Quality control in agar-dilution sensitivity-testing by direct assay of the antibiotic in the solid medium.

When antibiotic sensitivity of microorganisms is tested by agar dilution, single antibiotics are incorporated in solid media and 'spot' inocula of test strains are applied to the antibiotic media and to a plate without antibiotic. The usual method of quality control is to include control strains of known sensitivity.1 However, control strains are usually sensitive to a number of antibiotics, and the absence of growth does not indicate which antibiotic is in a particular plate. In addition, strains chosen for controls are often either very sensitive or highly resistant to antibiotics so that they do not indicate moderate discrepancies in antibiotic concentrations. The direct assay of the antibiotic content of the solid media described here shows that each antibiotic is present at the correct concentration and provides some check on the identity of the antibiotic.