In Vitro Sensitivity of Salmonella to Ten Antimicrobial Agents Including Sulfamethoxazole and Trimethoprim, Alone and in Combination

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Abstract
The activities of trimethoprim (TMP) and sulfamethoxazole (SMZ), alone and in combination (SMZ-TMP), and of the following antibiotics were tested against 115 clinical isolates of nontyphoid Salmonella species: tobramycin, gentamicin, ampicillin, amoxicillin, neomycin, kanamycin, chloramphenicol, and tetracycline. The methods of disk diffusion, microtiter broth dilution, and agar dilution were employed for all single antimicrobial agents as well as for SMZ-TMP studies. Growth curves were performed in broth. SMZ-TMP, TMP, gentamicin, tobramycin, and neomycin were the most active drugs in vitro. All strains were inhibited by ≤1 μg of TMP per ml, but >100 μg of SMZ per ml was required for at least 10% of strains. SMZ and TMP in a ratio of 10:0.5, respectively, inhibited all isolates and were synergistic for 105 strains. All strains inhibited by the combination of 10:0.5 SMZ-TMP had a zone diameter of ≥22 mm by using a combination disk containing 1.25 μg of TMP and 23.75 μg of SMZ. Seven isolates were resistant to >100 μg/ml of ampicillin or amoxicillin; all isolates were sensitive to chloramphenicol at ≤6.3 μg/ml. SMZ-TMP appears to be active against nontyphoid salmonellae in vitro; this is usually due to a synergistic effect.