria; and various other organisms including Actinomyces, Nocardia, and Propionibacterium (now Cutibacterium). Oxygen requirements further categorize the gram-positive rods and have practical implications for culture and clinical presentation. Bacillus species are aerobic or facultatively anaerobic, growing well in the presence of oxygen. Clostridium species are obligate anaerobes, killed by oxygen exposure and requiring special anaerobic culture techniques; their clinical infections typically occur in settings of tissue hypoxia such as deep wounds with necrotic tissue. Listeria and Corynebacterium are facultative anaerobes capable of growth in either aerobic or anaerobic conditions. Some Actinomyces species are microaerophilic, preferring reduced oxygen environments. Several gram-positive rods comprise normal human flora and cause disease only under specific circumstances. Propionibacterium acnes (now Cutibacterium acnes) is abundant in sebaceous areas of the skin and contributes to acne pathogenesis; it occasionally causes infections of prosthetic devices, particularly shunts and prosthetic joints, through its ability to form biofilms. Clostridium species are normal inhabitants of the intestinal tract; disruption of the normal flora by antibiotics allows overgrowth of Clostridioides difficile, causing antibiotic-associated colitis. Actinomyces species colonize the oral cavity and GI tract; they cause disease when they gain access to deeper tissues through trauma or mucosal disruption. Understanding which gram-positive rods represent normal flora and which are always pathogenic guides interpretation of culture results. Panel A: Classification scheme of gram-positive rods showing division into spore-forming (Bacillus as aerobic, Clostridium as anaerobic) and non-spore-forming (Listeria, Corynebacterium, Actinomyces, Propionibacterium) categories, with representative microscopic appearance for each genus showing rod morphology and spore characteristics where applicable. Panel B: Spore versus vegetative cell comparison showing the resistant spore structure with thick coat, cortex, and core containing DNA in metabolically dormant state, compared to active vegetative cell with normal cellular structures, and environmental conditions triggering germination (appropriate temperature, moisture, nutrients, anaerobic conditions for clostridia). Panel C: Oxygen requirements spectrum illustrated as a gradient showing obligate aerobes (Bacillus) on one end requiring oxygen for growth, facultative anaerobes (Listeria, Corynebacterium) in the middle capable of growing with or without oxygen, and obligate anaerobes (Clostridium) on the other end killed by oxygen, with typical culture conditions indicated for each. Panel D: Normal flora distribution showing human body diagram with skin (Propionibacterium acnes in sebaceous areas), oral cavity (Actinomyces species), and gastrointestinal tract (various Clostridium species), with notes on when these organisms become pathogenic. --- ### II. Bacillus anthracis (Anthrax) Bacillus anthracis, the causative agent of anthrax, is one of the most significant bacterial pathogens due to its historical importance, its use as a biological weapon, and the severe diseases it causes. Anthrax has affected humans and livestock for millennia and was the first bacterial disease for which Koch fulfilled his postulates, proving the germ theory of disease. The organism's ability to form highly resistant spores that can persist in soil for decades, combined with its production of lethal toxins, makes it a formidable pathogen and a Category A bioterrorism agent. Bacillus anthracis is a large, gram-positive rod with characteristic morphology described as boxcar-shaped, appearing as rectangular cells with squared-off ends. Unlike most other Bacillus species, B. anthracis is non-hemolytic on blood agar and non-motile, features that help distinguish it from the commonly encountered environmental contaminant B. cereus. The organism forms central, non-bulging spores that are the infectious form; vegetative cells are typically seen in clinical specimens, while spores predominate in environmental samples. Uniquely among pathogenic bacteria, B. anthracis possesses a capsule composed of poly-D-glutamic acid rather than polysaccharide. This polypeptide capsule is antiphagocytic and contributes significantly to virulence; non-encapsulated strains are avirulent. The pathogenicity of B. anthracis derives primarily from its tripartite exotoxin system, which consists of three proteins that work in combination. Protective antigen (PA) is the central component that binds to host cell receptors and facilitates entry of the other two components into the cytoplasm; it is called protective antigen because antibodies against it are protective. Edema factor (EF) is a calmodulin-dependent adenylyl cyclase that increases intracellular cyclic AMP, causing the massive tissue edema characteristic of anthrax. Lethal factor (LF) is a zinc metalloprotease that cleaves mitogen-activated protein kinase kinases (MAPKKs), disrupting cell signaling and causing macrophage death. PA combines with EF to form edema toxin, and with LF to form lethal toxin; neither EF nor LF is toxic alone. Anthrax presents in several distinct clinical forms depending on the route of spore entry. Cutaneous anthrax, accounting for approximately 95% of naturally occurring cases, results from spore inoculation through cuts or abrasions, typically in people handling contaminated animal products (hides, wool, hair). A painless papule develops at the inoculation site, progresses through vesicular and ulcerative stages, and forms a characteristic black eschar (the word anthrax derives from the Greek for coal, referring to this black lesion). Substantial edema surrounds the eschar. Untreated cutaneous anthrax has approximately 20% mortality, but with treatment, mortality is less than 1%. Inhalational anthrax, the most lethal form, results from inhaling spores, classically as an occupational hazard of wool-sorters. Spores are carried to mediastinal lymph nodes where they germinate and multiply, producing toxin. The classic radiographic finding is a widened mediastinum due to hemorrhagic mediastinitis. The clinical course is biphasic: initial flu-like symptoms are followed by sudden deterioration with respiratory distress, shock, and often death within 24 to 36 hours. Gastrointestinal anthrax results from ingesting contaminated meat and presents with severe abdominal pain, bloody diarrhea, and ascites. Injection anthrax has emerged among intravenous drug users using heroin contaminated with B. anthracis spores. Panel A: Bacillus anthracis microscopic morphology showing large boxcar-shaped gram-positive rods with squared ends arranged in chains, central non-bulging spores visible in some cells, and encapsulated organisms demonstrating the poly-D-glutamic acid capsule using special staining (McFadyean reaction with polychrome methylene blue). Panel B: Anthrax toxin mechanism illustrated showing protective antigen (PA) binding to cell surface receptor and forming a heptameric pore, edema factor (EF) and lethal factor (LF) entering through the pore, EF acting as adenylyl cyclase increasing cAMP causing edema, and LF cleaving MAPKKs causing macrophage death, with the combinations forming edema toxin (PA + EF) and lethal toxin (PA + LF) indicated. Panel C: Clinical forms of anthrax displayed: cutaneous anthrax showing progression from papule to vesicle to black eschar with surrounding edema, inhalational anthrax showing chest radiograph with widened mediastinum and hemorrhagic mediastinitis, and gastrointestinal anthrax showing intestinal involvement with ulceration and bloody ascites. Panel D: Anthrax diagnosis and treatment algorithm showing culture characteristics (non-hemolytic on blood agar, non-motile, Medusa head colonies), PCR confirmation, chest imaging findings, treatment with ciprofloxacin plus doxycycline plus antitoxin for inhalational disease, and post-exposure prophylaxis with ciprofloxacin plus vaccination. --- ### III. Bacillus cereus and Other Bacillus Species While Bacillus anthracis receives the most attention due to its severity and bioterrorism potential, Bacillus cereus is far more commonly encountered clinically, causing both food poisoning and opportunistic infections. Other Bacillus species occasionally cause disease but more often represent environmental contamination of clinical specimens. Understanding the clinical contexts in which Bacillus species cause true infection versus contamination is essential for appropriate management. Bacillus cereus causes two distinct food poisoning syndromes mediated by different toxins. The emetic syndrome results from ingestion of preformed cereulide toxin, a heat-stable cyclic peptide that accumulates in improperly stored food. The classic association is with reheated fried rice; B. cereus spores survive cooking, germinate as the rice cools, and produce toxin during storage at room temperature. Symptoms begin one to six hours after ingestion and consist primarily of nausea and vomiting, similar to Staphylococcus aureus food poisoning. Recovery is typically rapid and complete within 24 hours. The diarrheal syndrome results from heat-labile enterotoxins produced by B. cereus in the small intestine after ingestion of vegetative cells. Symptoms begin 8 to 16 hours after ingestion and consist of watery diarrhea and abdominal cramping without vomiting, similar to Clostridium perfringens food poisoning. This syndrome is also self-limited. Both syndromes are diagnosed clinically; laboratory confirmation is rarely necessary. Beyond food poisoning, B. cereus causes serious infections in specific settings. Endophthalmitis following penetrating eye trauma is the most feared B. cereus infection; the organism grows rapidly and produces destructive toxins and enzymes that can lead to blindness within 24 to 48 hours, even with aggressive treatment. Traumatic wounds contaminated with soil may develop B. cereus infection, particularly if devitalized tissue is present. Bacteremia and line-associated infections occur in intravenous drug users and in patients with central venous catheters, particularly in intensive care settings. Immunocompromised patients, especially those with hematologic malignancies and neutropenia, are at increased risk for invasive B. cereus infection, which can cause severe pneumonia and disseminated disease. Treatment of B. cereus food poisoning is supportive, as both syndromes are self-limited. For serious infections, B. cereus presents a therapeutic challenge because it produces beta-lactamases that inactivate penicillins and cephalosporins. Vancomycin is the drug of choice for serious infections, often combined with a carbapenem or fluoroquinolone. For endophthalmitis, immediate vitrectomy with intravitreal vancomycin and systemic antibiotics is required, though visual outcomes are often poor. Other Bacillus species, including B. subtilis, B. licheniformis, and B. megaterium, occasionally cause infections similar to those caused by B. cereus but are more often contaminants. The decision to treat depends on clinical context: isolation from normally sterile sites in patients with risk factors, particularly with multiple positive cultures, suggests true infection, while single positive cultures in low-risk patients more often represent contamination. Panel A: B. cereus food poisoning comparison showing emetic syndrome (preformed toxin, fried rice association, 1-6 hour onset, vomiting predominant, heat-stable cereulide toxin) versus diarrheal syndrome (enterotoxin produced in vivo, 8-16 hour onset, watery diarrhea predominant, heat-labile toxin), with typical food sources illustrated for each. Panel B: B. cereus pathogenesis in different settings showing contaminated fried rice leading to food poisoning, penetrating eye injury leading to destructive endophthalmitis with rapid vision loss, contaminated heroin or soil leading to wound infection, and central venous catheter serving as source of bacteremia in hospitalized patients. Panel C: B. cereus endophthalmitis progression showing timeline from injury to severe intraocular inflammation within 12-24 hours, hypopyon visible in anterior chamber, vitreal involvement, and rapid progression to permanent visual loss without emergent intervention (vitrectomy and intravitreal antibiotics). Panel D: Treatment approach for B. cereus showing food poisoning management (supportive care, self-limited), and serious infection management (vancomycin first-line, consider adding carbapenem or fluoroquinolone), with note about beta-lactam resistance due to beta-lactamase production, and distinction between true infection (multiple positive cultures, risk factors, clinical signs) versus contamination (single positive culture, no risk factors). --- ### IV. Clostridium Overview The genus Clostridium encompasses numerous species of gram-positive, obligately anaerobic, spore-forming rods that cause some of the most dramatic and rapidly progressive infectious diseases known. These organisms are ubiquitous in the environment, particularly in soil, and several species comprise normal intestinal flora. Clostridial diseases are primarily toxin-mediated, with bacterial toxins causing the clinical manifestations rather than direct tissue invasion. Understanding clostridial biology, including spore formation and toxin production, is essential for comprehending the pathogenesis, treatment, and prevention of these diseases. Clostridia share several key characteristics that influence their clinical behavior. They are gram-positive rods, though older cultures may lose the gram-positive staining and appear gram-variable or even gram-negative. Spore formation is a defining characteristic, with spores typically described as terminal (at the end of the cell, giving a drumstick or tennis racket appearance) or subterminal (near the end but not at the terminus). Spore position and whether the spore causes cell bulging help distinguish species. As obligate anaerobes, clostridia require the absence of oxygen for growth and cannot survive prolonged oxygen exposure. This requirement explains why clostridial infections characteristically occur in settings of tissue hypoxia: deep wounds with necrotic tissue, ischemic tissue, or environments protected from atmospheric oxygen such as the intestinal tract or improperly canned foods. The major pathogenic Clostridium species each produce characteristic diseases through specific toxin mechanisms. Clostridium tetani produces tetanospasmin, which causes the spastic paralysis of tetanus. Clostridium botulinum produces botulinum toxin, causing the flaccid paralysis of botulism. Clostridium perfringens produces alpha-toxin (a phospholipase C) that causes the tissue destruction of gas gangrene, as well as an enterotoxin causing food poisoning. Clostridioides difficile (recently reclassified from the Clostridium genus) produces toxins A and B that cause antibiotic-associated colitis. Clostridium septicum is strongly associated with malignancy, particularly colorectal cancer and hematologic malignancies, and causes spontaneous myonecrosis. Laboratory culture of clostridia requires anaerobic conditions, achieved using anaerobic chambers, anaerobic jars with gas-generating packets, or pre-reduced anaerobically sterilized (PRAS) media. Growth is generally slower than for aerobic organisms, often requiring several days of incubation. Special selective media facilitate isolation of specific species: cycloserine-cefoxitin fructose agar (CCFA) selects for C. difficile, which produces fluorescent yellow colonies. For clinical purposes, diagnosis of clostridial diseases often relies on clinical presentation and detection of toxins rather than culture; toxin assays or molecular detection of toxin genes provide faster and more clinically relevant results for diseases like botulism and C. difficile infection. Panel A: Clostridium microscopic morphology showing gram-positive rods (noting gram-variable appearance may occur), spore position variations including terminal spores with drumstick appearance (C. tetani), subterminal spores with bulging (C. botulinum), and subterminal non-bulging spores (C. perfringens), each with labeled example species. Panel B: Clostridium species and disease comparison showing C. tetani (tetanus, spastic paralysis), C. botulinum (botulism, flaccid paralysis), C. perfringens (gas gangrene, food poisoning), C. difficile (antibiotic-associated colitis), and C. septicum (associated with malignancy, spontaneous myonecrosis), with key toxin for each organism indicated. Panel C: Anaerobic environment requirements illustrated showing conditions favoring clostridial growth: deep puncture wound with necrotic tissue and impaired blood supply, improperly canned food with vacuum seal creating anaerobic conditions, and intestinal tract (anaerobic environment where clostridia are normal flora), demonstrating why clostridial diseases occur in specific contexts. Panel D: Laboratory culture of clostridia showing anaerobic chamber with gas mixture (nitrogen, hydrogen, carbon dioxide), anaerobic jar with gas-generating sachet, CCFA plate with yellow fluorescent C. difficile colonies, and note about extended incubation time required (days rather than overnight). --- ### V. Clostridium tetani (Tetanus) Tetanus is a devastating neurological disease caused by the toxin of Clostridium tetani. Despite being entirely preventable through vaccination, tetanus remains a significant cause of morbidity and mortality worldwide, particularly in developing countries where vaccine coverage is incomplete. Understanding the unique pathophysiology of tetanospasmin and the clinical presentation of tetanus enables recognition and appropriate management of this life-threatening condition. The pathogenesis of tetanus begins with wound contamination by C. tetani spores from soil or other environmental sources. Spores germinate into vegetative bacteria under anaerobic conditions, which are favored by deep puncture wounds, wounds with necrotic tissue, crush injuries, and compound fractures. The vegetative bacteria remain at the wound site and do not invade tissues; indeed, the wound may appear trivial or even healed by the time neurological symptoms appear. C. tetani produces tetanospasmin, the second most potent toxin known (after botulinum toxin). Tetanospasmin is released when bacterial cells lyse and is taken up by motor neuron terminals at the neuromuscular junction. The toxin then undergoes retrograde axonal transport to the spinal cord, a process that takes several days and explains the incubation period of typically 3 to 21 days (shorter incubation periods generally indicate more severe disease). Tetanospasmin is an A-B toxin that specifically cleaves synaptobrevin (VAMP-2), a SNARE protein essential for synaptic vesicle fusion and neurotransmitter release. Within the spinal cord, the toxin specifically affects inhibitory interneurons (Renshaw cells) that normally release glycine and gamma-aminobutyric acid (GABA) to modulate motor neuron activity. By blocking release of these inhibitory neurotransmitters, tetanospasmin removes the normal inhibition of motor neurons, resulting in unopposed excitatory input and sustained muscle contraction. This mechanism explains the characteristic spastic paralysis of tetanus, which contrasts with the flaccid paralysis of botulism where the same SNARE protein is cleaved but at peripheral motor nerve terminals. The clinical features of tetanus reflect the characteristic distribution and progression of muscle hypertonicity. Trismus, or lockjaw, is often the first symptom, resulting from masseter muscle spasm that prevents mouth opening. Risus sardonicus describes the sustained contraction of facial muscles producing a grimacing expression. Opisthotonus is the dramatic arching of the back caused by spasm of the paraspinal muscles; in severe cases, patients may arch so severely that only the head and heels touch the bed. Reflex spasms are triggered by minor stimuli such as noise, light, or touch, and can be severe enough to cause vertebral fractures or respiratory arrest. Autonomic dysfunction, including alternating hypertension and hypotension, tachycardia, diaphoresis, and hyperthermia, occurs in severe cases and is a major cause of mortality. Treatment of tetanus requires multiple simultaneous interventions. Human tetanus immune globulin (TIG) neutralizes circulating toxin but cannot reverse toxin already bound within neurons; early administration is therefore critical. Wound debridement removes the source of toxin production. Antibiotics, preferably metronidazole, kill vegetative bacteria and prevent further toxin production; penicillin is an alternative but may theoretically enhance GABA antagonism. Supportive care is paramount: benzodiazepines (such as diazepam or midazolam) provide both muscle relaxation and anticonvulsant effects by enhancing GABA activity at remaining receptors. Severe cases require intensive care with sedation, neuromuscular blockade, and mechanical ventilation, often for weeks, as recovery requires regeneration of affected nerve terminals. Prevention through vaccination is far preferable; DTaP (diphtheria, tetanus, acellular pertussis) is given in childhood, with Tdap (tetanus, diphtheria, acellular pertussis) or Td (tetanus, diphtheria) boosters every 10 years or sooner after tetanus-prone wounds. Panel A: Tetanus pathogenesis pathway showing wound contaminated with C. tetani spores, germination under anaerobic conditions in necrotic tissue, tetanospasmin release from lysed bacteria, toxin uptake at motor nerve terminals and retrograde axonal transport to spinal cord, and action at inhibitory interneurons blocking glycine and GABA release with resulting unopposed motor neuron firing and spastic paralysis. Panel B: Tetanospasmin molecular mechanism showing the A-B toxin structure with binding domain (B), translocation domain, and catalytic domain (A), binding to ganglioside receptors, endocytosis and retrograde transport, and cleavage of synaptobrevin (VAMP-2) preventing synaptic vesicle fusion and neurotransmitter release at inhibitory synapses. Panel C: Clinical features of tetanus illustrated: trismus (lockjaw) with patient unable to open mouth, risus sardonicus facial grimace, opisthotonus with severe back arching, generalized rigidity, and reflex spasms triggered by stimuli, with comparison to botulism showing that both affect SNARE proteins but produce opposite clinical syndromes. Panel D: Tetanus treatment and prevention showing multimodal approach: tetanus immune globulin to neutralize circulating toxin (early administration critical), wound debridement to remove source, metronidazole to kill bacteria, benzodiazepines for muscle relaxation, ICU care with ventilation for severe cases, and vaccination schedule with DTaP in childhood and Tdap/Td boosters every 10 years or after wounds. --- ### VI. Clostridium botulinum (Botulism) Botulism is a paralytic illness caused by botulinum toxin, the most potent biological toxin known. Despite its extreme lethality, botulinum toxin has been purified for therapeutic and cosmetic use, illustrating how understanding of toxin mechanisms can be applied beneficially. Several distinct forms of botulism occur, each with different routes of intoxication but all producing the same clinical syndrome of descending flaccid paralysis through inhibition of acetylcholine release at neuromuscular junctions. The forms of botulism are distinguished by the mechanism of intoxication. Foodborne botulism, the classic form, results from ingestion of preformed toxin in improperly preserved foods; home-canned vegetables, fermented fish, and other foods with anaerobic environments and inadequate heat treatment during preparation are typical sources. Infant botulism, the most common form in developed countries, occurs when infants ingest C. botulinum spores (classically from honey, but also from environmental exposure) that germinate and produce toxin in the intestinal tract; the immature infant gut microbiome lacks colonization resistance present in adults. Wound botulism results from contamination of wounds with C. botulinum spores that germinate and produce toxin locally; this form has increased dramatically among intravenous drug users, particularly those injecting black tar heroin subcutaneously. Adult intestinal (enteric) botulism is rare and occurs in adults with abnormal intestinal anatomy or flora, allowing colonization similar to infant botulism. Iatrogenic botulism can result from therapeutic or cosmetic botulinum toxin injections, either through overdose or spread from the injection site. Botulinum toxin, like tetanospasmin, is a zinc metalloprotease that cleaves SNARE proteins required for neurotransmitter release, but it acts at peripheral cholinergic nerve terminals rather than in the central nervous system. Seven serotypes (A through G) are recognized based on antigenic differences; serotypes A, B, E, and rarely F cause human disease. The toxin is taken up at motor nerve terminals and cleaves SNARE proteins (synaptobrevin, SNAP-25, or syntaxin depending on serotype), preventing acetylcholine release and causing flaccid paralysis. Because the toxin acts at the neuromuscular junction rather than in the spinal cord, paralysis is flaccid (loss of muscle tone) rather than spastic. The clinical presentation of botulism follows a characteristic pattern of descending, symmetric flaccid paralysis beginning with the cranial nerves. Early symptoms include diplopia, ptosis, blurred vision, dysphagia, and dysarthria. Paralysis descends to affect the neck, arms, thorax, and legs. Autonomic manifestations include dry mouth, constipation, and urinary retention. The most dangerous consequence is respiratory muscle paralysis, which can cause respiratory failure and death if not anticipated and managed with mechanical ventilation. Mental status remains clear because the toxin does not cross the blood-brain barrier. Infant botulism presents as the "floppy baby syndrome" with poor feeding, weak cry, diminished reflexes, and generalized hypotonia. Diagnosis of botulism is primarily clinical, as laboratory confirmation takes days and should not delay treatment. Detection of toxin in serum, stool, or suspect food samples using mouse bioassay or mass spectrometry-based methods confirms the diagnosis. Treatment centers on antitoxin and supportive care. Heptavalent botulinum antitoxin (HBAT), derived from equine serum, neutralizes all seven serotypes and should be administered as early as possible; it prevents further toxin binding but cannot reverse already established paralysis. For infant botulism, BabyBIG (Botulism Immune Globulin Intravenous, human-derived) is used. Supportive care, particularly ventilatory support, is critical and may be required for weeks to months as recovery depends on regeneration of nerve terminals and formation of new neuromuscular junctions. Prevention focuses on proper food handling and avoidance of honey in infants under one year of age. Panel A: Types of botulism illustrated: foodborne showing improperly canned food with preformed toxin ingestion, infant botulism showing honey jar and immature infant gut colonized by C. botulinum producing toxin, wound botulism showing injection drug use with contaminated heroin and spores germinating in wound, and iatrogenic showing therapeutic/cosmetic Botox injection with potential systemic spread. Panel B: Botulinum toxin mechanism showing toxin binding to receptors at neuromuscular junction, internalization into motor nerve terminal, cleavage of SNARE proteins (synaptobrevin, SNAP-25, or syntaxin depending on serotype), prevention of acetylcholine vesicle fusion, and resulting failure of muscle contraction causing flaccid paralysis, contrasted with tetanus where same mechanism at different location causes opposite clinical effect. Panel C: Clinical presentation of botulism showing characteristic descending paralysis: cranial nerve findings appearing first (ptosis, diplopia, facial weakness, dysphagia, dysarthria), then descending to neck weakness, arm weakness, respiratory muscle weakness (most dangerous), and leg weakness, with clear mental status throughout, and infant presentation as "floppy baby" with poor feeding and weak cry. Panel D: Botulism treatment timeline showing early antitoxin administration (HBAT for adults, BabyBIG for infants) to prevent further toxin binding (effective only if given before toxin binds), supportive care with mechanical ventilation often required for weeks to months, and gradual recovery as new neuromuscular junctions form, with prevention message about proper food canning and avoiding honey in infants under 1 year. --- ### VII. Clostridium perfringens and Gas Gangrene Clostridium perfringens is a ubiquitous organism found in soil, in the intestinal tract of humans and animals, and in many environmental niches. It causes two distinct disease syndromes: gas gangrene (clostridial myonecrosis), a rapidly progressive and often fatal soft tissue infection, and food poisoning, a self-limited diarrheal illness. The organism's ability to grow extremely rapidly and to produce a potent array of tissue-destructive toxins underlies the dramatic clinical presentation of gas gangrene. Clostridium perfringens possesses several distinguishing microbiological characteristics. Unlike most clostridia, which grow slowly, C. perfringens is one of the fastest-growing pathogenic bacteria known, with a doubling time of approximately 10 minutes under optimal conditions. This rapid growth contributes to the fulminant course of gas gangrene. Spores are subterminal and rarely observed in clinical specimens. On blood agar, C. perfringens produces a characteristic double zone of hemolysis: an inner zone of complete beta-hemolysis due to theta-toxin (perfringolysin O) surrounded by an outer zone of incomplete hemolysis due to alpha-toxin. The Nagler reaction demonstrates alpha-toxin (lecithinase) activity: C. perfringens produces opalescence when grown on egg yolk agar, which is inhibited on the side of the plate containing antitoxin. Gas gangrene (clostridial myonecrosis) is a life-threatening emergency requiring immediate intervention. The disease typically follows traumatic injury that introduces C. perfringens spores into deep tissue with devitalized muscle and impaired blood supply. Historically associated with battlefield injuries, gas gangrene now most commonly follows severe trauma, surgery (particularly intestinal or biliary), or inadequate management of compound fractures. Alpha-toxin (phospholipase C, lecithinase) is the key virulence factor, hydrolyzing lecithin (phosphatidylcholine) in cell membranes, causing massive tissue destruction, hemolysis, and platelet aggregation leading to thrombosis that further compromises blood supply. Gas production by bacterial fermentation causes crepitus on palpation and gas visible on radiographs. The clinical progression is dramatic: pain out of proportion to exam findings, followed by rapidly spreading edema, skin discoloration (initially pale, then bronze, then black), bullae formation, and characteristic foul-smelling serosanguinous discharge. Systemic toxicity with shock and multi-organ failure develops rapidly. Treatment of gas gangrene requires urgent surgical debridement with removal of all necrotic tissue, often necessitating amputation to save the patient's life. Antibiotic therapy with high-dose penicillin G (which inhibits alpha-toxin production more effectively than it kills bacteria) plus clindamycin (which inhibits toxin synthesis) is standard. Hyperbaric oxygen therapy may be beneficial as an adjunct by increasing tissue oxygen tension to levels inhibitory to clostridial growth, though it should never delay surgical debridement. Even with optimal treatment, mortality remains 20 to 30 percent or higher. Clostridium perfringens food poisoning is entirely different in character from gas gangrene. It results from ingestion of large numbers of C. perfringens vegetative cells, typically in meat dishes that have been inadequately cooked or held at improper temperatures. Spores survive cooking, germinate as the food cools, and multiply rapidly during holding at warm temperatures. In the small intestine, vegetative cells sporulate and release an enterotoxin (distinct from alpha-toxin) that binds to intestinal epithelium and disrupts tight junctions. Symptoms begin 8 to 16 hours after ingestion and consist of watery diarrhea and abdominal cramping without vomiting or fever. The illness is self-limited, resolving within 24 hours, and treatment is supportive. Panel A: C. perfringens microbiological characteristics showing gram-positive rods with subterminal spores (rarely seen), double zone of hemolysis on blood agar (inner complete hemolysis from theta-toxin, outer incomplete from alpha-toxin), Nagler reaction on egg yolk agar showing lecithinase activity with opalescence inhibited by antitoxin on one side of the plate, and rapid growth rate noted as fastest-growing pathogenic bacterium. Panel B: Gas gangrene pathogenesis showing traumatic wound introducing spores into devitalized tissue, germination under anaerobic conditions, alpha-toxin (phospholipase C) destroying cell membranes causing tissue necrosis and hemolysis, platelet aggregation and thrombosis further compromising blood supply, gas production from fermentation, and rapid spread through tissue planes. Panel C: Gas gangrene clinical progression shown as timeline: injury with wound contamination, early symptoms (pain out of proportion within hours), progression with edema, skin discoloration (pale to bronze to black), bullae formation, crepitus, and serosanguinous discharge, X-ray showing gas in soft tissues, and systemic toxicity with shock, with mortality statistics indicated. Panel D: Treatment of gas gangrene showing simultaneous interventions: emergent surgical debridement with removal of all necrotic tissue (amputation may be lifesaving), high-dose IV penicillin G plus clindamycin, consideration of hyperbaric oxygen as adjunct (never delays surgery), and contrasted with C. perfringens food poisoning showing entirely different presentation (watery diarrhea, cramping, self-limited) from ingestion of enterotoxin produced during sporulation in intestine. --- ### VIII. Clostridioides difficile Clostridioides difficile (formerly Clostridium difficile, recently reclassified based on genomic analysis) is the most common cause of healthcare-associated infectious diarrhea and a major public health concern. The organism causes disease through an unusual pathogenic mechanism: disruption of the normal intestinal microbiota, most commonly by antibiotics, allows C. difficile to proliferate and produce toxins that damage the intestinal mucosa. Understanding the epidemiology, pathogenesis, and management of C. difficile infection (CDI) is essential given its prevalence in healthcare settings. Clostridioides difficile produces highly resistant spores that are the principal mode of transmission. Spores survive on environmental surfaces for months, resist alcohol-based hand sanitizers, and are spread by the fecal-oral route in healthcare facilities. Once ingested, spores survive gastric acid and germinate into vegetative cells in the colon. Under normal circumstances, the complex intestinal microbiota provides colonization resistance that prevents C. difficile proliferation. However, antibiotic therapy disrupts this microbiota, creating an ecological niche that C. difficile readily fills. Nearly all antibiotics have been associated with CDI, but clindamycin, fluoroquinolones, and broad-spectrum cephalosporins pose particularly high risk. Other risk factors include advanced age, hospitalization, proton pump inhibitor use, and immunocompromise. The emergence of the hypervirulent NAP1/BI/027 strain, which produces larger amounts of toxin and the additional binary toxin, has been associated with more severe disease and outbreaks. The pathogenesis of CDI centers on two large toxins, toxin A (TcdA) and toxin B (TcdB), both of which are glucosyltransferases that inactivate Rho family GTPases in host cells. Rho GTPases regulate the actin cytoskeleton, cell adhesion, and tight junction integrity; their inactivation causes cell rounding, loss of tight junction integrity, and ultimately cell death. Toxin A was historically considered the enterotoxin causing fluid secretion, while toxin B was considered the more potent cytotoxin, but both toxins contribute to disease pathogenesis. Some strains, particularly NAP1/BI/027, also produce binary toxin (CDT), which has ADP-ribosyltransferase activity and may contribute to disease severity. The characteristic pathologic finding is pseudomembranous colitis: raised, yellowish-white plaques (pseudomembranes) composed of fibrin, mucus, neutrophils, and cellular debris adherent to inflamed colonic mucosa. Clinical manifestations of CDI range from mild diarrhea to life-threatening colitis. Mild to moderate disease presents with watery diarrhea (classically malodorous), lower abdominal cramping, and low-grade fever. Severe disease is defined by marked leukocytosis (white blood cell count greater than 15,000), rising serum creatinine, or severe abdominal pain. Fulminant disease includes hypotension, shock, ileus, or toxic megacolon; ileus may result in absence of diarrhea, making diagnosis challenging. Pseudomembranous colitis is diagnosed definitively by colonoscopy showing characteristic pseudomembranes. Diagnosis of CDI requires both detection of the organism or its toxins and presence of clinically significant diarrhea. Current algorithms typically use a multi-step approach: screening with glutamate dehydrogenase (GDH) antigen detection (sensitive but not specific for toxigenic strains), followed by toxin enzyme immunoassay (EIA) or nucleic acid amplification testing (NAAT) for toxin genes. NAAT is highly sensitive but may detect colonization without disease, so clinical correlation is essential. Treatment depends on disease severity. For initial non-severe episodes, oral vancomycin or fidaxomicin is recommended; metronidazole is no longer first-line due to inferior efficacy. For severe disease, oral vancomycin at higher doses is used. For fulminant disease, oral vancomycin plus intravenous metronidazole is indicated, with possible addition of vancomycin retention enemas if ileus is present; surgical consultation for colectomy may be lifesaving. First recurrence is treated similarly to the initial episode; multiple recurrences may benefit from fidaxomicin, bezlotoxumab (a monoclonal antibody against toxin B), or fecal microbiota transplantation (FMT), which has shown remarkable efficacy by restoring colonization resistance. Panel A: C. difficile transmission and risk factors showing spores in healthcare environment on surfaces, hands of healthcare workers, and medical equipment, fecal-oral transmission to patient, antibiotic disruption of normal colonic microbiota creating ecological niche for C. difficile proliferation, with major risk factors listed (antibiotics especially clindamycin/fluoroquinolones/cephalosporins, hospitalization, advanced age, PPI use, immunocompromise). Panel B: C. difficile toxin mechanism showing toxin A and toxin B as glucosyltransferases binding to cell surface receptors, internalization, and inactivation of Rho GTPases causing disruption of actin cytoskeleton, loss of tight junction integrity, cell rounding and death, with binary toxin (CDT) additional factor in some strains, resulting in mucosal damage, inflammation, and fluid secretion. Panel C: Clinical spectrum of CDI illustrated from mild to fulminant: mild disease (watery malodorous diarrhea, cramping), moderate disease (same with low-grade fever), severe disease (leukocytosis >15,000, elevated creatinine, severe pain), fulminant disease (hypotension, shock, ileus, toxic megacolon), and colonoscopic appearance showing pseudomembranes as raised yellowish plaques on inflamed colonic mucosa. Panel D: CDI diagnosis and treatment algorithm showing two-step testing (GDH screening then toxin EIA or NAAT), treatment stratified by severity (oral vancomycin or fidaxomicin for non-severe, higher-dose vancomycin for severe, vancomycin plus IV metronidazole for fulminant with surgical consultation), and recurrence management (fidaxomicin, bezlotoxumab, fecal microbiota transplantation for multiple recurrences). --- ### IX. Listeria monocytogenes Listeria monocytogenes is a facultatively intracellular gram-positive rod that causes listeriosis, a disease with particular propensity to affect pregnant women, neonates, the elderly, and immunocompromised individuals. Its unique biological properties, including the ability to grow at refrigerator temperatures and to spread directly from cell to cell, distinguish it from other bacterial pathogens. Understanding the pathogenesis and epidemiology of listeriosis is essential for prevention and management. Listeria monocytogenes possesses several distinctive microbiological characteristics. It is a short, gram-positive rod that may appear coccoid, potentially leading to confusion with streptococci. On blood agar, it produces a narrow zone of beta-hemolysis. A key feature is tumbling motility at room temperature (20 to 25 degrees Celsius), caused by peritrichous flagella that are expressed at lower temperatures but not at 37 degrees Celsius. Most remarkably, L. monocytogenes grows at refrigeration temperatures (as low as 4 degrees Celsius), a property called psychrotrophic growth that enables it to multiply in refrigerated foods and distinguishes it from most other foodborne pathogens. It is catalase-positive, unlike streptococci. The intracellular life cycle of L. monocytogenes represents a paradigm of bacterial pathogenesis. Following ingestion of contaminated food, bacteria cross the intestinal epithelium by invading enterocytes using surface proteins called internalins. Internalin A (InlA) binds E-cadherin on intestinal epithelial cells, while internalin B (InlB) binds the hepatocyte growth factor receptor c-Met on hepatocytes. Once internalized, the bacterium resides in a phagosome but rapidly escapes into the cytoplasm using listeriolysin O (LLO), a cholesterol-dependent pore-forming toxin, along with phospholipases. In the cytoplasm, L. monocytogenes hijacks the host cell actin polymerization machinery using the surface protein ActA, which activates the Arp2/3 complex to polymerize actin filaments at one pole of the bacterium. This "actin rocket" propels the bacterium through the cytoplasm at speeds up to 1.5 micrometers per second. When the bacterium reaches the cell membrane, it pushes outward, forming a protrusion that is engulfed by the adjacent cell, allowing direct cell-to-cell spread without exposure to the extracellular environment, antibodies, or complement. Listeriosis presents differently depending on the patient population. In healthy adults, infection typically causes febrile gastroenteritis with watery diarrhea, which is self-limited. In immunocompromised patients and the elderly, invasive disease presents as meningitis or bacteremia (septicemia). Listeria meningitis is the third most common cause of bacterial meningitis in adults (after pneumococcus and meningococcus) and has a predilection for brainstem involvement (rhombencephalitis). In pregnant women, listeriosis causes mild flu-like illness in the mother but can be devastating to the fetus, causing spontaneous abortion, stillbirth, premature delivery, or neonatal sepsis and meningitis (granulomatosis infantisepticum). The bacteria cross the placenta and infect the fetus through bacteremia. Diagnosis requires culture of blood, cerebrospinal fluid, or other normally sterile sites. Gram stain of CSF may show gram-positive rods, which should raise immediate suspicion for Listeria, though sensitivity is limited. Treatment requires ampicillin, often combined with gentamicin for synergistic bactericidal effect in serious infections such as meningitis. Critically, cephalosporins have no activity against Listeria, so empiric meningitis regimens in patients at risk for listeriosis must include ampicillin or an alternative active agent. Trimethoprim-sulfamethoxazole is an alternative for penicillin-allergic patients. Prevention focuses on avoiding high-risk foods (soft cheeses, delicatessen meats, smoked seafood, unpasteurized dairy products) in high-risk populations, particularly pregnant women. Panel A: Listeria monocytogenes characteristics showing short gram-positive rods that may appear coccoid, beta-hemolysis on blood agar (narrow zone), tumbling motility at room temperature demonstrated in hanging drop preparation, and growth curve showing psychrotrophic ability to multiply at refrigeration temperatures (4 degrees Celsius) distinguishing it from most foodborne pathogens. Panel B: Listeria intracellular life cycle illustrated in sequential steps: attachment and invasion via internalins (InlA binding E-cadherin, InlB binding c-Met), internalization into phagosome, escape using listeriolysin O (LLO) and phospholipases, ActA-mediated actin polymerization forming rocket tail that propels bacterium through cytoplasm, membrane protrusion, and engulfment by adjacent cell allowing cell-to-cell spread without extracellular exposure. Panel C: Listeriosis clinical presentations in different populations: healthy adults (febrile gastroenteritis, self-limited), immunocompromised and elderly (meningitis with predilection for brainstem causing rhombencephalitis, bacteremia/septicemia), and pregnancy (mild maternal illness but devastating fetal consequences including abortion, stillbirth, premature delivery, neonatal sepsis/meningitis), with transplacental transmission illustrated. Panel D: Listeriosis diagnosis and treatment showing CSF Gram stain with gram-positive rods (must consider Listeria), blood and CSF culture for definitive diagnosis, treatment with ampicillin plus gentamicin (noting cephalosporins have no activity), and prevention through food safety in high-risk populations (avoid soft cheeses, deli meats, smoked seafood, unpasteurized products) especially during pregnancy. --- ### X. Corynebacterium and Other Gram-Positive Rods Corynebacterium diphtheriae, the causative agent of diphtheria, was once a major cause of childhood morbidity and mortality but has been largely controlled through vaccination in developed countries. The organism produces a powerful exotoxin that causes the characteristic clinical features of diphtheria. Other Corynebacterium species and various other gram-positive rods cause diverse clinical syndromes and occasionally serious infections, particularly in immunocompromised patients. Corynebacterium diphtheriae displays distinctive microbiological features. It is a pleomorphic gram-positive rod with club-shaped morphology (the genus name derives from "coryne," Greek for club) and characteristically arranges in V or L shapes described as "Chinese letters" or palisades. Metachromatic granules (Babes-Ernst or volutin bodies) composed of polymetaphosphate can be demonstrated with special stains (methylene blue or Albert stain) and represent stored nutrient reserves. The organism grows on Loeffler medium and can be selectively isolated on cystine-tellurite agar, where it produces characteristic black colonies. Importantly, only strains lysogenized by a corynebacteriophage carrying the tox gene produce diphtheria toxin; non-toxigenic strains cause only local infection without systemic toxicity. Diphtheria toxin is a classic A-B toxin that inhibits protein synthesis through ADP-ribosylation of elongation factor 2 (EF-2). The B fragment binds to the heparin-binding epidermal growth factor precursor receptor on host cells, facilitating toxin internalization. Following endocytosis and acidification of the endosome, the A fragment translocates to the cytoplasm where it catalyzes transfer of ADP-ribose from NAD+ to diphthamide, a unique amino acid in EF-2. This modification inactivates EF-2, halting protein synthesis and causing cell death. The toxin is extraordinarily potent; a single molecule can kill a cell. Classic respiratory diphtheria presents as pharyngitis with formation of a tough, adherent, gray pseudomembrane on the tonsils and pharynx. The membrane consists of dead epithelial cells, fibrin, bacteria, and inflammatory cells; attempts to remove it cause bleeding. Toxin absorbed from the local site of infection causes systemic effects. Myocarditis, occurring in 10 to 25 percent of cases, may cause arrhythmias, heart block, and heart failure. Neurological manifestations, occurring weeks after infection, include cranial nerve palsies (particularly affecting the palate and pharynx) and peripheral neuropathy. Cutaneous diphtheria presents as chronic non-healing ulcers, typically in tropical regions. Treatment of diphtheria requires both neutralization of circulating toxin with diphtheria antitoxin (equine-derived) and antibiotics (penicillin or erythromycin) to eliminate the organism and prevent further toxin production. Antitoxin must be given promptly, as it neutralizes only circulating toxin and cannot reverse toxin already bound to cells. Supportive care for respiratory compromise, myocarditis, and neuropathy is essential. Prevention through vaccination with diphtheria toxoid (inactivated toxin) is highly effective; DTaP is given in childhood and Tdap or Td boosters maintain immunity in adults. Other Corynebacterium species occasionally cause significant disease. Corynebacterium jeikeium is a multidrug-resistant nosocomial pathogen causing bacteremia and device-related infections in immunocompromised patients, particularly those with hematologic malignancies; vancomycin is the treatment of choice. Corynebacterium urealyticum causes urinary tract infections characterized by alkaline urine and struvite stone formation due to urease production. Corynebacterium minutissimum causes erythrasma, a superficial skin infection of intertriginous areas that fluoresces coral-red under Wood lamp examination due to porphyrin production. Other notable gram-positive rods include Actinomyces species, which cause actinomycosis with characteristic sulfur granules and draining sinus tracts; Nocardia species, partially acid-fast filamentous bacteria causing pulmonary disease, brain abscess, and disseminated infection in immunocompromised patients; and Propionibacterium (Cutibacterium) acnes, which contributes to acne pathogenesis and causes indolent prosthetic device infections. Panel A: Corynebacterium diphtheriae microscopic appearance showing club-shaped gram-positive rods arranged in V-shapes, L-shapes, and palisades resembling Chinese letters, metachromatic (volutin) granules visible with methylene blue stain as dark-staining bodies, and colonial morphology on tellurite agar showing characteristic black colonies due to tellurite reduction. Panel B: Diphtheria toxin mechanism illustrated showing the A-B toxin structure with binding (B) fragment attaching to HB-EGF receptor, receptor-mediated endocytosis, translocation of catalytic (A) fragment across endosomal membrane at low pH, and ADP-ribosylation of EF-2 (specifically at diphthamide residue) using NAD+ as substrate, resulting in protein synthesis inhibition and cell death. Panel C: Diphtheria clinical features showing pharyngeal diphtheria with characteristic gray adherent pseudomembrane on tonsils and pharynx (bleeding if removed), bull neck appearance from cervical lymphadenopathy and edema, and systemic toxin effects including myocarditis (arrhythmias, heart block, heart failure) and neurological complications (palatal paralysis, peripheral neuropathy), with treatment approach (antitoxin plus antibiotics) and prevention (vaccination). Panel D: Other gram-positive rods overview showing Actinomyces (actinomycosis with sulfur granules and sinus tracts, treated with prolonged penicillin), Nocardia (partially acid-fast filaments causing pulmonary/CNS/disseminated disease in immunocompromised, treated with TMP-SMX), and Propionibacterium (Cutibacterium) acnes (acne pathogenesis, prosthetic device infections with indolent course), with key distinguishing features and clinical associations for each. --- ## Summary - Gram-positive rods are classified as spore-forming (aerobic Bacillus, anaerobic Clostridium) or non-spore-forming (Listeria, Corynebacterium, Actinomyces); spores provide environmental resistance and are the infectious form for several diseases - B. anthracis causes anthrax through a tripartite toxin system (PA + EF + LF); cutaneous disease (black eschar), inhalational disease (widened mediastinum, high mortality), and GI disease; treatment requires ciprofloxacin plus additional agents; prevention includes vaccination - B. cereus causes emetic (preformed toxin in rice, vomiting) and diarrheal (enterotoxin, watery diarrhea) food poisoning; it also causes serious ocular and invasive infections, especially in immunocompromised; resistant to beta-lactams - Tetanus results from tetanospasmin blocking inhibitory neurotransmitters in the spinal cord, causing spastic paralysis with trismus, risus sardonicus, and opisthotonus; treatment includes TIG, metronidazole, wound care, and supportive care; prevention through DTaP/Tdap vaccination - Botulism results from botulinum toxin blocking acetylcholine release at neuromuscular junctions, causing descending flaccid paralysis; forms include foodborne, infant (honey), and wound; treatment with antitoxin and ventilatory support - C. perfringens causes gas gangrene (alpha-toxin mediating rapid myonecrosis requiring emergent debridement) and food poisoning (enterotoxin causing self-limited diarrhea) - C. difficile causes antibiotic-associated colitis through toxins A and B; treatment stratified by severity with oral vancomycin or fidaxomicin; recurrences may benefit from FMT - L. monocytogenes is an intracellular pathogen causing meningitis in elderly and immunocompromised and devastating fetal/neonatal infection; treatment with ampicillin (cephalosporins inactive) - C. diphtheriae causes diphtheria through toxin-mediated inhibition of protein synthesis (EF-2 ADP-ribosylation); characteristic pseudomembrane, myocarditis, neuropathy; prevented by vaccination --- ## Key Terms | Term | Definition | |------|------------| | Endospore | Highly resistant dormant bacterial structure formed by Bacillus and Clostridium species; survives environmental extremes and is the infectious form for many clostridial diseases | | Anthrax | Disease caused by Bacillus anthracis; forms include cutaneous (black eschar), inhalational (widened mediastinum), and gastrointestinal; caused by tripartite toxin (PA, EF, LF) | | Tetanospasmin | Toxin of C. tetani that blocks glycine and GABA release from inhibitory interneurons by cleaving synaptobrevin, causing spastic paralysis | | Botulinum toxin | Toxin of C. botulinum that blocks acetylcholine release at neuromuscular junctions by cleaving SNARE proteins, causing flaccid paralysis | | Gas gangrene | Rapidly progressive clostridial myonecrosis caused primarily by C. perfringens alpha-toxin (phospholipase C); characterized by tissue necrosis, gas production, and systemic toxicity | | Pseudomembranous colitis | Severe manifestation of C. difficile infection characterized by raised yellowish pseudomembranes on colonic mucosa composed of fibrin, mucus, and inflammatory debris | | Listeriolysin O | Pore-forming toxin of Listeria monocytogenes that allows escape from the phagosome into the host cell cytoplasm | | Diphtheria toxin | A-B toxin of C. diphtheriae that ADP-ribosylates elongation factor 2 (EF-2), inhibiting protein synthesis and causing cell death | | Fecal microbiota transplantation | Treatment for recurrent C. difficile infection involving transfer of stool from healthy donor to restore colonization resistance | | Psychrotrophic | Ability to grow at refrigeration temperatures; characteristic of Listeria monocytogenes that enables it to multiply in refrigerated foods | | Internalin | Surface proteins of Listeria monocytogenes (InlA, InlB) that mediate invasion of host cells by binding to E-cadherin and c-Met receptors | | ActA | Listeria surface protein that activates host cell actin polymerization machinery to propel the bacterium through the cytoplasm and enable cell-to-cell spread | --- *This content is subject to the [MIT License](https://opensource.org/licenses/MIT). © 2024–2026 Hibbert School of Medicine.*