In a conventional workflow, where is susceptibility testing typically performed?

• A. On gram-stained smear
• B. In broth immediately from the culture bottle
• C. On subcultured colonies on agar
• D. Using rapid diagnostic test directly on blood culture

Answer: (C)

Accurate susceptibility testing requires testing a pure organism with a standardized inoculum. In practice, that means taking the positive culture and subculturing it onto agar to obtain discrete colonies, then preparing a uniform suspension from those colonies and using it for disk diffusion or broth microdilution. This standardization ensures comparable inoculum density and a single species, which gives reliable, interpretable results against breakpoints.

Testing directly from a Gram-stained smear won’t work because it mixes cells from potentially more than one organism and provides no viable, standardized inoculum. Using broth directly from the culture bottle likewise introduces mixed flora and variable cell density, making results unreliable. While rapid diagnostic tests on blood culture can rapidly suggest resistance patterns, conventional susceptibility testing is performed on subcultured colonies on agar to maintain accuracy and reproducibility.