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

Abstract

To determine the role played by previous growth in the presence of osmolytes on the subsequent survival and sub-lethal injury of L. monocytogenes during long-term chilled storage in a model buffer system. Four Listeria monocytogenes strains were grown separately 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 saline (pH 5.5) and stored for four weeks at 4 degrees C. Initially, and at weekly intervals, samples were plated on both Tryptic Soy Agar and Tryptic Soy Agar 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 to stationary phase, except one which increased ( approximately 2 log cfu ml-1, P < 0.05) in the presence of NaCl, were not influenced significantly by previous growth conditions (P > 0.05). During subsequent chilled storage, however, numbers of all strains grown in the presence of NaCl remained constant while those grown in its absence decreased. The rate and magnitude of the decrease in cell numbers was strain dependent. The initial percentage of sub-lethal injury increased significantly in all strains when grown previously in the presence of L-carnitine (P < 0.05). During subsequent chilled storage sub-lethal injury increased for all strains in a manner that was strain dependent, but not related to the previous growth conditions. 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 buffer. This study shows that risks associated with L. monocytogenes in chilled food may be influenced by the individual life histories of the cells.