Mechanisms of resistance to ampicillin, chloramphenicol and quinolones in multiresistant Salmonella typhimurium strains isolated from fish

Mechanisms of antibiotic resistance and epidemiological relationships were investigated for five multiresistant strains of Salmonella typhimurium isol ated from fish in India. Four strains showed resistance to nalidixic acid, chloramphenicol, tetracycline, co-trimoxazole, gentamicin and beta-lactam a ntibiotics. The remaining strain was susceptible to all beta-lactam antibio tics tested and to co-trimoxazole but resistant to the other antibiotics te sted. Epidemiological analysis performed by REP-PCR showed that the five is olates belonged to the same clone. Resistance to nalidixic acid was related to a single mutation in the gyrA gene. Chloramphenicol resistance was rela ted to the production of chloramphenicol acetyl-transferase. An OXA-1 beta- lactamase, located in an integron, was responsible for resistance to ampici llin. These results indicate the health hazard posed by the fact that S. ty phimurium may acquire or develop several mechanisms of resistance to a vari ety of antibiotics, including quinolones, and can thus cause disease in hum ans which may be difficult to treat.