# Clinical Cases: Gram-Negative Enteric Bacteria ## Case 1: Hemolytic Uremic Syndrome ### Presentation A 4-year-old girl is brought to the emergency department with a 5-day history of bloody diarrhea. The illness began with watery diarrhea and abdominal cramps after a family barbecue where she ate a hamburger. Over the past 2 days, the diarrhea became grossly bloody. Today her mother noticed decreased urine output and the child appears pale. Vital signs show temperature 37.8°C, heart rate 120 bpm, and blood pressure 100/70 mmHg. Laboratory studies reveal hemoglobin 7.2 g/dL with schistocytes on peripheral smear, platelet count 45,000/μL, creatinine 3.2 mg/dL, and LDH 1,850 U/L. ### Clinical Image ![Peripheral blood smear showing schistocytes](image_01.png) *Peripheral blood smear demonstrating fragmented red blood cells (schistocytes) characteristic of microangiopathic hemolytic anemia in hemolytic uremic syndrome.* **Image Source**: Lecture image - hematology laboratory findings ### Questions 1. **What is the most likely diagnosis, and what organism is most commonly responsible?** 2. **What is the mechanism by which this organism causes the clinical syndrome?** 3. **Why are antibiotics contraindicated in this condition?** 4. **What are the key supportive management strategies for this patient?** ### Answers 1. **Diagnosis and organism**: The most likely diagnosis is **hemolytic uremic syndrome (HUS)** caused by **Shiga toxin-producing Escherichia coli (STEC)**, most commonly serotype O157:H7. The clinical triad of HUS includes microangiopathic hemolytic anemia (schistocytes, elevated LDH), thrombocytopenia, and acute kidney injury. The history of bloody diarrhea following consumption of undercooked ground beef is classic for EHEC/STEC infection. 2. **Pathogenic mechanism**: EHEC produces **Shiga toxin (Stx)**, which has an A-B structure. The B subunit binds to globotriaosylceramide (Gb3) receptors on glomerular endothelial cells, and the A subunit cleaves ribosomal RNA to halt protein synthesis and cause cell death. Endothelial damage triggers microvascular thrombosis, platelet consumption, and red blood cell fragmentation as they pass through damaged vessels. The resulting microvascular occlusion in the kidneys causes acute kidney injury. 3. **Antibiotic contraindication**: Antibiotics are contraindicated because bacterial killing may increase the release of Shiga toxin from lysed bacteria, potentially worsening the risk of HUS. Studies have shown that antibiotic treatment of EHEC infection is associated with increased progression to HUS. Treatment should be supportive, with close monitoring for HUS development. 4. **Supportive management**: Management includes aggressive intravenous fluid resuscitation to maintain renal perfusion, careful electrolyte management (especially potassium in the setting of acute kidney injury), blood transfusions for symptomatic anemia (avoiding platelets unless active bleeding, as they may worsen microvascular thrombosis), and renal replacement therapy (dialysis) if indicated. Close monitoring of fluid balance, blood pressure, and neurologic status is essential. --- ## Case 2: Typhoid Fever ### Presentation A 32-year-old man presents with 2 weeks of progressively worsening fever, headache, and abdominal discomfort. He returned from a trip to South Asia 3 weeks ago. The fever has been rising in a stepwise pattern over the past week, now reaching 40°C. He reports constipation rather than diarrhea, and significant fatigue. Physical examination reveals a toxic-appearing patient with relative bradycardia (pulse 80 bpm despite high fever), hepatosplenomegaly, and faint salmon-colored maculopapular lesions on the trunk. ### Clinical Image ![Rose spots on the trunk](image_02.png) *Rose spots - faint salmon-colored maculopapular lesions on the trunk representing bacterial emboli in the skin, characteristic of typhoid fever.* **Image Source**: Lecture image - typhoid fever clinical findings ### Questions 1. **What is the most likely diagnosis based on the clinical presentation?** 2. **What is the most sensitive diagnostic test during the first week of illness?** 3. **What complications may develop if this condition is left untreated?** 4. **What is the appropriate antibiotic treatment, and what resistance patterns should be considered?** ### Answers 1. **Most likely diagnosis**: This presentation is classic for **typhoid fever** caused by **Salmonella typhi**. Key features include the stepwise rising fever, relative bradycardia (pulse-temperature dissociation), hepatosplenomegaly, rose spots on the trunk, and travel to an endemic region. The incubation period of 1-3 weeks and constitutional symptoms with abdominal discomfort (often constipation rather than diarrhea early in disease) are characteristic. 2. **Most sensitive diagnostic test**: **Blood culture** is most sensitive during the first week of illness, when bacteremia is most prominent. However, **bone marrow culture** has the highest overall sensitivity (>90%) and remains positive even after antibiotic initiation. Stool cultures become positive in the second and third weeks. The Widal test (serologic) has limited sensitivity and specificity and is not recommended for diagnosis in endemic areas. 3. **Untreated complications**: Without treatment, complications develop during the third week and include **intestinal hemorrhage and perforation** from necrosis of Peyer's patches in the terminal ileum. These are surgical emergencies with high mortality. Other complications include myocarditis, meningitis, osteomyelitis, and chronic carriage (colonization of the gallbladder leading to fecal shedding for more than one year). 4. **Treatment and resistance**: First-line treatment options include **fluoroquinolones** (such as ciprofloxacin), **third-generation cephalosporins** (ceftriaxone), and **azithromycin**. However, increasing fluoroquinolone resistance, particularly in South Asia, is a major concern. Susceptibility testing should guide therapy. Treatment duration is typically 10-14 days. Chronic carriers may require prolonged antibiotic courses or cholecystectomy. --- ## Case 3: Carbapenem-Resistant Klebsiella Pneumonia ### Presentation A 68-year-old man with diabetes mellitus and chronic kidney disease has been in the intensive care unit for 3 weeks following emergency abdominal surgery for perforated diverticulitis. He has been on mechanical ventilation and has had multiple courses of broad-spectrum antibiotics. He now develops new fever to 39.5°C with increased purulent secretions from his endotracheal tube. Chest radiograph shows a new right lower lobe infiltrate. Sputum culture grows Klebsiella pneumoniae that is resistant to all beta-lactams including carbapenems, fluoroquinolones, and aminoglycosides. ### Clinical Image ![Mucoid Klebsiella colonies](image_03.png) *Culture plate showing mucoid, sticky colonies of Klebsiella pneumoniae demonstrating the positive string test (hypermucoviscosity phenotype), characteristic of encapsulated strains.* **Image Source**: Lecture image - Klebsiella laboratory identification ### Questions 1. **What mechanism most likely accounts for the carbapenem resistance in this isolate?** 2. **What infection control measures should be implemented?** 3. **What are the treatment options for this carbapenem-resistant infection?** 4. **What factors in this patient's history predisposed him to infection with a multidrug-resistant organism?** ### Answers 1. **Resistance mechanism**: This isolate likely produces a **carbapenemase**, a beta-lactamase enzyme that hydrolyzes carbapenems along with other beta-lactams. Common carbapenemase families include **KPC** (Klebsiella pneumoniae carbapenemase), which is endemic in the United States, **NDM** (New Delhi metallo-beta-lactamase), and **OXA-48**. These enzymes are often carried on plasmids and can spread horizontally between bacteria. Carbapenem-resistant Enterobacteriaceae (CRE) infections have limited treatment options and high mortality. 2. **Infection control measures**: Essential measures include **contact precautions** (gown and gloves for all patient contact), ideally **single room isolation**, strict **hand hygiene** compliance, dedicated equipment, and **environmental cleaning**. Active surveillance cultures should be considered to identify colonized patients. The laboratory and infection prevention team should be notified immediately. Antimicrobial stewardship programs help reduce selection pressure for resistant organisms. 3. **Treatment options**: Treatment of CRE requires newer agents such as **ceftazidime-avibactam** (effective against KPC and OXA-48 but not metallo-beta-lactamases), **meropenem-vaborbactam** (effective against KPC), or **cefiderocol** (a siderophore cephalosporin with activity against many CRE including metallo-beta-lactamase producers). For NDM-producing organisms, **polymyxins** (colistin) despite nephrotoxicity, or **tigecycline** may be options. Combination therapy is often used. Infectious disease consultation is essential. 4. **Predisposing factors**: Multiple factors predisposed this patient to MDR infection: prolonged **ICU stay**, **mechanical ventilation**, multiple courses of **broad-spectrum antibiotics** (selecting for resistant organisms), **intra-abdominal surgery** disrupting normal flora, underlying **diabetes mellitus** and **chronic kidney disease** (impairing immune function), and the healthcare environment itself (exposure to resistant organisms circulating in the facility).