Adaptive Resistance to Biocides inSalmonella entericaandEscherichia coliO157 and Cross-Resistance to Antimicrobial Agents

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Abstract
The mechanisms by which bacteria resist killing by antibiotics and biocides are still poorly defined, although repeated exposure to sublethal concentrations of antibacterial agents undoubtedly contributes to their development. This study aimed both to investigate the potential ofSalmonella entericaandEscherichia coliO157 for adaptive resistance to commonly used biocides and to determine any cross-resistance to antibiotics. Strains were repeatedly passaged in media containing increasing concentrations of a biocide or antibiotic until adaptive resistance was obtained. A wide panel of antimicrobial agents was then screened by using the adapted strain to determine cross-resistance, if any. Adaptive resistance was readily achieved for bothS. entericaandE. coliO157. Cross-resistance in adaptively resistantS. entericavaried with the serotype;Salmonella entericaserovar Enteritidis expressed cross-resistance to chloramphenicol, whereasSalmonella entericaserovar Typhimurium expressed cross-resistance to chlorhexidine. Benzalkonium chloride-resistantSalmonella entericaserovar Virchow showed elevated resistance to chlorhexidine; however, chlorhexidine-resistantSalmonellaserovar Virchow did not demonstrate reciprocal cross-resistance to benzalkonium chloride, suggesting specific rather than generic resistance mechanisms.E. coliO157 strains acquired high levels of resistance to triclosan after only two sublethal exposures and, when adapted, repeatedly demonstrated decreased susceptibilities to various antimicrobial agents, including chloramphenicol, erythromycin, imipenem, tetracycline, and trimethoprim, as well as to a number of biocides. These observations raise concern over the indiscriminate and often inappropriate use of biocides, especially triclosan, in situations where they are unnecessary, whereby they may contribute to the development of microbial resistance mechanisms.

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