The Effects of Environmental Stress on the Antibiotic Resistance Phenotype of Salmonella Typhimurium

Bacteria work to reduce the effects of sub-lethal stress by making phenotypic and genotypic changes. Phenotypic changes may provide resistance to further stress events and may also lead to cross-protection against a number of environmental challenges including resistance to antibiotics which bacteria see as a form of environmental stress. The effects of stress (temperature, pH, osmotic effects and starvation) on the antibiotic resistance phenotypes of S. Typhimurium DT104 were investigated in vitro. The antibiotics; tetracycline (T), streptomycin (S) and trimethoprim and sulfamethoxazole (SXT) were selected as resistance to T, S and SXT has been frequently observed in Salmonella isolated from farm animals in Northern Ireland. The investigation was carried out using standard antibiotic disk diffusion methodologies adapted to incorporate a stress inducing step. In terms of outcomes, starvation stress had the most effect on the susceptibility of Salmonella to the three antibiotics tested. Of the three antibiotics tested, Streptomycin was the least effective under starvation, osmotic and low temperature stresses. These results indicate that some types of sub-lethal stress may contribute to the expression of antibiotic resistance in Salmonella.