Impact of Sodium Reduction on the Microbiota of Ready-to-Eat Meats

Abstract

The growth of Listeria monocytogenes and spoilage microorganisms is a concern in sodium-reduced ready-to-eat (RTE) meats. To assess the microbial risk of sodium-reduced RTE meats, the microbiota of retail RTE meats was profiled and the growth of L. monocytogenes and an autochthonous microbiota was examined under experimental conditions relevant to sodium reduction. To determine the effect of sodium reduction on the growth of L. monocytogenes, ham was manufactured with different concentrations of sodium chloride. Results confirmed that L. monocytogenes had shorter lag phase on ham formulated with less NaCl when no autochthonous microbiota was present. The autochthonous microbiota effectively controlled the growth of L. monocytogenes through competition regardless of the concentration of sodium chloride. The antilisterial effect of carnocyclin A (CCLA) was examined. Global gene expression, protein production and morphological changes were profiled in L. monocytogenes exposed to a sublethal dose of CCLA in a low sodium environment. CCLA exposure resulted in down-regulation of genes involved in cell division, cell wall synthesis and motility in L. monocytogenes after 4 h and 30 h of exposure. Absence of flagella from the cell surface was observed. The filamentation of L. monocytogenes was examined following exposure of cells to stress including acid, salt and antimicrobial peptides. Listeria formed filaments in low concentrations of acids and salts but not with purified CCLA. L. monocytogenes was able to form filaments both in vitro and in situ. Down-regulation of cell division gene ftsX could be involved in filamentation. The effect of a mixture of sodium lactate and sodium diacetate (NaL/DA) on gene expression in L. monocytogenes was examined. Global gene expression was examined after Listeria cells were exposed to NaL/DA in broth. The genes encoding glycolytic enzymes were down-regulated. The DNA repair gene, cell division gene and cell structure synthesis genes were up-regulated. The addition of NaL/DA inhibited the growth of Listeria on low sodium ham products. In conclusion, this work demonstrated that the use of preservatives (CCLA and NaL/DA) and the presence of an autochthonous microbiota can effectively inhibit the growth and down-regulate the expression of genes essential for cellular functions in L. monocytogenes.