A 7-year-old boy is brought to the emergency room because of severe, acute diarrhea. He is drowsy with a dull, lethargic appearance. He has sunken eyes, poor skin turgor, and dry oral mucous membranes and tongue. He has a rapid, thready pulse with a systolic blood pressure of 60 mm Hg and his respirations are 33/min. His capillary refill time is 6 sec. He has had no urine output for the past 24 hours. Which of the following is the most appropriate next step in treatment?

Start IV fluid resuscitation with normal saline or Ringer’s lactate, along with monitoring of vitals and urine output

Start IV fluid resuscitation by administering colloid solutions

Provide oral rehydration therapy to correct dehydration

Give initial IV bolus of 2 L of Ringer’s lactate, followed by packed red cells, fresh frozen plasma, and platelets in a ratio of 1:1:1

A 47-year-old woman presents to a local medical shelter while on a mission trip with her church to help rebuild homes after a hurricane. She has been experiencing severe nausea, vomiting, and diarrhea for the last 2 days and was feeling too fatigued to walk this morning. On presentation, her temperature is 99.2°F (37.3°C), blood pressure is 95/62 mmHg, pulse is 121/min, and respirations are 17/min. Physical exam reveals decreased skin turgor, and a stool sample reveals off-white watery stools. Gram stain reveals a gram-negative, comma-shaped organism that produces a toxin. Which of the following is consistent with the action of the toxin most likely involved in the development of this patient's symptoms?

Increased adenylyl cyclase activity

Increased membrane permeability

Cleavage of junctional proteins

Activation of receptor tyrosine kinase

A 25-year-old man presents to the physician with 2 days of profuse, watery diarrhea. He denies seeing blood or mucus in the stools. On further questioning, he reveals that he eats a well-balanced diet and generally prepares his meals at home. He remembers having some shellfish from a street vendor 3 days ago. He takes no medications. His past medical history is unremarkable. Which of the following mechanisms most likely accounts for this patient’s illness?

Tyrosine kinase phosphorylation

Tyrosine kinase dephosphorylation

Osmotic effect of intestinal contents

Inflammation of the gastrointestinal wall

A stool sample was taken from a 19-year-old male who presented with profuse watery diarrhea. He recently returned from a trip to Central America. A microbiologist identified the causative agent as a gram-negative, oxidase-positive, comma-shaped bacteria that is able to grow well in a pH > 8. Which of the following is a mechanism of action of the toxin produced by this bacteria?

Overactivation of adenylate cyclase by activation of Gs subunit by ADP-ribosylation

Overactivation of adenylate cyclase by inhibition of Gi subunit by ADP-ribosylation

Inactivation of the 60S ribosomal subunit by cleaving an adenine from the 28S rRNA

Overactivation of guanylate cyclase

Degradation of cell membranes by hydrolysis of the phospholipids

A 57-year-old HIV-positive male with a history of intravenous drug abuse presents to the emergency room complaining of arm swelling. He reports that he developed progressively worsening swelling and tenderness over the right antecubital fossa three days prior. He recently returned from a trip to Nicaragua. His past medical history is notable for an anaphylactoid reaction to vancomycin. His temperature is 101.4°F (38.6°C), blood pressure is 140/70 mmHg, pulse is 110/min, and respirations are 20/min. Physical examination reveals an erythematous, fluctuant, and tender mass overlying the right antecubital fossa. Multiple injection marks are noted across both upper extremities. He undergoes incision and drainage and is started on an antibiotic that targets the 50S ribosome. He is discharged with plans to follow up in one week. However, five days later he presents to the same emergency room complaining of abdominal cramps and watery diarrhea. Which of the following classes of pathogens is most likely responsible for this patient’s current symptoms?

Anaerobic flagellated protozoan

A 17-year-old girl comes to the physician because of a 12-hour history of profuse watery diarrhea with flecks of mucus that started shortly after she returned from a trip to South America. She has not had any fever or nausea. Pulse is 104/min and blood pressure is 110/65 mm Hg. Physical examination shows dry mucous membranes and decreased skin turgor. Stool culture shows gram-negative, comma-shaped, flagellated bacilli. Therapy with oral rehydration solution is initiated. Which of the following is the most likely mechanism of this patient's diarrhea?

Luminal chloride hypersecretion due to overactivation of adenylate cyclase

Impaired intestinal motility due to degeneration of autonomic nerves

Excessive water excretion due to osmotically active solutes in the lumen

Fluid and electrolyte loss due to inflammation of luminal surface epithelium

Reduced ability of water absorption in the colon due to rapid intestinal transit

A 45-year-old man comes to the physician because of a 1-day history of progressive pain and blurry vision of his right eye. He has difficulties opening the eye because of pain. His left eye is asymptomatic. He wears contact lenses. He has bronchial asthma treated with inhaled salbutamol. He works as a kindergarten teacher. His temperature is 37°C (98.6°F), pulse is 85/min, and blood pressure is 135/75 mm Hg. Examination shows a visual acuity in the left eye of 20/25 and the ability to count fingers at 3 feet in the right eye. A photograph of the right eye is shown. Which of the following is the most likely diagnosis?

Staphylococcus aureus keratitis

A 23-year-old man comes to the physician because of a 2-day history of profuse watery diarrhea and abdominal cramps. Four days ago, he returned from a backpacking trip across Southeast Asia. Physical examination shows dry mucous membranes and decreased skin turgor. Stool culture shows gram-negative, oxidase-positive, curved rods that have a single polar flagellum. The pathogen responsible for this patient's condition most likely has which of the following characteristics?

Grows well in medium with pH of 9

Acts by activation of guanylate cyclase

Forms spores in unfavorable environment

Infection commonly precedes Guillain-Barré syndrome

Causes necrosis of Peyer patches of distal ileum

Vibrio species for USMLE Step 2 CK: High-Yield Microbiology Lessons

Vibrio Basics - Comma-Shaped Critters

Gram-negative, motile, comma-shaped rods.
Oxidase-positive (key differentiator from Enterobacteriaceae).
Facultative anaerobes, found in aquatic/marine environments.
- Transmission: consumption of contaminated water or raw/undercooked seafood (especially oysters).
Culture on Thiosulfate-Citrate-Bile Salts-Sucrose (TCBS) agar.

Vibrio cholerae bacteria with flagella and pili

⭐ Most pathogenic species are halophilic (require salt for growth), except for V. cholerae and V. mimicus.

Vibrio cholerae - The Diarrhea Dynamo

Vibrio cholerae bacteria with flagella and pili

Organism: Gram-negative, motile, comma-shaped rod. Oxidase-positive.
Transmission: Fecal-oral route, typically via contaminated water or raw/undercooked shellfish.
Disease: Cholera-an acute, severe diarrheal illness. Presents with voluminous, painless, watery stools ("rice-water stool"), leading to rapid dehydration, electrolyte imbalance, and potential hypovolemic shock.

Diagnosis: Stool culture on Thiosulfate-citrate-bile salts-sucrose (TCBS) agar, where it ferments sucrose to produce yellow colonies.
Treatment: Prompt and aggressive fluid and electrolyte replacement is critical. Antibiotics (e.g., doxycycline, azithromycin) can reduce disease duration.

⭐ Oral rehydration therapy (ORT) is highly effective because the glucose-Na+ cotransport mechanism (SGLT1) in the gut remains unaffected by the cholera toxin, allowing for efficient water absorption.

Other Vibrios - Seafood Sickness Squad

Primarily associated with consumption of raw/undercooked seafood (especially oysters) or exposure to contaminated seawater.
Both are halophilic (salt-loving), unlike V. cholerae.

Species	Clinical Features	Notes
V. parahaemolyticus	Gastroenteritis: Explosive, watery diarrhea; self-limiting.	Most common cause of seafood-associated gastroenteritis in the US.
V. vulnificus	1. Wound Infections: Rapidly progressive cellulitis, hemorrhagic bullae, necrotizing fasciitis. 2. Septicemia: High fever, shock.	High mortality, especially with underlying liver disease (e.g., cirrhosis, hemochromatosis).

Vibrio vulnificus hemorrhagic bullae on hands

Diagnosis & Treatment - Taming the Tide

Specimen: Stool sample for culture; blood culture for septicemia (V. vulnificus).
Culture: Selective Thiosulfate-Citrate-Bile Salts-Sucrose (TCBS) agar is crucial.
- V. cholerae: ferments sucrose → yellow colonies.
- V. parahaemolyticus/vulnificus: non-fermenters → green/blue-green colonies.
Primary Treatment: Aggressive Rehydration
- Oral Rehydration Salts (ORS) for most cases.
- IV fluids (Ringer's Lactate) for severe dehydration, shock, or altered mental status.
Antibiotics (adjunctive): Reduce fluid loss & duration.
- Primary: Doxycycline (single dose), Azithromycin.
- Alternative: Ciprofloxacin.

⭐ Prompt fluid and electrolyte replacement is the single most important life-saving intervention in cholera management.

High‑Yield Points - ⚡ Biggest Takeaways

Vibrio cholerae causes profuse, watery diarrhea (rice-water stool) via the cholera toxin.

The toxin permanently activates the Gs protein, leading to ↑ cAMP and massive fluid secretion.

Transmission is fecal-oral, typically through contaminated water or improperly cooked shellfish.

Grows on TCBS agar (Thiosulfate-Citrate-Bile Salts-Sucrose) and in alkaline media.

Treatment is aggressive oral or IV rehydration.

Vibrio vulnificus can cause life-threatening wound infections and sepsis.

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Vibrio species