Determination of the effect of antimicrobics in combination against Listeria monocytogenes.

The in vitro activity of selected penicillins, extended spectrum cephalosporins, vancomycin, gentamicin, erythromycin, tetracycline, rifampin, and trimethoprim and sulfamethoxazole (alone and in combination) was determined by microtiter technique for 20 isolates of Listeria monocytogenes. The activity of selected combinations of antimicrobics was determined by the microtiter checkboard technique. Trimethoprim-sulfamethoxazole (1:20 ratio) was the most active agent in inhibitory tests and also showed bactericidal activity. The combinations of gentamicin with either ampicillin or vancomycin and that of erythromycin with tetracycline showed bactericidal effect in synergy studies. Combining ampicillin with an extended spectrum cephalosporin showed no antagonism, whereas, combining rifampin with trimethoprim or with trimethoprim/sulfamethoxazole led only to indifference or antagonism. These observations may have importance in selection of therapy in animal models or in selected clinical situations.