In vitro Activity of Conventional and Newer Antimicrobial Agents against <i>Brucella melitensi</i><i>s</i> Clinical Isolates from Kuwait

Abstract

The in vitro activity of 15 antimicrobial agents was tested against 21 clinical isolates of Brucella melitensis from Kuwait. The minimum inhibitory concentration for 90% of strains (MIC 90) was determined. Tetracycline and doxycycline showed similar activity (MIC 90 ≤ 0.25). The corresponding values for other antimicrobials were: rifampin (4), tri-methoprim-sulfamethoxazole (4/76), ciprofloxacin (0.25), and ampicillin (8). Among the aminoglycosides, MIC 90 was lower for gentamicin (0.5) than that for tobramycin (1), streptomycin (2) or amikacin (4). Of the cephalosporins, MIC 90 for ceftriaxone (2) and cefotaxime (2) was lower than that for cefuroxime (32), cephalothin (64) or cefoxitin (64). The effect of antimicrobials in combinations was assessed by fractional inhibitory concentration and fractional bactericidal concentration indices in three strains. Though variations among strains were present, synergy ( < 0.5) was observed between doxycycline plus trimethoprim-sulfame-thoxazole, rifampin, streptomycin, gentamicin or cefotaxime; ciprofloxacin plus streptomycin or gentamicin, and cefotaxime plus gentamicin. Other combinations of ciprofloxacin, cefotaxime or trimethoprim-sulfamethoxazole with those agents that were combined with doxycycline showed indifference &(< 0.5–4). Antagonism &(< 4) was seen in only one strain for the combination of ciprofloxacin plus rifampin and trimethoprim-sulfamethoxazole plus cefotaxime. Time-kill studies showed that doxycycline or gentamicin alone decreased the viable counts below 2 log 10 after 24 h. Combinations of doxycycline plus gentamicin or ciprofloxacin plus gentamicin decreased the viable counts below 2 log 10 within 6 h.