Influence of long-term survival on resistance of Salmonella enterica serovar typhimurium and Escherichia coli O157:H7 to physical or chemical food processes

Abstract

<p>Under adverse conditions in the natural environment or in food processing facilities bacteria may enter a long-term-survival (LTS) phase and maintain a stable concentration of viable cells for months or years. In this LTS phase, pathogens may become resistant to antimicrobial processes that would typically inactivate them in the exponential- or stationary phase of their life cycle. In this regard, the resistance of LTS phase Salmonella enterica serovar Typhimurium, Salmonella PT 30 and Escherichia coli O157:H7 to ultraviolet (UV) radiation, desiccation or cinnamaldehyde in model systems and in food products was investigated. Exponential-, stationary, and LTS phase cells were cultured in tryptic soy broth supplemented with 0.6% yeast extract (TSBYE; 35°C) for 4 h, 24 h and 14 days, respectively. In the UV resistance experiment, cells from each physiological state were exposed to UV light in saline (80 µW/cm2) and apple juice (820 µW/cm2). The LTS S. Typhimurium consistently exhibited the highest UV resistance. In both saline and apple juice, D-values of LTS S. Typhimurium were significantly higher than D-values of the other two physiological states of cells. The LTS cells also had the least sub-lethal injury in the surviving population. Both S. Typhimurium and S. PT30 in the LTS phase were the most resistant to desiccation on paper discs and on raw almonds, and to hot air drying of almonds at 100°C. The LTS E. coli O157:H7 exhibited the highest resistance to cinnamaldehyde in carrot juice and apple juice compared to stationary phase cells. These results indicate that the LTS state cross-protects S. enterica and E. coli O157:H7 against UV radiation, desiccation and cinnamaldehyde. Additionally, we demonstrated that LTS cells are highly tolerant to antibiotics compared to stationary phase cells and that inhibition of protein synthesis results in the death of those cells.</p>