Homologous stress adaptation, antibiotic resistance, and biofilm forming ability of Salmonella enterica serovar Heidelberg (ATCC8326) on different food-contact surfaces following exposure to sub-lethal chlorine concentrations

Authors: OBE, T - Mississippi State University; NANNAPANENI, R - Mississippi State University; SHARMA, C - Mississippi State University; KIESS, A - Mississippi State University

Submitted to: Poultry Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/8/2017
Publication Date: 3/1/2018
Citation: Obe, T., Nannapaneni, R., Sharma, C.S., Kiess, A. 2018. Homologous stress adaptation, antibiotic resistance, and biofilm forming ability of Salmonella enterica serovar Heidelberg (ATCC8326) on different food-contact surfaces following exposure to sub-lethal chlorine concentrations. Poultry Science. 97(3):951-961.

Interpretive Summary: Salmonella enterica is well known as the leading cause of foodborne infections globally and its antibiotic resistance has continued to be a global public health issue. Several studies have documented the ability of bacteria to adapt to stress posed by antimicrobials, which they use to induce cross-protection against other stressful conditions. Since chlorine is the antimicrobial agent that is most common in sanitizing solutions, adaptation to chlorine could constitute a potential threat to food safety by inducing cross-protection to antibiotics that are clinically important. In the present study, the homologous stress adaptation of Salmonella Heidelberg exposed to chlorine at a sub-lethal concentration was determined by observing changes in minimum inhibitory concentration (MIC). The biofilm formation of the adapted cells compared to non-adapted cells on different food-contact surfaces was also measured. Antibiotic susceptibility patterns and acid resistance of adapted cells were further investigated. Our findings support the initial hypothesis that Salmonella Heidelberg would adapt to sub-lethal concentrations of chlorine. The adapted cells were better biofilm formers on both food-contact surfaces tested. Also, Salmonella Heidelberg cells that were adapted to sub-lethal concentrations of chlorine exhibited a certain degree of reduced susceptibility to some of the antibiotics tested.

Technical Abstract: 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 (CLSI) guidelines and low pH. Adapted cells exhibited reduced susceptibility to some of the antibiotics tested as compared to control. There was no difference in the acidic tolerance of the adapted and control cells. The findings of this study suggest that exposure to sub-lethal 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.