In vitro action of combinations of antimicrobial agents with EDTA-tromethamine on Proteus vulgaris of a canine origin.

Combinations of EDTA-tromethamine and 7 antimicrobial agents (chloramphenicol, nalidixic acid, oxytetracycline, penicillin, polymyxin-B, streptomycin, or a combination of sulfamethazine, sulfapyridine, and sulfathiazole) were tested for synergistic activities against Proteus vulgaris of canine origin. Three in vitro tests were used, including minimal inhibitory concentrations of the drugs, a 2-dimensional microtiter checkerboard technique, and bacterial inhibition studies. A synergistic inhibitory action was observed with combinations of EDTA-tromethamine plus penicillin, oxytetracycline, nalidixic acid, chloramphenicol, polymyxin-B, or sulfamethazine-sulfapyridine-sulfathiazole by at least one test system. A synergistic effect was not recorded with combinations of EDTA-tromethamine and streptomycin. Poor correlation between data from the 3 test systems was noticed, despite the fact that each monitors bacteriostatic end points.