Antimicrobial susceptibility of clinical isolates of Brucella

Abstract

A number of antimicrobial agents have been used in the treatment of human brucellosis with varying effectiveness. The purpose of this study was to test the in vitro susceptibility of isolates of four Brucella species to a variety of antimicrobial agents, and to study in vitro synergy of combinations of agents. Minimal inhibitory concentrations (MICs) were determined using conventional broth microdilution methods and commercially available systems. Conventional checkerboard synergy microdilutions were prepared for gentamicin or streptomycin plus tetracycline, and rifampicin plus tetracycline. Synergy or antagonism was determined by the fractional inhibitory concentration index. Penicillin G and ampicillin showed in vitro activity against Brucella (MIC 90 4 μg/ml), whereas the antipseudomonal penicillins were less active (carbenicillin MIC 90 12 μg/ml, piperacillin MIC 90 32 μg/ml). Among the third generation cephalosporins tested, cefotaxime (MIC 90 2 μg/ml) demonstrated greatest activity. As a class, aminoglycosides were equivalent (MIC 90 1–4 μg/ml). All strains were sensitive to tetracycline (MIC 90 0.25 μg/ml), trimethoprim-sulfamethoxazole (MIC 90 1/19 μg/ml), and rifampin (MIC 90 1 μg/ml). Erythromycin (MIC 90 > 8 μmg/ml) and vancomycin (MIC 90 > 16 μmg/ml) demonstrated no activity. In vitro synergy (fractional inhibitory concentration index < 0.5) was demonstrated with tetracycline plus rafampin in six of eight isolates tested.