A laboratory performs Kirby-Bauer testing for trimethoprim-sulfamethoxazole on Mueller-Hinton agar prepared with casein hydrolysate containing excess thymidine. Which error is expected in the reported results?
- A Falsely large zones suggesting susceptibility when the organism is truly resistant
- B Falsely small zones suggesting resistance when the organism is truly susceptible ✓
- C No effect, because thymidine affects only beta-lactam agents
- D Zones appear normal but colonies within the zone grow larger
Correct answer: B. Falsely small zones suggesting resistance when the organism is truly susceptible
Explanation
Trimethoprim-sulfonamide combinations act by blocking folate-dependent thymidine synthesis. Excess thymidine in the medium bypasses this blockade, allowing growth right up to the disc and producing falsely small zones, so a susceptible organism is misreported as resistant. This is why CLSI specifies Mueller-Hinton agar low in thymidine and thymidine phosphorylase. The defect is specific to folate antagonists, not beta-lactams.
Reference: Bailey and Scott's Diagnostic Microbiology, 14th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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