WBR292

Author	[[PageAuthor::Gerald Chi (Reviewed by Yazan Daaboul, M.D.)]]
Exam Type	USMLE Step 1
Main Category	Microbiology
Sub Category	Gastrointestinal
Prompt	A 26-year-old man presents to the emergency department with abdominal cramps, general malaise, fever, and profound diarrhea 3 hours after a potluck dinner. Three of his friends also have similar symptoms. Stool samples demonstrate numerous polymorphonuclear leukocytes and are positive for growth on a selective medium under CO2 incubation at 42 °C (108 °F). The microorganism responsible for this patient's condition is also found to possess an enzyme that catalyzes the transport of electrons from NADPH to oxygen. Which of the following descriptions best explains the pathogenesis of this microorganism?
Answer A	ADP-ribosylation of the Gs alpha subunit proteins using nicotinamide adenine dinucleotide (NAD)
Answer A Explanation	Cholera toxin involves ADP-ribosylation of the Gs alpha subunit proteins.
Answer B	ADP-ribosylation of elongation factor 2 (EF-2) thus inhibiting protein synthesis
Answer B Explanation	Exotoxin A from Pseudomonas aeruginosa involves ADP-ribosylation of elongation factor 2 (EF-2).
Answer C	Invasion of colonic mucosa with disruption of epithelial cells
Answer C Explanation	Campylobacter jejuni infection involves invasion of colonic mucosa with disruption of epithelial cells.
Answer D	Activation of guanylyl cyclase that enhances intestinal secretion
Answer D Explanation	Heat-stable toxin (STa) of ETEC involves activation of guanylyl cyclase that enhances intestinal secretion.
Answer E	Preformed toxins in ingested food
Answer E Explanation	Staphylococcus aureus, Bacillus cereus, and Clostridium botulinum produce preformed toxins in ingested food.
Right Answer	C
Explanation	Campylobacter jejuni is a curved, helical-shaped, non-spore forming, non-glucose-fermenting, microaerophilic, gram-negative bacteria. It is one of the most common causes of human gastroenteritis worldwide. Infection with C. jejuni occurs following invasion of the microorganism of the colonic mucosa with disruption of epithelial cells. C. jejuni has also been associated with subsequent development of Guillain-Barré syndrome, which usually develops two to three weeks after the initial illness. C. jejuni is commonly associated with ingestion of poultry as it naturally colonizes the digestive tract of many bird species. C. jejuni is grown on specially selective agar plates at 42°C, the normal avian body temperature, rather than at 37°C, the temperature at which the majority of other pathogenic bacteria are grown. Since the colonies are oxidase positive, they will usually only grow in scanty amounts on the plates. Microaerophilic conditions are required for abundant growth. A selective blood agar medium (Skirrow's medium) may also be used. The following include the mportant oxidase-positive pathogens: Campylobacter Helicobacter Legionella Neisseria Pseudomonas Vibrio Educational Objective: Campylobacter jejuni grows at 42°C and causes intestinal disease by invasion of the colonic mucosa. References: First Aid 2015 page 138.
Approved	Yes
Keyword	Campylobacter jejuni, Bacteria, Gram negative, Gastroenteritis, Toxin
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