A 32-year-old man presents with a history of diarrhea several days after eating a hot dog at a neighborhood barbeque. He notes that the diarrhea is visibly bloody, but he has not experienced a fever. He adds that several other people from his neighborhood had similar complaints, many of which required hospitalization after eating food at the same barbeque. His temperature is 37°C (98.6°F ), respiratory rate is 16/min, pulse is 77/min, and blood pressure is 100/88 mm Hg. A physical examination is performed and is within normal limits. Blood is drawn for laboratory testing. The results are as follows: Hb%: 12 gm/dL Total count (WBC): 13,100/mm3 Differential count: Neutrophils: 80% Lymphocytes: 15% Monocytes: 5% ESR: 10 mm/hr Glucose, Serum: 90 mg/dL BUN: 21 mg/dL Creatinine, Serum: 1.96 mg/dL Sodium, Serum: 138 mmol/L Potassium, Serum: 5.2 mmol/L Chloride, Serum: 103 mmol/L Bilirubin, Total: 2.5 mg/dL Alkaline Phosphatase, Serum: 66 IU/L Aspartate aminotransferase (AST): 32 IU/L Alanine aminotransferase (ALT): 34 IU/L Urinalysis is normal except for RBC casts. What is the most concerning possible complication?

Disseminated intravascular coagulation

A 59-year-old man comes to the physician because of urinary frequency and perineal pain for the past 3 days. During this time, he has also had pain with defecation. He is sexually active with his wife only. His temperature is 39.1°C (102.3°F). His penis and scrotum appear normal. Digital rectal examination shows a swollen, exquisitely tender prostate. His leukocyte count is 13,400/mm3. A urine culture obtained prior to initiating treatment is most likely to show which of the following?

Gram-negative, lactose-fermenting rods in pink colonies

Gram-negative, oxidase-positive rods in green colonies

Gram-negative, encapsulated rods in mucoid colonies

Gram-negative, aerobic, intracellular diplococci

Weakly staining, obligate intracellular bacilli

An investigator is studying the immune response and the spleen in a mouse model infected with Escherichia coli. Which of the following anatomical sites in the spleen is important for the secondary maturation and affinity maturation of B cells that will ultimately target Escherichia coli?

Periarteriolar lymphatic sheaths

Escherichia coli for USMLE Step 3: High-Yield Microbiology Lessons

E. coli Basics - The Gutsy Gram-Negative

Morphology: Gram-negative rod, facultative anaerobe.
Habitat: Normal commensal of the GI tract; however, pathogenic strains cause a wide range of diseases.
Key Biochemical Markers:
- Lactose fermenter → forms pink colonies on MacConkey agar.
- Catalase-positive, Oxidase-negative.
- Indole-positive.
Antigenic Structure: Serotyped by O (somatic), H (flagellar), and K (capsular) antigens.

⭐ E. coli is the #1 cause of urinary tract infections (UTIs) and gram-negative sepsis.

E. coli and other bacteria on MacConkey agar

Extraintestinal Disease - Beyond the Bowel

Urinary Tract Infection (UTI): The most common cause of both community and hospital-acquired UTIs.
- Uropathogenic E. coli (UPEC) utilize P fimbriae for adhesion to the urothelium, preventing washout.
Neonatal Meningitis: A leading cause, second only to Group B Strep.
- Strains possessing the K1 capsular antigen are typically responsible.
Sepsis: LPS endotoxin in the outer membrane can trigger septic shock, often secondary to a UTI or intra-abdominal infection.

⭐ E. coli is the number one cause of urinary tract infections.

E. coli ascending urinary tract & infection mechanisms

Diarrheagenic Strains - A Tale of Tummy Troubles

E. coli Pathogenicity Mechanisms

A comparative overview of key E. coli strains causing gastroenteritis.

Strain	Pathogenesis	Clinical Features	Notes
ETEC	Traveler's Diarrhea	Profuse watery diarrhea	Toxins: Heat-Labile (LT → ↑cAMP), Heat-Stable (ST → ↑cGMP)
EPEC	Pediatric Diarrhea	Watery diarrhea, malabsorption	Adherence/effacement of microvilli in the small intestine.
EHEC	Hemorrhagic Colitis	Bloody diarrhea, severe cramps	O157:H7 serotype. Produces Shiga-like toxin. No fever.
EIEC	Invasive	Bloody diarrhea, fever, pus	Invades large intestine mucosa; clinically similar to Shigella.

⭐ EHEC (O157:H7) can cause Hemolytic Uremic Syndrome (HUS): triad of anemia, thrombocytopenia, and acute renal failure. Avoid antibiotics as they can ↑ HUS risk.

EHEC & HUS - Bloody Business

Source: Undercooked ground beef, unpasteurized milk, raw leafy greens.
Toxin: Shiga-like toxin (verotoxin) halts protein synthesis by inactivating the 60S ribosomal subunit.
Presentation: Bloody, voluminous diarrhea (dysentery) with abdominal pain, but typically no fever.
⚠️ Warning: Antibiotics are contraindicated; they can induce toxin release and ↑ risk of HUS.

⭐ E. coli O157:H7 is the most common serotype; it is distinguished from other E. coli by not fermenting sorbitol on MacConkey agar.

Peripheral blood smear showing schistocytes in HUS

Diagnosis & Treatment - Lab Coat & Rx

Lab Diagnosis
- Culture: Lactose-fermenter on MacConkey agar (pink colonies) & classic green metallic sheen on Eosin Methylene Blue (EMB) agar.
- Indole positive (distinguishes from Klebsiella).
Treatment (Rx)
- UTI: TMP-SMX, nitrofurantoin, fosfomycin.
- Systemic/Sepsis: 3rd-gen cephalosporins (e.g., ceftriaxone).
- EHEC: Supportive care. ⚠️ Antibiotics may ↑ HUS risk.

⭐ EHEC (O157:H7) is unique as it does not ferment sorbitol on Sorbitol-MacConkey (SMAC) agar, appearing as colorless colonies.

High‑Yield Points - ⚡ Biggest Takeaways

#1 cause of Urinary Tract Infections (UTIs) and a common trigger for gram-negative sepsis.

Lactose-fermenting gram-negative rod that grows pink on MacConkey agar.

ETEC causes watery "Traveler's Diarrhea," while EHEC (O157:H7) causes bloody diarrhea and Hemolytic Uremic Syndrome (HUS).

EHEC's Shiga-like toxin inactivates the 60S ribosome, causing endothelial damage.

A primary cause of neonatal meningitis.

Virulence is driven by pili for UTI attachment and LPS endotoxin causing septic shock.

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