Microdilution transfer plate technique for determining in vitro synergy of antimicrobial agents

A microdilution transfer plate technique for determining in vitro synergy of antimicrobial agents is described. Combinations of gentamicin-nalidixic acid against Proteus mirabilis and rifampin-amphotericin B against Candida albicans are used as examples to demonstrate the technique. Results correlate with published data obtained by conventional methods. The technique is effective for evaluating the in vitro effects of antimicrobial agent combinations against both bacteria and fungi. The technique enables one to produce a checkerboard gradient in a fast, convenient, and reproducible way; results are easily visualized.