WBR293

Author	[[PageAuthor::Gerald Chi (Reviewed by Yazan Daaboul, M.D.)]]
Exam Type	ExamType::USMLE Step 1
Main Category	MainCategory::Microbiology
Sub Category	SubCategory::Gastrointestinal
Prompt	[[Prompt::A 27-year-old man presents to the clinic with abdominal cramps, general malaise, fever, and diarrhea 5 hours after consuming hamburgers. He has dry mucosa and decreased skin turgor. Few leukocytes are found in his fecal specimen. Growth of colorless colonies on sorbitol-MacConkey agar is noted. Which of the following descriptions best explains the pathogenesis of the organism responsible for this patient's condition?]]
Answer A	AnswerA::ADP-ribosylation of the Gs alpha subunit proteins
Answer A Explanation	AnswerAExp::Cholera toxin involves ADP-ribosylation of the Gs alpha subunit proteins.
Answer B	AnswerB::ADP-ribosylation of elongation factor 2 (EF-2)
Answer B Explanation	AnswerBExp::Exotoxin A from Pseudomonas aeruginosa involves ADP-ribosylation of elongation factor 2 (EF-2).
Answer C	AnswerC::Cleavage of N-glycoside bond of adenine in 28S ribosomal RNA
Answer C Explanation	AnswerCExp::Shiga-like toxin-1 (SLT-1) of EHEC cleaves N-glycoside bond of adenine in 28S rRNA of the 60S ribosome unit to prevent EF-1-dependent aminoacyl tRNA binding, thereby inhibiting protein synthesis.
Answer D	AnswerD::Attachment of epithelium and effacement of microvilli
Answer D Explanation	AnswerDExp::In EPEC, attachment of epithelium and effacement of microvilli is mediated via phosphorylation of Tir and subsequent intimin binding.
Answer E	AnswerE::Local invasion of colonic mucosa
Answer E Explanation	AnswerEExp::In EIEC, diarrhea is caused by local invasion of colonic mucosa.
Right Answer	RightAnswer::C
Explanation	[[Explanation::Escherichia coli O157:H7 (EHEC) is an enterohemorrhagic strain of Escherichia coli. Infection may lead to hemorrhagic diarrhea and subsequent development of hemolytic-uremic syndrome (HUS). Transmission is via the fecal-oral route, and the majority of cases are associated with ingestion of undercooked, contaminated ground beef or pork. E. coli O157:H7 is negative for invasiveness (Sereny test), adheres through the E. coli common pilus (ECP), and does not produce heat-stable or heat-labile toxins. In addition, E. coli O157:H7 is usually sorbitol negative, whereas other E. coli ferment sorbitol. E. coli O157:H7 also lacks the ability to hydrolyze 4-methylumbelliferyl-β-D-glucuronide (MUG) and does not grow at 45 °C in the presence of 0.15% bile salts (cannot be isolated by using standard fecal coliform methods that include incubation at 45 °C). Among the virulence factors are periplasmic catalase and shiga-like toxins. Shiga-like toxin (SLT-1) is an iron-regulated toxin that catalytically inactivates 60S ribosomal subunits of eukaryotic cells. It cleaves N-glycoside bond of adenine in 28S rRNA of the 60S ribosome unit to prevent EF-1-dependent aminoacyl tRNA binding, thereby inhibiting protein synthesis. Educational Objective: Escherichia coli O157:H7 (EHEC) is an enterohemorrhagic strain of Escherichia coli. Transmission is via the fecal-oral route, and the majority of cases are associated with ingestion of undercooked, contaminated ground beef or pork. Shiga-like toxin-1 (SLT-1) of EHEC cleaves N-glycoside bond of adenine in 28S rRNA of the 60S ribosome unit to prevent EF-1-dependent aminoacyl tRNA binding, thereby inhibiting protein synthesis. References: First Aid 2015 page 138.]]
Approved	Approved::Yes
Keyword	WBRKeyword::Escherichia coli, WBRKeyword::Gastroenteritis, WBRKeyword::Shiga-like toxin
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