Mechanism of Resistance to Several Antimicrobial Agents inSalmonellaClinical Isolates Causing Traveler's Diarrhea

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Abstract
The evolution of antimicrobial resistance inSalmonellaisolates causing traveler's diarrhea (TD) and their mechanisms of resistance to several antimicrobial agents were analyzed. From 1995 to 2002, a total of 62Salmonellastrains were isolated from stools of patients with TD. The antimicrobial susceptibility to 12 antibiotics was determined, and the molecular mechanisms of resistance to several of them were detected as well. The highest levels of resistance were found against tetracycline and ampicillin (21 and 19%, respectively), followed by resistance to nalidixic acid (16%), which was mainly detected from 2000 onward. Molecular mechanisms of resistance were analyzed in 16 isolates. In these isolates, which were resistant to ampicillin, two genes encoding β-lactamases were detected:oxa-1(one isolate) andtem-like (seven isolates [in one strain concomitantly with acarb-2]). Resistance to tetracycline was mainly related totetA(five cases) and totetBandtetG(one case each). Resistance to chloramphenicol was related to the presence of thefloRandcmlAgenes and to chloramphenicol acetyltransferase activity in one case each. Different genes encoding dihydrofolate-reductases (dfrA1,dfrA12,dfrA14, anddfrA17) were detected in trimethoprim-resistant isolates. Resistance to nalidixic acid was related to the presence of mutations in the amino acid codons 83 or 87 of thegyrAgene. Further surveillance of theSalmonellaspp. causing TD is needed to detect trends in their resistance to antimicrobial agents, as we have shown in our study with nalidixic acid. Moreover, such studies will lead to better treatment and strategies to prevent and limit their spread.

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