Potential Development of Resistance to Food Antimicrobials by Foodborne Pathogens

The potential for development of resistance of four strains of Listeria monocytogenes (101, 108, 310 and Scott A) and Salmonella Typhimurium DT 104 (2380, 2576, 2582, 2486) to food antimicrobials sodium benzoate (SB), potassium sorbate (PS), sodium diacetate (SD) and sodium lactate (SL) at pH 6.0 was studied. Cells were not subjected to any pre-stress condition. An agar dilution assay was performed to determine susceptibility and adaptation to different and increasing antimicrobial concentrations. A microbroth dilution assay was used to determine tolerance development after two consecutives exposures to same antimicrobial concentrations, and one further exposure to a step higher concentration. A growth curve was made with four strains (101, 108, 2380, 2486) to compare growth of adapted vs. non-adapted (parent) strains at same high concentrations in different medium environments. In the agar dilution assay, both microorganisms demonstrated increased resistance after growth in the presence of the food antimicrobials up to 0.5% PS, 4.0% SL and 1.0% SD. For SB, Listeria cells adapted up to 0.4% (except strain 108) and Salmonella cells to up to 0.5%. Using a microbroth dilution assay, all strains showed less susceptibility to low levels (0.1-0.3%) of PS and BS. Results indicated that 4.0% SL had essentially no effect on any cells growth. For all strains there was no change in absorbance (OD₆₃₀) at > 1.0% SD. Adapted strains grown in TSB with and without dextrose resulted in higher log CFU/ml compared to non-adapted strains when exposed to previous and higher antimicrobial concentrations. Both microorganisms showed potential for resistance to SL and to lower concentrations of BS and PS at certain pH.