Comparison of Recovery Methods for Freeze-Injured Listeria monocytogenes, Salmonella Typhimurium, and Campylobacter coli in Cell Suspensions and Associated with Pork Surfaces

Abstract

Cells injured as a result of freezing, heating, and acidification treatments may not grow during conventional microbiological procedures owing to the presence of selective agents, compounds, or dyes in the media, impairing the cell's ability to repair itself and grow. Injured cells can be recovered by combining selective and nonselective media into a single system. With such combinations, the diffusion of the selective compounds or dyes is controlled, allowing for the resuscitation of injured cells of interest while also inhibiting the growth of undesirable background microflora. In this study, Listeria monocytogenes, Salmonella Typhimurium, and Campylobacter coli suspended in buffer or associated with pork surfaces were subjected to a freeze-thaw cycle (−15°C for 24 h, 4°C for 4 h). Following treatments, freeze-injured cells were plated on appropriate media for the overlay (OV), thin agar layer (TAL), and Lutri plate (LP) recovery methods. The levels of L. monocytogenes and Salmonella Typhimurium recovered from cell suspensions and pork surfaces by the TAL, OV, and LP methods following freeze treatments were not statistically different (P > 0.05) from recovery levels associated with nonselective media. Conversely, levels of pathogens on selective media were significantly reduced compared with those for the other methods employed. The TAL method's recovery of C. coli was not significantly different from that achieved with the nonselective media. Overall, the results presented in this study demonstrate that the TAL method not only was easier to perform, but also allowed improved isolation of single colonies for further characterization. This study may provide researchers with better methods to determine the effectiveness of industry-employed chilling processes in reducing pathogenic bacteria associated with red meat surfaces.