WBR294

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 28-year-old man presents to the clinic with abdominal cramps, general malaise, fever, and diarrhea 6 hours after consuming ground beef. Few leukocytes are found in his fecal specimen. Growth of colorless colonies on sorbitol-MacConkey agar is noted. The pathogen produces toxins which contain enzymatic subunits that interfere with protein synthesis. Which of the following antibiotics has a similar mechanism regarding to the toxin's action on translation?]]
Answer A	AnswerA::Linezolid
Answer A Explanation	AnswerAExp::Linezolid and aminoglycosides block the formation of initiation complex, thereby inhibiting protein synthesis.
Answer B	AnswerB::Tetracycline
Answer B Explanation	AnswerBExp::Tetracycline blocks the entry and binding of aminoacyl transfer RNA, thereby inhibiting protein synthesis.
Answer C	AnswerC::Quinupristin/dalfopristin
Answer C Explanation	AnswerCExp::Quinupristin/dalfopristin and other streptogramins block peptidyl transferase which forms peptide bonds between adjacent amino acids using tRNAs during the translation process.
Answer D	AnswerD::Clindamycin
Answer D Explanation	AnswerDExp::Clindamycin and macrolides block translocation by binding to the 50S rRNA of the large bacterial ribosome subunit.
Answer E	AnswerE::Chloramphenicol
Answer E Explanation	AnswerEExp::Chloramphenicol blocks peptidyl transferase which forms peptide bonds between adjacent amino acids using tRNAs during the translation process.
Right Answer	RightAnswer::B
Explanation	[[Explanation::E. coli O157:H7 (EHEC) is negative for invasiveness (Sereny test), adheres through the E. coli common pilus (ECP), and does not produce either heat-stable or heat-labile toxins. In addition, E. coli O157:H7 is usually sorbitol-negative, whereas other E. coli ferment sorbitol. Among the virulence factors, 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. This mechanism of action is similar to tetracyclines, which also inhibit protein synthesis by blocking the entry and binding of aminoacyl transfer RNA. Educational Objective: Among the virulence factors of E. coli O157:H7, Shiga-like toxin (SLTs) is an iron-regulated toxin that catalytically cleaves N-glycoside bond of adenine in 28S rRNA of the 60S ribosome unit to prevent aminoacyl tRNA binding, thereby inhibiting protein synthesis. This mechanism of action is similar to tetracyclines, which also inhibit protein synthesis by blocking the entry and binding of aminoacyl transfer RNA. References: First Aid 2015 page 138.]]
Approved	Approved::Yes
Keyword	WBRKeyword::Protein synthesis, WBRKeyword::Shiga-like toxin, WBRKeyword::Tetracycline, WBRKeyword::Gastroenteritis
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