Homologous stress adaptation, antibiotic resistance, and biofilm forming ability of Salmonella enterica serovar Heidelberg ATCC8326 on different food-contact surfaces following exposure to sublethal chlorine concentrations1.

Salmonella enterica serovar Heidelberg (American Type Culture Collection; ATCC 8326) was examined for the ability to adapt to the homologous stress of chlorine through exposure to increasing chlorine concentrations (25 ppm daily increments) in tryptic soy broth (TSB). The tested strain exhibited an acquired tolerance to chlorine in TSB with the tolerant cells growing in concentrations up to 400 ppm. In addition, the chlorine stressed cells displayed rugose morphology on tryptic soy agar (TSA) plates at 37°C. The minimum inhibitory concentration (MIC) of chlorine for adapted (rugose and smooth) cells was determined to be 550 ppm and 500 ppm, respectively whereas the MIC for the control was 450 ppm. The biofilm forming ability of the adapted and control cells were examined on both plastic and stainless steel surface at room temperature and 37°C. The rugose variant, in contrast to the smooth (adapted and control) showed the ability to form strong biofilms (P ≤ 0.05) on a plastic surface at room temperature and 37°C. Rugose cells compared to smooth and control attached more (P ≤ 0.05) to steel surfaces as well. The possibility of cross-adaptation was examined by exposing the adapted and control cells to different antibiotics according to the Clinical & Laboratory Standards Institute guidelines. Adapted cells exhibited reduced susceptibility to some of the antibiotics tested as compared to control. The findings of this study suggest that exposure to sublethal chlorine concentration during the sanitization procedure can result in tolerant Salmonella cells. Chlorine may confer cross-protection that aids in the survival of the tolerant population to other environmental stresses.