Other Gram-negative bacilli

Case 1: Typhoid Fever (Salmonella typhi)

A 35-year-old male presents with high fever, abdominal pain, and rose-colored spots on his skin. He reports eating food from street vendors before symptoms began. A blood culture is positive in the first two weeks, and a stool culture is positive in the third week. The Widal test shows high titers of O and H antigens.

Questions and Answers:

  1. What is the most likely diagnosis?

    • Answer: Typhoid fever (Salmonella typhi)

  2. What is the expected TSI (Triple Sugar Iron) agar reaction for this organism?

    • Answer: Alkaline slant / Acid butt (K/A), small amount of H₂S, no gas

  3. What is the major virulence factor that protects S. typhi from host immune responses?

    • Answer: Vi antigen (polysaccharide capsule)

  4. How is S. typhi primarily transmitted?

    • Answer: Fecal-oral route via contaminated food or water

  5. What is the preferred treatment for this infection?

    • Answer: Ceftriaxone or Azithromycin

Case 2: Salmonella Gastroenteritis (Salmonella enteritidis)

A 27-year-old woman experiences nausea, vomiting, watery diarrhea, and fever 12 hours after eating undercooked chicken. Stool culture on Hektoen Enteric Agar reveals green colonies with black centers.

Questions and Answers:

  1. What is the most likely bacterial pathogen?

    • Answer: Salmonella enteritidis (Salmonella gastroenteritis)

  2. What is the primary source of this infection?

    • Answer: Poultry, eggs, dairy products

  3. How does Salmonella appear on XLD (Xylose Lysine Deoxycholate) agar?

    • Answer: Red colonies with black centers (H₂S production)

  4. Why are sickle cell disease patients more prone to invasive Salmonella infections?

    • Answer: Defective spleen function impairs bacterial clearance

  5. What is the appropriate treatment for Salmonella gastroenteritis?

    • Answer: Supportive therapy (fluids & electrolytes); antibiotics are not routinely needed

Case 3: Bacillary Dysentery (Shigellosis) (Shigella spp.)

A 5-year-old child develops bloody diarrhea with mucus, fever, and abdominal cramps. Stool culture on XLD Agarshows red colonies without black centers.

Questions and Answers:

  1. What is the most likely bacterial pathogen?

    • Answer: Shigella spp. (Shigellosis/Bacillary Dysentery)

  2. What is the infecting dose of Shigella?

    • Answer: Only 100–200 bacilli (very low infective dose)

  3. What virulence factor is responsible for the intestinal damage?

    • Answer: Shiga toxin (inhibits 60S ribosome, leading to cell death)

  4. What biochemical tests help differentiate Shigella from Salmonella?

    • Answer: Shigella is non-motile, does not produce H₂S, and is indole-negative

  5. What is the first-line treatment for severe shigellosis?

    • Answer: Azithromycin or Ciprofloxacin

Case 4: Traveler’s Diarrhea (Enterotoxigenic Escherichia coli - ETEC)

A 30-year-old man develops watery diarrhea, nausea, and abdominal cramps after traveling abroad. Stool culture on MacConkey Agar shows pink colonies, and the Indole test is positive.

Questions and Answers:

  1. What is the most likely causative organism?

    • Answer: Enterotoxigenic Escherichia coli (ETEC)

  2. What toxins are responsible for this illness?

    • Answer: Heat-labile (LT) and heat-stable (ST) enterotoxins

  3. How does ETEC appear on EMB (Eosin Methylene Blue) agar?

    • Answer: Green metallic sheen colonies

  4. How is this infection transmitted?

    • Answer: Contaminated food and water (fecal-oral route)

  5. What is the primary treatment for this infection?

    • Answer: Oral rehydration therapy (ORT); Fluoroquinolones for severe cases

Case 5: Hemorrhagic Colitis & HUS (Enterohemorrhagic E. coli - EHEC)

A 7-year-old child presents with bloody diarrhea and acute kidney failure after eating an undercooked hamburger. Sorbitol MacConkey Agar shows colorless colonies.

Questions and Answers:

  1. What is the most likely bacterial pathogen?

    • Answer: Enterohemorrhagic E. coli (EHEC O157:H7)

  2. What virulence factor causes hemolytic uremic syndrome (HUS)?

    • Answer: Shiga-like toxin (Verotoxin)

  3. Why does EHEC produce colorless colonies on Sorbitol MacConkey Agar?

    • Answer: EHEC O157:H7 does not ferment sorbitol

  4. What test can confirm EHEC O157:H7?

    • Answer: PCR for Shiga toxin genes or ELISA for toxin detection

  5. Why are antibiotics not recommended for this infection?

    • Answer: They may increase toxin release and worsen HUS

Case 6: UTI Due to Enterobacteriaceae (Uropathogenic E. coli - UPEC)

A 24-year-old female presents with dysuria, frequent urination, and cloudy urine. Urine culture on MacConkey Agarshows pink colonies, and the Indole test is positive.

Questions and Answers:

  1. What is the most likely bacterial pathogen?

    • Answer: Uropathogenic Escherichia coli (UPEC)

  2. What virulence factor allows adherence to the urinary tract?

    • Answer: P fimbriae (P pili)

  3. What is the expected TSI reaction for this pathogen?

    • Answer: Acid slant / Acid butt (A/A), gas production, no H₂S

  4. How does this organism appear on EMB agar?

    • Answer: Green metallic sheen colonies

  5. What is the first-line treatment for uncomplicated UTI?

    • Answer: Nitrofurantoin or Trimethoprim-Sulfamethoxazole (TMP-SMX)