Effect of mixed cultures on antibiotic susceptibility testing.

Careful studies of the antibiotic susceptibilities of mixtures of bacteria likely to be encountered in clinical cultures have shown that the results obtained are completely unreliable. Mixtures of resistant and sensitive species appeared either as "resistant" or "sensitive" depending upon the organisms and the drug. A number of sensitive species gave reactions interpreted as resistant when tested in combination. Since reactions of bacterial mixtures are completely unpredictable, the authors emphasize that antibiotic susceptibility testing be limited to pure cultures.