Relationship of Growth Conditions to Desiccation Tolerance of Salmonella enterica, Escherichia coli, and Listeria monocytogenes.

ABSTRACT

Growth on solid media as sessile cells is believed to increase the desiccation tolerance of Salmonella enterica. However, the reasons behind increased resistance have not been well explored. In addition, the same effect has not been examined for other foodborne pathogens such as pathogenic Escherichia coli or Listeria monocytogenes. The purpose of this research was twofold: first, to determine the role of oxygenation during growth on the desiccation resistance of S. enterica, E. coli, and L. monocytogenes, and second, to determine the effect of sessile versus planktonic growth on the desiccation resistance of these pathogens. Three different serotypes each of Salmonella, E. coli, and L. monocytogenes were cultured in Trypticase soy broth with 0.6% yeast extract (TSBYE), with (aerobic) shaking or on TSBYE with agar under either aerobic or anaerobic conditions and harvested in the stationary phase. After adding cell suspensions to cellulose filter disks, pathogen survival was determined by enumeration before drying (0 h) and after drying for 24 h. Results showed statistical differences in harvested initial populations before drying. For Salmonella, a correlation was found between high initial population and greater survival on desiccation (P = 0.05). In addition, statistical differences (P ≤ 0.05) between survival based on growth type were identified. However, differences found were not the same for the three pathogens, or between their serotypes. In general, Salmonella and E. coli desiccation resistance followed the pattern of aerobic agar media ≥ liquid media ≥ anaerobic agar media. For L. monocytogenes serotypes, resistance to desiccation was not statistically different based on mode of growth. These results indicate growth on solid media under aerobic conditions is not always necessary for optimal desiccation survival, but may be beneficial when the desiccation resistance of the test serotype is unknown.