MLT GlobeBacteriology: Culture media, specimens & AST – page 7
126 Bacteriology MCQs on Culture media, specimens & AST with answers and explanations.
An E. coli is resistant to cefoxitin, cefotaxime and ceftazidime, remains susceptible to cefepime, and shows no zone increase with clavulanic acid. The most likely mechanism is:
AmpC enzymes hydrolyse cephamycins such as cefoxitin, are not inhibited by clavulanate and spare cefepime. ESBLs are inhibited by clavulanate and do not attack cefoxitin.
In broth microdilution, the trimethoprim-sulfamethoxazole wells show light 'trailing' growth over several concentrations. According to CLSI, the MIC is read as the lowest concentration giving:
Organisms can grow for a few generations before folate antagonists act, causing trailing. CLSI therefore reads sulfonamides and trimethoprim at ≥80% growth reduction, not at complete clearing.
Daptomycin MICs are falsely high if the broth is not supplemented with:
Daptomycin needs free calcium to bind to the bacterial membrane, so CLSI requires Mueller-Hinton broth adjusted to 50 mg/L calcium. Lysed horse blood is used for streptococci.
An epidemiological cutoff value (ECV) for an organism-drug pair is used to:
The ECV is the highest MIC of the wild-type population; isolates above it are 'non-wild type'. Unlike clinical breakpoints, ECVs do not predict treatment outcome.
Oxacillin and cefoxitin tests for Staphylococcus aureus should be incubated at 33–35 °C and not above 35 °C because:
Expression of mecA-mediated resistance is temperature dependent, and heteroresistant subpopulations may be missed above 35 °C. S. aureus grows well at 37 °C, so poor growth is not the reason.
When reading disk diffusion zones for Proteus mirabilis, a thin veil of swarming growth extends into the zone of inhibition. According to CLSI, the technologist should:
CLSI states that a thin veil of swarming Proteus should be ignored and the zone read at the clear margin of heavy growth. MacConkey is not a standard AST medium.