The role of L-carnitine and glycine betaine in the survival and sub-lethalinjury of non-growing Listeria monocytogenes cells during chilled storage

Aims: To determine the role played by previous grow th in the presence of o smolytes on the subsequent survival and sub-lethal injury of L. monocytogen es during long-term chilled storage in a model buffer system. Methods and Results: Four Listeria monocytogenes strains were grown separat ely to stationary phase in Listeria minimal medium (DM) alone or in DM with 4% NaCl alone, or both these media supplemented with 1 mM L-carnitine and/ or 1 mM glycine betaine. Cells were resuspended in phosphate buffered salin e (pH 5.5) and stored for four weeks at 4 degreesC. Initially, and at weekl y intervals, samples were plated on both Tryptic Soy Agar and Tryptic Soy A gar with 4% NaCl to determine total numbers and degree of sub-lethal injury in the populations. The numbers of cells within all strains after growth t o stationary phase, except one which increased (similar to2 log cfu ml(-1), P < 0.05) in the presence of NaCl, were not influenced significantly by pr evious growth conditions (P > 0.05). During subsequent chilled storage, how ever, numbers of all strains grown in the presence of NaCl remained constan t while those grown in its absence decreased. The rate and magnitude of the decrease in cell numbers was strain dependent. The initial percentage of s ub-lethal injury increased significantly in all strains when grown previous ly in the presence of L-carnitine (P < 0.05). During subsequent chilled sto rage sub-lethal injury increased for all strains in a manner that was strai n dependent, but not related to the previous growth conditions. Conclusions: Previous growth in the presence of osmolytes of NaCl, but not osmolytes alone, increases the subsequent survival, but not percentage sub- lethal injury, of L. monocytogenes during subsequent chilled storage in buf fer. Significance and Impact of the Study: This study shows that risks associate d with L. monocytogenes in chilled food may be influenced by the individual life histories of the cells.