## Laboratory Diagnosis and Epidemiology of Vibrio cholerae ### Correct Statements (Options A, B, C) **High-Yield:** TCBS agar is the selective and differential medium for Vibrio isolation (Option A). Vibrio cholerae grows as flat, yellow, mucoid colonies. The medium suppresses gram-positive bacteria and most coliforms, making it ideal for isolation from stool specimens. **Key Point:** Vibrio cholerae O1 El Tor biotype is the current pandemic strain (Option B), responsible for the 7th pandemic since 1961. It has largely replaced the classical biotype globally and remains endemic in many regions including India and Bangladesh. **Clinical Pearl (Option C):** The organism is indeed motile at 37°C via a single polar flagellum. At 25°C (environmental/aquatic temperatures), it expresses lateral (peritrichous) flagella — and remains motile. The statement in Option C is correct in describing the flagellar switch but is presented as a true fact, making it a correct statement (not the exception). ### Why Option D is Incorrect (The EXCEPT Answer) **Warning:** Option D states that "non-O1/non-O139 strains do not produce cholera toxin." This is **not universally true**. While non-O1/non-O139 strains are generally non-toxigenic and rarely cause epidemic cholera, some non-O1/non-O139 strains **can** harbor the ctx prophage and produce cholera toxin (CT). The statement as written is an oversimplification and factually incorrect as an absolute claim. - Cholera toxin (CT) is encoded by the CTXφ bacteriophage, which can theoretically infect any Vibrio cholerae strain carrying the VPI-1 pathogenicity island. - Non-O1/non-O139 strains have been documented to produce CT in sporadic cases. - Therefore, the claim that non-O1/non-O139 strains **do not** produce cholera toxin is false. ### Laboratory Identification Summary | Feature | Finding | Significance | |---|---|---| | Gram stain | Gram-negative, curved rod | Morphology | | Oxidase test | Positive | Confirms Vibrio | | TCBS agar | Yellow, flat, mucoid colonies | Selective/differential | | Motility (37°C) | Motile (polar flagellum) | Pathogenic behavior | | Motility (25°C) | Motile (lateral flagella) | Environmental adaptation | | O1/O139 antisera | Positive for epidemic strains | Toxigenic strain confirmation | | Non-O1/non-O139 | Can occasionally produce CT | Not always non-toxigenic | [cite: Jawetz, Melnick & Adelberg's Medical Microbiology 28e Ch 24; Mandell, Douglas, and Bennett's Principles and Practice of Infectious Diseases 9e Ch 214; Park's Textbook of Preventive and Social Medicine 26e Ch 8]
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