# Lecture 6: Gram-Negative Enteric Bacteria ## Unit 2.8: Microbiology --- ## Learning Objectives By the end of this lecture, students will be able to: 1. Describe the Enterobacteriaceae family and shared characteristics 2. Explain Escherichia coli pathotypes and clinical syndromes 3. Describe Salmonella and typhoid fever 4. Explain Shigella and bacillary dysentery 5. Describe other clinically significant enteric organisms 6. Explain antimicrobial resistance in Enterobacteriaceae --- ## Lecture Outline ### I. Enterobacteriaceae Overview The family Enterobacteriaceae comprises a large and diverse group of gram-negative bacteria that inhabit the gastrointestinal tract of humans and animals and are found throughout the environment. This family includes some of the most commonly encountered clinical pathogens, responsible for urinary tract infections, gastrointestinal diseases, pneumonia, bacteremia, and various other infections. Understanding the shared characteristics of these organisms provides a foundation for laboratory identification, while knowledge of individual genera and species enables prediction of clinical behavior and appropriate treatment. Members of the Enterobacteriaceae share several defining characteristics that unite this diverse family. They are gram-negative rods, though cell size and shape vary among genera. All members are oxidase-negative, a critical distinction that separates them from other gram-negative rods such as Pseudomonas (oxidase-positive). They are catalase-positive and facultatively anaerobic, capable of both aerobic respiration and fermentation. A defining metabolic characteristic is the ability to ferment glucose, though fermentation of other sugars varies among species and is used for laboratory identification. Most species are motile by means of peritrichous flagella, though notable exceptions include Shigella and Klebsiella, which are non-motile. Many species are part of the normal intestinal flora, while others are always pathogenic when present. Laboratory identification of Enterobacteriaceae begins with their growth characteristics and lactose fermentation status. These organisms grow readily on standard media and on selective media designed to inhibit gram-positive organisms. MacConkey agar is the workhorse medium for Enterobacteriaceae identification, containing bile salts and crystal violet to inhibit gram-positive bacteria, lactose as a fermentable carbohydrate, and neutral red as a pH indicator. Lactose-fermenting organisms produce acid that lowers pH, causing colonies to appear pink or red, while lactose non-fermenters produce colorless colonies. Lactose fermenters include Escherichia coli, Klebsiella, and Enterobacter, while lactose non-fermenters include Salmonella, Shigella, and Proteus. Triple sugar iron (TSI) agar is used to characterize fermentation patterns of glucose, lactose, and sucrose, as well as hydrogen sulfide production. The IMViC tests (Indole, Methyl Red, Voges-Proskauer, Citrate utilization) provide additional biochemical differentiation. Modern laboratories increasingly use automated biochemical identification systems or MALDI-TOF mass spectrometry for rapid species identification. The antigenic structure of Enterobacteriaceae is important for serotyping, epidemiology, and understanding pathogenesis. The O antigen (somatic antigen) is the outermost polysaccharide component of lipopolysaccharide (LPS), extending from the cell surface. Different O antigens define serogroups used to classify strains; for example, E. coli O157 denotes a strain with the 157th identified O antigen. The H antigen is the flagellar antigen, composed of the protein flagellin. H antigens define serotypes within serogroups; E. coli O157:H7 has both the O157 somatic antigen and the H7 flagellar antigen. The K antigen is the capsular antigen present in encapsulated species such as Klebsiella and some E. coli strains. Serotyping is particularly important for epidemiological tracking of outbreaks and for identifying specific pathogenic variants, such as the association of E. coli O157:H7 with hemolytic uremic syndrome. Panel A: Gram stain appearance and colonial morphology of Enterobacteriaceae showing gram-negative rods of various sizes, with colonies on MacConkey agar demonstrating lactose-fermenters (pink colonies of E. coli and Klebsiella) versus lactose non-fermenters (colorless colonies of Salmonella and Shigella), with the key oxidase-negative reaction displayed. Panel B: Major genera of Enterobacteriaceae organized in a hierarchical diagram showing Escherichia, Salmonella, Shigella, Klebsiella, Proteus, Enterobacter, Serratia, Citrobacter, and Yersinia, with key species and associated diseases for each genus indicated. Panel C: Laboratory identification flowchart beginning with gram-negative rod isolated, oxidase test (negative for Enterobacteriaceae), MacConkey agar showing lactose fermentation status, followed by TSI slant interpretation and biochemical tests (IMViC), leading to species identification with typical patterns for major organisms displayed. Panel D: Antigenic structure of Enterobacteriaceae showing a bacterial cell with labeled O antigen (outermost LPS polysaccharide, somatic), H antigen (flagella), and K antigen (capsule where present), with example of serotyping nomenclature explained using E. coli O157:H7. --- ### II. Escherichia coli Escherichia coli is the most thoroughly studied bacterial species and serves as a model organism for molecular biology research. In its commensal role, E. coli is a harmless and even beneficial inhabitant of the human intestinal tract. However, specific strains have acquired virulence factors that enable them to cause a variety of diseases, from urinary tract infections to life-threatening meningitis and hemolytic uremic syndrome. Understanding the different pathotypes of E. coli and their associated syndromes is essential for clinical practice. As a commensal organism, E. coli is the most abundant facultative anaerobe in the human colon, present at concentrations of approximately 10 to the seventh to 10 to the eighth organisms per gram of feces. It colonizes the gastrointestinal tract within hours of birth and normally persists throughout life. Commensal E. coli provides benefits to the host, including synthesis of vitamin K, competitive exclusion of pathogenic bacteria through colonization resistance, and stimulation of immune system development. These commensal strains lack the specific virulence factors that characterize pathogenic E. coli. Extra-intestinal pathogenic E. coli (ExPEC) strains cause disease outside the gastrointestinal tract, most commonly urinary tract infections, neonatal meningitis, and bacteremia. Uropathogenic E. coli (UPEC) is the leading cause of community-acquired urinary tract infections, responsible for approximately 80 percent of cases. UPEC strains possess specific virulence factors that enable colonization of the urinary tract: P pili (or P fimbriae) bind to glycolipid receptors containing galactose-galactose residues on uroepithelial cells, particularly in the kidneys, and are associated with pyelonephritis; type 1 pili bind mannose residues and are important for bladder colonization; alpha-hemolysin is a pore-forming toxin that damages host cells and releases iron for bacterial use; and siderophores (aerobactin, enterobactin) scavenge iron in the iron-limited urinary environment. UPEC can cause the full spectrum of UTI, from asymptomatic bacteriuria to cystitis to pyelonephritis to urosepsis. E. coli strains expressing the K1 capsular antigen are the leading cause of neonatal meningitis and the second most common cause of neonatal sepsis after Group B Streptococcus. The K1 capsule is a polymer of polysialic acid that is structurally identical to polysialic acid found in human neural tissue, providing immune evasion through molecular mimicry and poor immunogenicity. K1-encapsulated strains are acquired from the maternal genital tract during delivery and can invade from the bloodstream into the central nervous system. The K1 capsule is also associated with severe bacteremia in neonates. Treatment of K1 E. coli meningitis requires antibiotics that penetrate the central nervous system and are active against the organism, typically ampicillin (if susceptible) or a third-generation cephalosporin such as cefotaxime, often combined with an aminoglycoside for synergistic bactericidal effect. Panel A: E. coli as normal intestinal flora illustrated showing anatomical diagram of the colon with E. coli as the predominant facultative anaerobe among the diverse microbiota, benefits listed (vitamin K synthesis, colonization resistance, immune development), and quantification of typical abundance (10^7-10^8 per gram feces). Panel B: Uropathogenic E. coli (UPEC) virulence factors and pathogenesis showing the bacterium with P pili binding Gal-Gal receptors on uroepithelial cells in the kidney causing pyelonephritis, type 1 pili binding mannose in the bladder causing cystitis, alpha-hemolysin creating pores in host cell membranes, and siderophores capturing iron, with spectrum of UTI (asymptomatic bacteriuria to urosepsis) indicated. Panel C: K1 antigen and neonatal meningitis showing the polysialic acid K1 capsule surrounding the bacterium, structural similarity to human neural polysialic acid molecules providing immune evasion, acquisition from maternal genital tract during delivery, bloodstream invasion, and crossing of the blood-brain barrier to cause meningitis, with treatment options listed. Panel D: Extra-intestinal pathogenic E. coli (ExPEC) disease spectrum illustrated on a human body diagram showing UTI (bladder and kidney), bacteremia (bloodstream), neonatal meningitis (brain), and less commonly pneumonia (lungs) and intra-abdominal infections, with risk factors and predisposing conditions for each site. --- ### III. Diarrheagenic E. coli In addition to causing extra-intestinal infections, specific E. coli strains cause gastrointestinal disease through distinct pathogenic mechanisms. These diarrheagenic E. coli pathotypes are defined by their virulence factors and the clinical syndromes they produce. Understanding these pathotypes is essential because they differ markedly in epidemiology, clinical presentation, and management, including whether antibiotic treatment is beneficial or potentially harmful. Enterotoxigenic E. coli (ETEC) is the most common cause of traveler's diarrhea and an important cause of childhood diarrhea in developing countries. ETEC strains produce heat-labile toxin (LT) and/or heat-stable toxin (ST), which act on intestinal epithelium to cause secretory diarrhea without invading or destroying the mucosa. Heat-labile toxin is structurally and functionally similar to cholera toxin: it is an A-B toxin with a pentameric B subunit that binds GM1 ganglioside and an A subunit that ADP-ribosylates the stimulatory G protein Gsα, permanently activating adenylyl cyclase and elevating intracellular cyclic AMP. Elevated cAMP causes chloride secretion and inhibits sodium absorption, producing watery diarrhea. Heat-stable toxin activates guanylate cyclase, elevating cyclic GMP, which produces similar secretory effects. ETEC infections typically present as acute watery, non-bloody diarrhea with abdominal cramps, beginning one to two days after ingestion of contaminated food or water. The illness is usually self-limited, lasting three to five days, and treatment is primarily supportive with oral rehydration. Antibiotics may shorten the course of illness but are not routinely required for mild cases. Enteropathogenic E. coli (EPEC) is an important cause of infant diarrhea in developing countries, typically affecting children under two years of age. EPEC does not produce toxins but causes diarrhea through a distinctive mechanism called the attaching and effacing lesion. Using a type III secretion system, EPEC injects effector proteins into intestinal epithelial cells, including Tir (translocated intimin receptor), which inserts into the host cell membrane. The bacterial surface protein intimin then binds to Tir, creating intimate attachment. This process triggers dramatic rearrangement of the host cell cytoskeleton, with actin accumulation beneath the attached bacterium forming a pedestal-like structure. The attaching and effacing lesion destroys microvilli, reducing the absorptive surface area and causing watery diarrhea without blood. Treatment is primarily supportive. Enterohemorrhagic E. coli (EHEC), also called Shiga toxin-producing E. coli (STEC), causes bloody diarrhea and the potentially fatal hemolytic uremic syndrome (HUS). The most common serotype is O157:H7, though other serotypes also produce Shiga toxin. EHEC produces attaching and effacing lesions similar to EPEC but additionally produces Shiga toxin (Stx), which is virtually identical to the toxin produced by Shigella dysenteriae type 1. Shiga toxin has an A-B structure, with the B subunit binding to globotriaosylceramide (Gb3) on host cell surfaces and the A subunit cleaving ribosomal RNA to halt protein synthesis and cause cell death. Infection typically begins with watery diarrhea that progresses to bloody diarrhea (hemorrhagic colitis) over one to two days. The stool is characteristically bloody without significant mucus or leukocytes, distinguishing it from inflammatory diarrhea. Approximately 5 to 10 percent of patients, particularly children under five and elderly individuals, develop HUS, characterized by microangiopathic hemolytic anemia (fragmented red blood cells called schistocytes), thrombocytopenia, and acute kidney injury. HUS results from Shiga toxin damaging glomerular endothelial cells, triggering microvascular thrombosis. Critically, antibiotic treatment of EHEC infection is contraindicated because bacterial killing may increase toxin release and worsen the risk of HUS. Treatment is supportive with close monitoring for HUS development. Panel A: ETEC pathogenesis and clinical features showing heat-labile toxin (LT) with AB5 structure binding GM1 ganglioside, ADP-ribosylating Gsα, activating adenylyl cyclase, raising cAMP, and causing chloride secretion and water loss; heat-stable toxin (ST) activating guanylate cyclase and raising cGMP; and clinical presentation of traveler's diarrhea with watery non-bloody stool, oral rehydration therapy, and typical self-limited course. Panel B: EPEC attaching and effacing lesion illustrated in sequential steps: initial adherence via bundle-forming pili, type III secretion injecting Tir and other effectors into host cell, Tir insertion into host membrane, intimin binding to Tir creating intimate attachment, actin recruitment forming pedestal beneath bacterium, microvillus effacement, and resulting malabsorption causing watery diarrhea. Panel C: EHEC/STEC pathogenesis showing combination of attaching and effacing lesion plus Shiga toxin production, Shiga toxin structure with B subunit binding Gb3 and A subunit cleaving rRNA, clinical progression from watery to bloody diarrhea, and warning that antibiotics are contraindicated due to increased HUS risk. Panel D: Hemolytic uremic syndrome (HUS) pathophysiology and presentation showing Shiga toxin damaging glomerular endothelial cells, microvascular thrombosis, red blood cell fragmentation producing schistocytes, platelet consumption causing thrombocytopenia, and kidney damage from microvascular occlusion, with clinical triad (microangiopathic hemolytic anemia, thrombocytopenia, acute renal failure) and supportive management emphasized. --- ### IV. Salmonella The genus Salmonella comprises gram-negative enteric bacteria that cause a spectrum of disease ranging from self-limited gastroenteritis to life-threatening typhoid fever. Although Salmonella taxonomy is complex (all pathogenic Salmonella belong to a single species, Salmonella enterica, with multiple subspecies and over 2500 serovars), clinical classification into typhoidal and non-typhoidal Salmonella provides a practical framework for understanding disease manifestations and guiding management. Non-typhoidal Salmonella (NTS) causes acute gastroenteritis, one of the most common foodborne illnesses worldwide. NTS serovars, including Salmonella typhimurium and Salmonella enteritidis, are acquired through consumption of contaminated food, particularly poultry, eggs, and produce, or through contact with animals (reptiles are an important reservoir). After an incubation period of 12 to 48 hours, patients develop diarrhea (which may be watery or bloody), fever, abdominal cramps, and sometimes nausea and vomiting. Unlike Shigella, which is highly invasive and remains localized to the intestine, Salmonella invades intestinal epithelium but typically causes inflammation limited to the mucosa and submucosa. The illness is usually self-limited, resolving in four to seven days without specific treatment. Antibiotic therapy is not recommended for uncomplicated NTS gastroenteritis in healthy adults because it does not shorten illness duration and may prolong the carrier state. However, antibiotics are indicated for patients at high risk of invasive disease (infants, elderly, immunocompromised, those with prosthetic devices or vascular abnormalities) and for those with bacteremia or extraintestinal focal infections. Typhoid fever, caused by Salmonella typhi (and paratyphoid fever caused by Salmonella paratyphi A, B, and C), is a severe systemic illness distinct from NTS gastroenteritis. Typhoid fever is acquired through ingestion of food or water contaminated with human feces, as humans are the only reservoir. Unlike NTS, S. typhi crosses the intestinal barrier, survives within macrophages, and disseminates throughout the reticuloendothelial system, causing systemic infection. After an incubation period of one to three weeks, patients develop a characteristic stepwise (or stairstep) fever that rises over several days, often accompanied by relative bradycardia (pulse slower than expected for degree of fever). Constitutional symptoms including headache, malaise, and abdominal discomfort are prominent. During the second week, sustained high fever occurs with hepatosplenomegaly and the appearance of rose spots, faint salmon-colored maculopapular lesions on the trunk representing bacterial emboli in the skin. Without treatment, complications develop in the third week, including intestinal hemorrhage and perforation from necrosis of Peyer's patches. Typhoid can be fatal in 10 to 30 percent of untreated cases. Diagnosis of typhoid fever is confirmed by culture, with blood cultures being most sensitive during the first week of illness. Bone marrow culture has the highest sensitivity (greater than 90 percent) and remains positive even after antibiotic initiation. Stool cultures become positive in the second and third weeks. The Widal test, which detects antibodies against O and H antigens, has limited sensitivity and specificity and is not recommended for diagnosis in endemic areas where background seropositivity is common. Treatment of typhoid fever requires antibiotics, with fluoroquinolones (such as ciprofloxacin), third-generation cephalosporins (ceftriaxone), and azithromycin being first-line agents, though increasing antimicrobial resistance, particularly to fluoroquinolones, is a major concern. Chronic carriers, who shed S. typhi in stool for more than one year following acute infection, represent a reservoir for transmission. Carriage typically involves colonization of the gallbladder, and treatment with prolonged antibiotic courses or cholecystectomy may be required to eliminate carriage. Vaccines are available for typhoid prevention and are recommended for travelers to endemic areas; the Vi polysaccharide vaccine and the live attenuated oral Ty21a vaccine are both effective. Panel A: Non-typhoidal Salmonella (NTS) gastroenteritis showing sources of infection (poultry, eggs, produce, reptile contact), invasion of intestinal epithelium with inflammatory response limited to mucosa, clinical presentation (diarrhea, fever, cramps, 12-48 hour incubation), typical self-limited course (4-7 days), and management approach (supportive care without antibiotics for uncomplicated cases, antibiotics for high-risk patients or invasive disease). Panel B: Typhoid fever pathogenesis showing ingestion of S. typhi, crossing intestinal barrier at Peyer's patches, uptake by macrophages with survival and replication within, dissemination through bloodstream to liver, spleen, bone marrow (reticuloendothelial system), and secondary bacteremia, with humans as only reservoir and fecal-oral transmission cycle. Panel C: Typhoid fever clinical timeline illustrated: incubation period (1-3 weeks), week 1 (stepwise rising fever with relative bradycardia, malaise, headache), week 2 (sustained fever, rose spots on trunk, hepatosplenomegaly), week 3 (complications including intestinal hemorrhage and perforation from Peyer's patch necrosis), with diagnostic yield of blood culture (week 1), stool culture (weeks 2-3), and bone marrow culture (highest sensitivity throughout). Panel D: Typhoid treatment and prevention showing antibiotic options (fluoroquinolones, ceftriaxone, azithromycin) with note about increasing resistance, chronic carrier state with gallbladder colonization and treatment (prolonged antibiotics or cholecystectomy), and vaccines (Vi polysaccharide and live oral Ty21a) recommended for travelers to endemic areas. --- ### V. Shigella Shigella species cause bacillary dysentery, an acute invasive infection of the colonic mucosa characterized by bloody, mucoid diarrhea, fever, and abdominal cramps. Shigellosis is notable for its extremely low infectious dose, with as few as 10 to 100 organisms capable of causing disease, reflecting the organism's acid resistance and highly efficient invasion machinery. This low infectious dose enables person-to-person transmission and makes Shigella a significant public health concern, particularly in settings of poor sanitation. The four Shigella species differ in geographic distribution and disease severity. Shigella sonnei is the most common species in developed countries, typically causing mild disease. Shigella flexneri is the most common species in developing countries and causes moderate disease. Shigella dysenteriae type 1 produces Shiga toxin and causes the most severe disease, associated with epidemics and a higher incidence of hemolytic uremic syndrome. Shigella boydii is relatively uncommon and geographically restricted. Despite being classified as separate species historically, Shigella organisms are actually E. coli strains that have acquired specific virulence factors and lost certain metabolic capabilities; molecular analysis reveals that Shigella and enteroinvasive E. coli (EIEC) are essentially the same pathovar. The pathogenesis of shigellosis involves invasion of colonic epithelium with intracellular multiplication and cell-to-cell spread. Unlike Salmonella, Shigella does not survive well in macrophages and does not cause systemic infection in immunocompetent hosts. Shigella crosses the intestinal barrier by invading M cells in the follicle-associated epithelium overlying Peyer's patches. Once across this barrier, Shigella invades colonic epithelial cells from the basolateral side using a type III secretion system to inject effector proteins (Ipa proteins) that trigger macropinocytosis and bacterial internalization. Within epithelial cells, Shigella escapes from the endocytic vacuole into the cytoplasm, where it replicates rapidly. Like Listeria monocytogenes, Shigella hijacks the host cell actin polymerization machinery using the surface protein IcsA (VirG), which activates N-WASP and the Arp2/3 complex to form an actin comet tail that propels the bacterium through the cytoplasm. When Shigella reaches the cell membrane, it pushes into the adjacent cell, allowing direct cell-to-cell spread without exposure to extracellular defenses. This cycle of invasion, replication, and spread, combined with intense inflammatory response including neutrophil infiltration, causes ulceration and destruction of the colonic mucosa. Clinical presentation of shigellosis begins one to three days after ingestion with watery diarrhea, fever, and abdominal cramps. Within one to two days, the characteristic dysenteric syndrome develops, with frequent small-volume stools containing blood and mucus, and tenesmus (painful, ineffective straining to defecate). The disease is typically self-limited, resolving in five to seven days, though symptoms may persist longer without treatment. Complications include severe dehydration, HUS (particularly with S. dysenteriae type 1), seizures in children (possibly related to toxin effects on the CNS), toxic megacolon, and intestinal perforation. Post-infectious reactive arthritis may develop, particularly in individuals with HLA-B27 antigen. Unlike EHEC infection, antibiotic treatment of shigellosis is recommended because it shortens the duration of illness, reduces shedding and transmission, and may prevent complications. First-line agents include azithromycin and fluoroquinolones, though antimicrobial resistance is increasing and susceptibility testing is important. Panel A: Shigella species comparison showing S. sonnei (most common in developed countries, mild disease), S. flexneri (most common in developing countries, moderate disease), S. dysenteriae type 1 (Shiga toxin production, severe disease, HUS risk, epidemics), and S. boydii (uncommon), with note that extremely low infectious dose (10-100 organisms) enables person-to-person transmission. Panel B: Shigella invasion mechanism illustrated in sequential steps: crossing intestinal barrier via M cells, basolateral invasion of epithelial cells using type III secretion injecting Ipa proteins, escape from endocytic vacuole into cytoplasm, intracellular replication, IcsA-mediated actin tail formation propelling bacterium through cytoplasm, and cell-to-cell spread without extracellular exposure, with intense inflammatory response and mucosal destruction. Panel C: Shigellosis clinical presentation showing dysentery syndrome with frequent small-volume bloody, mucoid stools, tenesmus (painful straining illustrated), fever, abdominal cramps, and timeline of illness from initial watery diarrhea to dysentery development, with complications listed (HUS with S. dysenteriae type 1, seizures in children, toxic megacolon, reactive arthritis). Panel D: Shigella treatment and prevention displaying recommended antibiotics (azithromycin, fluoroquinolones) with note that treatment shortens illness and reduces transmission (unlike EHEC where antibiotics are contraindicated), increasing resistance patterns requiring susceptibility testing, and prevention measures (hand hygiene, sanitation, avoiding fecal contamination of food and water). --- ### VI. Klebsiella Klebsiella pneumoniae is an encapsulated gram-negative rod that causes a variety of infections, ranging from pneumonia and urinary tract infections to the distinctive hypervirulent syndrome of pyogenic liver abscess. The organism's prominent polysaccharide capsule contributes to its virulence and produces the characteristic mucoid colonial appearance. Klebsiella has emerged as a major concern in healthcare settings due to the spread of carbapenem-resistant strains. Klebsiella pneumoniae is distinguished by its large, non-motile cells and abundant polysaccharide capsule, which produces mucoid, sticky colonies that can be pulled into a string more than 5 millimeters long (the string test or hypermucoviscosity phenotype). The capsule inhibits phagocytosis and complement-mediated killing and is the major virulence factor. On MacConkey agar, Klebsiella is a lactose fermenter, producing pink colonies. The capsular K antigens define serotypes; over 80 serotypes have been identified, with K1 and K2 associated with hypervirulent strains. Traditional Klebsiella pneumoniae infections include hospital-acquired pneumonia, urinary tract infections, and bacteremia. Klebsiella pneumonia is classically described in alcoholics and diabetics, though it can affect any hospitalized patient. The classic teaching describes a severe necrotizing pneumonia with "currant jelly sputum" (thick, bloody, mucoid sputum from tissue destruction and the abundant capsule), though this presentation is not universal. Radiographically, Klebsiella pneumonia may produce lobar consolidation with a bulging fissure due to inflammatory exudate accumulation, and cavitation from tissue necrosis. Klebsiella is a common cause of catheter-associated urinary tract infections in hospitalized patients. Bacteremia often arises from respiratory or urinary sources and carries significant mortality. Hypervirulent Klebsiella pneumoniae (hvKp) represents a distinct clinical entity that typically occurs in community settings, affecting patients without traditional healthcare exposure, particularly diabetics and individuals of East Asian descent. HvKp strains, usually serotypes K1 or K2, cause pyogenic liver abscess, a syndrome that has been termed Klebsiella pneumoniae invasive syndrome (KPIS). Patients present with fever, right upper quadrant pain, and frequently have diabetes mellitus. Alarmingly, hvKp has a propensity for metastatic spread, seeding distant sites including the eyes (endophthalmitis, which can cause rapid, devastating vision loss) and the central nervous system (meningitis, brain abscess). The diagnosis should be suspected in a diabetic patient with pyogenic liver abscess, particularly if bacteremic, and empiric treatment should include coverage for this organism while cultures are pending. Carbapenem-resistant Klebsiella pneumoniae represents a critical public health threat. Carbapenemase-producing Klebsiella, including strains producing KPC (Klebsiella pneumoniae carbapenemase), NDM (New Delhi metallo-beta-lactamase), and OXA-48, are resistant to nearly all beta-lactam antibiotics and often carry resistance determinants for other drug classes as well. These strains cause healthcare-associated infections with limited treatment options and high mortality. Infection control measures including contact precautions, screening of high-risk patients, hand hygiene, and antimicrobial stewardship are essential to prevent spread. Treatment options for CRE include newer beta-lactam/beta-lactamase inhibitor combinations such as ceftazidime-avibactam and meropenem-vaborbactam, as well as older agents with toxicity concerns such as polymyxins. Panel A: Klebsiella pneumoniae characteristics showing large gram-negative rods with prominent polysaccharide capsule visible on special staining, mucoid colonies on agar with demonstration of string test (hypermucoviscosity phenotype with >5 mm string), lactose fermentation (pink colonies on MacConkey), and note that organism is non-motile. Panel B: Traditional Klebsiella infections illustrated: pneumonia in alcoholic or diabetic patient with currant jelly sputum (thick, bloody, mucoid), chest X-ray showing lobar consolidation with bulging fissure and potential cavitation, catheter-associated urinary tract infection, and bacteremia often arising from respiratory or urinary source. Panel C: Hypervirulent Klebsiella pneumoniae (hvKp) syndrome showing typical patient (diabetic, East Asian descent, community-acquired), pyogenic liver abscess on CT imaging, metastatic spread to eyes (endophthalmitis causing rapid vision loss) and CNS (meningitis, brain abscess), serotypes K1/K2 association, and importance of considering this diagnosis in diabetic patient with liver abscess. Panel D: Carbapenem-resistant Klebsiella (CRE) showing major carbapenemase enzymes (KPC endemic in US, NDM from Indian subcontinent, OXA-48 from Mediterranean/Middle East), resistance to nearly all beta-lactams, treatment options (ceftazidime-avibactam, meropenem-vaborbactam, polymyxins as last resort), and infection control measures (contact precautions, screening, hand hygiene, stewardship). --- ### VII. Proteus and Other Enterics Beyond the major pathogens already discussed, several other genera within Enterobacteriaceae cause clinically significant infections, particularly in healthcare settings. Proteus, Enterobacter, Serratia, and Citrobacter are important causes of hospital-acquired infections and often possess intrinsic or acquired antimicrobial resistance that complicates treatment. Proteus mirabilis is a highly motile organism displaying characteristic swarming motility on agar plates, with waves of bacteria spreading outward in concentric rings. A key biochemical feature is the production of urease, which hydrolyzes urea to ammonia and carbon dioxide. Urease production has important clinical consequences in urinary tract infections: ammonia production alkalinizes the urine, causing precipitation of magnesium ammonium phosphate (struvite) and calcium phosphite (apatite) to form struvite stones (also called triple phosphate stones or infection stones). These stones can grow rapidly to fill the renal collecting system (staghorn calculi), causing obstruction and serving as a nidus for persistent infection that cannot be cleared without stone removal. Proteus is a common cause of complicated urinary tract infections, particularly in patients with indwelling catheters or structural abnormalities. Proteus vulgaris, which is more commonly resistant to ampicillin than P. mirabilis, causes similar infections. Enterobacter species, including Enterobacter cloacae and Enterobacter aerogenes, are opportunistic pathogens causing hospital-acquired infections including pneumonia, urinary tract infections, and bacteremia. A critical feature of Enterobacter is the presence of an inducible chromosomal AmpC beta-lactamase. At baseline, AmpC expression is low, and the organism may appear susceptible to third-generation cephalosporins. However, exposure to these antibiotics can induce high-level AmpC expression, leading to resistance emergence during therapy. This phenomenon can result in treatment failure as initially susceptible isolates become resistant. For serious Enterobacter infections, many experts recommend avoiding cephalosporins even if initial susceptibility testing suggests susceptibility, instead using carbapenems or other agents not affected by AmpC. Serratia marcescens is notable for its production of the red pigment prodigiosin when grown at room temperature, giving colonies a distinctive red color. In the clinical laboratory, cultures incubated at 37 degrees Celsius may not produce pigment, but it may appear on cultures left at room temperature. Serratia causes hospital-acquired infections including pneumonia, urinary tract infections, and bloodstream infections, particularly in intensive care unit settings. Outbreaks have been associated with contaminated medications and solutions. Serratia possesses intrinsic resistance to ampicillin, first-generation cephalosporins, and polymyxins, and may produce extended-spectrum beta-lactamases. Citrobacter species, including Citrobacter freundii and Citrobacter koseri, cause urinary tract infections, bacteremia, and other hospital-acquired infections. Citrobacter freundii, like Enterobacter, possesses an inducible AmpC beta-lactamase with similar treatment implications. Citrobacter koseri (formerly Citrobacter diversus) has a distinctive propensity to cause brain abscesses in neonates, a rare but serious infection with high morbidity. Panel A: Proteus mirabilis characteristics showing swarming motility on agar plate with characteristic concentric rings of growth, strong urease positivity (color change on urea agar), and pathogenesis of struvite stone formation: urease hydrolyzing urea to ammonia, alkalinization of urine, precipitation of magnesium ammonium phosphate forming struvite stones, and staghorn calculus filling renal collecting system on imaging. Panel B: Enterobacter and AmpC beta-lactamase illustrated showing the inducible AmpC mechanism: low baseline expression with apparent cephalosporin susceptibility, antibiotic exposure inducing high-level AmpC production, resulting resistance emergence during therapy, and recommendation to avoid cephalosporins for serious Enterobacter infections even if initially susceptible. Panel C: Serratia marcescens showing red pigment (prodigiosin) production at room temperature in test tube, hospital-acquired infection settings (ICU, ventilated patients), intrinsic resistance pattern (ampicillin, first-generation cephalosporins, polymyxins), and association with outbreaks from contaminated medications. Panel D: Citrobacter species comparison showing C. freundii (AmpC beta-lactamase like Enterobacter, nosocomial infections) and C. koseri (propensity for neonatal brain abscess, distinctive clinical syndrome requiring neurosurgical consultation), with general treatment approaches and importance of susceptibility testing. --- ### VIII. Yersinia The genus Yersinia contains three species of human pathogens: Yersinia enterocolitica, which causes gastroenteritis and mesenteric adenitis; Yersinia pseudotuberculosis, which causes mesenteric adenitis; and Yersinia pestis, the causative agent of plague, one of the most devastating infectious diseases in human history. These organisms share virulence mechanisms involving the type III secretion system but differ markedly in their epidemiology and clinical presentations. Yersinia enterocolitica causes gastrointestinal infections acquired through consumption of contaminated food, particularly undercooked pork, and unpasteurized milk. A distinctive feature of Y. enterocolitica is its ability to grow at refrigerator temperatures (4 degrees Celsius), a property called psychrotrophic growth. This creates a transfusion-associated risk: red blood cells stored at refrigerator temperatures can support Y. enterocolitica growth, leading to fatal sepsis if contaminated units are transfused. Clinical manifestations of Y. enterocolitica infection include enterocolitis with diarrhea (which may be bloody), fever, and abdominal pain. A distinctive presentation is mesenteric adenitis, with inflammation of mesenteric lymph nodes causing right lower quadrant abdominal pain and tenderness that mimics acute appendicitis (pseudoappendicitis syndrome). Patients may undergo appendectomy for presumed appendicitis, only to find a normal appendix with mesenteric adenitis. Post-infectious complications include reactive arthritis, particularly in patients with HLA-B27, and erythema nodosum. Treatment of uncomplicated enterocolitis is supportive; antibiotics are reserved for severe or systemic infection. Yersinia pestis causes plague, a disease transmitted primarily by flea bites from infected rodents. Plague has caused three major pandemics in recorded history, including the Black Death that killed an estimated one-third of Europe's population in the 14th century. Y. pestis has evolved from Y. pseudotuberculosis relatively recently in evolutionary terms, acquiring virulence factors that enable flea transmission and cause severe disease. The organism possesses the F1 capsular antigen, which is antiphagocytic, and a type III secretion system that injects Yop (Yersinia outer proteins) effectors into host cells, paralyzing phagocytosis. Plague presents in three principal clinical forms. Bubonic plague, the most common form, develops after a flea bite and features a painful, markedly swollen regional lymph node (bubo), typically inguinal or axillary. Patients are acutely ill with fever and may progress rapidly to sepsis. Septicemic plague may develop from bubonic plague or primarily without bubo formation, presenting as fulminant sepsis with potential for disseminated intravascular coagulation, shock, and acral necrosis (black discoloration of extremities that gave the Black Death its name). Pneumonic plague is the most dangerous form, involving primary pneumonic infection from inhaling infected respiratory droplets or secondary pneumonia from hematogenous spread. Pneumonic plague can be transmitted person-to-person through respiratory droplets, is uniformly fatal without treatment, and has a very short incubation period (one to three days). Because of its high lethality, potential for person-to-person transmission (in pneumonic form), and prior weaponization, Y. pestis is classified as a Category A bioterrorism agent. Treatment requires prompt antibiotic therapy with streptomycin, gentamicin, doxycycline, or fluoroquinolones; the prognosis depends critically on early initiation of treatment. Panel A: Yersinia enterocolitica characteristics and disease showing sources of infection (undercooked pork, unpasteurized milk), psychrotrophic growth at 4 degrees Celsius creating transfusion risk, clinical presentations of enterocolitis and mesenteric adenitis causing pseudoappendicitis syndrome (normal appendix found at surgery with enlarged mesenteric nodes), and post-infectious complications (reactive arthritis in HLA-B27 individuals, erythema nodosum). Panel B: Yersinia pestis virulence factors and transmission showing flea vector feeding on infected rodent then biting human, F1 capsular antigen providing antiphagocytic protection, type III secretion system injecting Yop proteins into macrophages and neutrophils to block phagocytosis, and geographic distribution of plague reservoirs (rodents in Western US, endemic areas worldwide). Panel C: Three forms of plague illustrated: bubonic plague with painful swollen lymph node (bubo) typically in groin or axilla from flea bite, septicemic plague with fulminant sepsis, DIC, and acral necrosis (blackened extremities), and pneumonic plague with severe pneumonia, person-to-person respiratory transmission, and nearly 100% mortality without treatment, noting this is the bioterrorism concern. Panel D: Plague treatment and public health response showing antibiotic options (streptomycin, gentamicin, doxycycline, fluoroquinolones) with emphasis on early treatment being critical, isolation precautions for pneumonic plague, post-exposure prophylaxis for contacts, and Category A bioterrorism agent designation requiring immediate public health notification. --- ### IX. Antimicrobial Resistance Antimicrobial resistance in Enterobacteriaceae has emerged as one of the most pressing challenges in infectious diseases. The widespread use of antibiotics in human medicine and agriculture has selected for resistance, while horizontal gene transfer has facilitated rapid spread of resistance determinants between organisms. Understanding the mechanisms of resistance and the treatment options for resistant infections is essential for current clinical practice. Beta-lactamases are enzymes that hydrolyze the beta-lactam ring of penicillins, cephalosporins, and related antibiotics, rendering them inactive. The evolution of beta-lactamases in Enterobacteriaceae has paralleled the development of new beta-lactam antibiotics. Simple TEM and SHV beta-lactamases that inactivate penicillins and first-generation cephalosporins were widespread by the 1970s. Extended-spectrum beta-lactamases (ESBLs), including CTX-M, TEM variants, and SHV variants, emerged in the 1980s and have become increasingly prevalent. ESBLs hydrolyze extended-spectrum cephalosporins (third- and fourth-generation), including ceftriaxone, ceftazidime, and cefepime, as well as aztreonam. ESBL-producing organisms, particularly E. coli and Klebsiella pneumoniae, are common causes of both hospital-acquired and increasingly community-acquired infections, including urinary tract infections, intra-abdominal infections, and bacteremia. ESBL-producing infections require treatment with carbapenems, as these are not hydrolyzed by ESBLs, or with newer beta-lactam/beta-lactamase inhibitor combinations. Carbapenem-resistant Enterobacteriaceae (CRE) represent the most serious resistance threat, as carbapenems are often the last-line beta-lactam agents. CRE possess carbapenemases, beta-lactamases that hydrolyze carbapenems along with other beta-lactams. Major carbapenemase families include KPC (Klebsiella pneumoniae carbapenemase), which is endemic in the United States and many other regions; NDM (New Delhi metallo-beta-lactamase), which emerged from the Indian subcontinent and has spread globally; and OXA-48, prevalent in the Mediterranean region and Middle East. CRE infections have limited treatment options and high mortality rates (40 to 50 percent in some series). Treatment requires infectious diseases consultation and typically involves newer agents such as ceftazidime-avibactam (which inhibits KPC and OXA-48 but not metallo-beta-lactamases), meropenem-vaborbactam (effective against KPC), cefiderocol (a siderophore cephalosporin with activity against many CRE including metallo-beta-lactamase producers), or combination therapy including polymyxins (colistin) despite their nephrotoxicity and neurotoxicity. Beyond beta-lactamase production, Enterobacteriaceae employ other resistance mechanisms. Fluoroquinolone resistance develops through mutations in DNA gyrase and topoisomerase IV (the targets of fluoroquinolones), plasmid-mediated quinolone resistance genes (qnr family), and efflux pump overexpression. Aminoglycoside resistance results from aminoglycoside-modifying enzymes that acetylate, adenylate, or phosphorylate these drugs. Trimethoprim-sulfamethoxazole resistance involves altered dihydrofolate reductase (the target of trimethoprim) and dihydropteroate synthase (the target of sulfonamides). The accumulation of multiple resistance mechanisms in individual strains creates multidrug-resistant (MDR) and extensively drug-resistant (XDR) phenotypes with extremely limited treatment options. Infection control measures are critical for preventing the spread of resistant Enterobacteriaceae in healthcare settings. Contact precautions (gown, gloves, and ideally single rooms) for patients colonized or infected with CRE and other MDR organisms prevent transmission via healthcare workers' hands and contaminated surfaces. Active surveillance cultures can identify colonized patients before transmission occurs. Hand hygiene remains the single most important measure for preventing healthcare-associated transmission. Antimicrobial stewardship programs that optimize antibiotic selection, dosing, and duration reduce selection pressure for resistance. Panel A: Beta-lactamase evolution illustrated as timeline showing penicillinases (TEM-1, SHV-1) emerging in 1970s and conferring resistance to penicillins, extended-spectrum beta-lactamases (ESBLs, especially CTX-M) emerging in 1980s-2000s conferring resistance to third-generation cephalosporins, and carbapenemases (KPC, NDM, OXA-48) emerging 2000s-present conferring resistance to carbapenems, with graphic showing the beta-lactam ring being hydrolyzed by each enzyme type. Panel B: ESBL-producing Enterobacteriaceae showing prevalence in E. coli and Klebsiella, clinical settings (UTI, bacteremia, hospital and community), laboratory detection methods (disk diffusion showing synergy with clavulanate, molecular testing), and treatment approach (carbapenems for serious infections, newer agents as alternatives). Panel C: Carbapenemase types and geographic distribution showing world map with KPC endemic in Americas and spreading, NDM from Indian subcontinent spreading globally, OXA-48 in Mediterranean and Middle East, and treatment options organized by enzyme type (ceftazidime-avibactam for KPC/OXA-48, cefiderocol for metallo-beta-lactamases, polymyxins as last resort). Panel D: Infection control pyramid showing layers of prevention: foundation of hand hygiene (most important), contact precautions for colonized/infected patients, active surveillance to identify carriers, antimicrobial stewardship to reduce selection pressure, and environmental cleaning, with mortality statistics for CRE infections emphasizing the importance of prevention. --- ### X. Treatment Guidelines The treatment of Enterobacteriaceae infections requires consideration of the infection site, severity, likely pathogens based on clinical context, and local resistance patterns. Empiric therapy must provide adequate coverage for probable pathogens while directed therapy based on culture and susceptibility results allows optimization and de-escalation. For uncomplicated cystitis in non-pregnant women, first-line agents include nitrofurantoin, trimethoprim-sulfamethoxazole (if local resistance is less than 20 percent), and fosfomycin. Fluoroquinolones are highly effective but should be reserved for other infections given their association with serious adverse effects and resistance selection. For complicated urinary tract infections and pyelonephritis, fluoroquinolones or oral beta-lactams may be used for outpatient therapy, while parenteral ceftriaxone, fluoroquinolones, or broader-spectrum agents are used for patients requiring hospitalization. For ESBL-producing organisms, carbapenems are the treatment of choice for serious infections, though newer agents and oral options may be appropriate for less severe infections. Intra-abdominal infections require coverage of both aerobic gram-negative bacilli (Enterobacteriaceae) and anaerobes (Bacteroides fragilis group). Mild-to-moderate community-acquired infections can be treated with ampicillin-sulbactam, cefoxitin, or ertapenem. Severe infections or those in patients with healthcare exposure require broader coverage with piperacillin-tazobactam, carbapenems, or combination therapy. Healthcare-associated intra-abdominal infections may involve resistant organisms including ESBL-producers and require broader empiric coverage with adjustment based on culture results. Specific enteric pathogens require targeted treatment approaches. Typhoid fever always requires antibiotic treatment, with fluoroquinolones, ceftriaxone, or azithromycin as first-line agents; increasing fluoroquinolone resistance requires attention to susceptibility results. Shigellosis is treated with azithromycin or fluoroquinolones to shorten illness and reduce transmission; again, resistance is an increasing concern. Non-typhoidal Salmonella gastroenteritis typically does not require antibiotics in healthy adults, as treatment does not shorten illness and may prolong the carrier state; antibiotics are indicated for severe disease, bacteremia, or high-risk patients. Critically, EHEC (Shiga toxin-producing E. coli) should not be treated with antibiotics, as this may increase toxin release and worsen the risk of hemolytic uremic syndrome; treatment is supportive with careful monitoring for HUS development. Treatment duration varies by infection type. Uncomplicated cystitis typically requires three to five days of therapy depending on the agent. Pyelonephritis is treated for seven to fourteen days. Bacteremia requires at least seven to fourteen days, with longer courses for endovascular infection or metastatic foci. Typhoid fever is treated for ten to fourteen days or longer for severe cases or complications. Panel A: Urinary tract infection treatment algorithm showing uncomplicated cystitis (nitrofurantoin, TMP-SMX if resistance less than 20%, fosfomycin as first-line; avoid fluoroquinolones for uncomplicated UTI), complicated UTI and pyelonephritis (fluoroquinolones, beta-lactams, or ceftriaxone depending on severity), and ESBL-producing organisms (carbapenems for serious infections), with dose and duration information. Panel B: Enteric pathogen treatment summary displayed as decision matrix: Typhoid fever (always treat; fluoroquinolones, ceftriaxone, or azithromycin), Shigella (usually treat to reduce transmission; azithromycin or fluoroquinolones), non-typhoidal Salmonella (usually do not treat; reserve for severe/high-risk), EHEC/STEC (do NOT treat; antibiotics may increase HUS risk; supportive care only), with reasoning for each approach. Panel C: Intra-abdominal infection coverage requirements showing need to cover Enterobacteriaceae plus anaerobes, regimen options (ampicillin-sulbactam or cefoxitin for mild-moderate; piperacillin-tazobactam or carbapenem for severe or healthcare-associated), and adjustment for ESBL risk in healthcare-associated infections. Panel D: Treatment duration guidelines displayed as timeline diagram showing uncomplicated cystitis (3-5 days), pyelonephritis (7-14 days), bacteremia (7-14 days, longer for complications), typhoid fever (10-14 days), and factors that may extend duration (slow clinical response, metastatic foci, immunocompromise). --- ## Summary - Enterobacteriaceae are gram-negative rods that are oxidase-negative, catalase-positive, and ferment glucose; lactose fermentation (pink colonies on MacConkey) distinguishes E. coli, Klebsiella, and Enterobacter from Salmonella, Shigella, and Proteus - E. coli causes UTI (UPEC with P pili), neonatal meningitis (K1 capsule), and diarrheal diseases (ETEC, EPEC, EHEC); EHEC produces Shiga toxin causing bloody diarrhea and HUS; antibiotics are contraindicated for EHEC - Salmonella includes non-typhoidal (gastroenteritis, usually self-limited without antibiotics) and typhoidal (systemic illness with stepwise fever, rose spots, risk of complications; always treat) - Shigella causes bacillary dysentery (bloody, mucoid diarrhea with tenesmus) via invasion and intracellular spread; very low infectious dose (10-100 organisms); treatment shortens illness and transmission - Klebsiella is encapsulated, causing pneumonia, UTI, and hypervirulent syndrome (liver abscess with metastatic spread); CRE strains are a critical threat - Proteus produces urease causing alkaline urine and struvite stones; swarming motility is characteristic - Yersinia enterocolitica causes mesenteric adenitis mimicking appendicitis; Y. pestis causes plague (bubonic, septicemic, pneumonic forms) - ESBL-producers require carbapenems for serious infections; CRE requires newer agents (ceftazidime-avibactam, meropenem-vaborbactam) or polymyxins --- ## Key Terms | Term | Definition | |------|------------| | Lactose fermenter | Enterobacteriaceae that ferments lactose, producing pink colonies on MacConkey agar (E. coli, Klebsiella, Enterobacter) | | O antigen | Somatic antigen; outermost polysaccharide of LPS; used in serotyping (e.g., O157) | | H antigen | Flagellar antigen composed of flagellin protein; used in serotyping (e.g., H7) | | Shiga toxin | A-B toxin that cleaves ribosomal RNA, causing cell death; produced by EHEC and S. dysenteriae type 1; associated with HUS | | Hemolytic uremic syndrome | Triad of microangiopathic hemolytic anemia, thrombocytopenia, and acute kidney injury; complication of EHEC and S. dysenteriae type 1 infection | | ESBL | Extended-spectrum beta-lactamase; enzymes that hydrolyze third-generation cephalosporins; require carbapenem treatment for serious infections | | CRE | Carbapenem-resistant Enterobacteriaceae; produce carbapenemases (KPC, NDM, OXA-48); limited treatment options with high mortality | | Typhoid fever | Systemic infection caused by Salmonella typhi; characterized by stepwise fever, rose spots, hepatosplenomegaly, and risk of intestinal perforation | | Dysentery | Inflammatory diarrhea characterized by bloody, mucoid stools with tenesmus; classic presentation of shigellosis | | Struvite stone | Magnesium ammonium phosphate kidney stone formed when urease-producing bacteria (Proteus) alkalinize urine | | AmpC beta-lactamase | Chromosomal cephalosporinase in Enterobacter and Citrobacter freundii; inducible expression can cause resistance emergence during cephalosporin therapy | | Mesenteric adenitis | Inflammation of mesenteric lymph nodes causing right lower quadrant pain; caused by Y. enterocolitica; mimics appendicitis | --- *This content is subject to the [MIT License](https://opensource.org/licenses/MIT). © 2024–2026 Hibbert School of Medicine.*