WBR293
| Author | [[PageAuthor::Gerald Chi (Reviewed by Yazan Daaboul, M.D.)]] |
|---|---|
| Exam Type | USMLE Step 1 |
| Main Category | Microbiology |
| Sub Category | Gastrointestinal |
| 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 | ADP-ribosylation of the Gs alpha subunit proteins |
| Answer A Explanation | Cholera toxin involves ADP-ribosylation of the Gs alpha subunit proteins. |
| Answer B | ADP-ribosylation of elongation factor 2 (EF-2) |
| Answer B Explanation | Exotoxin A from Pseudomonas aeruginosa involves ADP-ribosylation of elongation factor 2 (EF-2). |
| Answer C | Cleavage of N-glycoside bond of adenine in 28S ribosomal RNA |
| Answer C Explanation | 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 | Attachment of epithelium and effacement of microvilli |
| Answer D Explanation | In EPEC, attachment of epithelium and effacement of microvilli is mediated via phosphorylation of Tir and subsequent intimin binding. |
| Answer E | Local invasion of colonic mucosa |
| Answer E Explanation | In EIEC, diarrhea is caused by local invasion of colonic mucosa. |
| Right Answer | C |
| 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. |
| Approved | Yes |
| Keyword | Escherichia coli, Gastroenteritis, Shiga-like toxin |
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