Which procedure describes how to determine the MBC after a broth dilution assay?

• A. Perform Gram stain on the clear tubes and if no cells are visible under microscopy the lowest concentration without cells on the slide is the MBC.
• B. Store the clear tubes at 4°C for 48 hours and if turbidity does not develop during cold incubation consider that the highest concentration tested is the MBC without subculture steps.
• C. Take aliquots from tubes that showed no visible growth, plate on antibiotic-free agar, incubate, and identify the lowest original antibiotic concentration that produces no growth on the antibiotic-free plates as the MBC.
• D. Measure the optical density of the clear tubes with a spectrophotometer; the lowest concentration with an optical density below 0.05 is the MBC and no plating is required.

Answer: (C)