The adaptation of Listeria monocytogenes to osmotic stress
The food-borne pathogen Listeria monocytogenes is more salt tolerant in the complex medium brain heart infusion (BHI, 2.0M NaCI upper limit for growth) than in a chemically-defined medium (DM, 1.0M NaCI upper limit for growth). The components in BHI responsible for the characteristic salt tolerance of L. monocytogenes are peptone and glycine betaine. At high osmolarity, the growth stimulation by peptone was higher than expected from nutritional supplementation, indicating that an osmoprotective mechanism was also at play. Peptone provided a higher level of osmotic protection than the compatible solute glycine betaine which was a moderate osmoprotectant. Our growth data demonstrated that of the free amino acids and peptides contained in peptone it is the peptides which are the osmoprotectants for L monocytogenes. Furthermore, specific peptides, such as PGG (prolyl-glycyl- glycine) and PHP (prolyl-hydroxyproline), behaved in growth experiments as the compatible solute glycine betaine, i.e. stimulation of growth at high osmolarity and no effect at low osmolarity. Our analysis of the changes in the intracellular pools of amino acids, under conditions of sosmotic stress, when peptone or specific peptide are supplied to the growth medium, has shown the following features in the mechanism of adaptation of L. monocytogenes to osmotic stress: i) Peptides are taken up, by at least two specific transport systems. ii) Subsequently, peptides are hydrolysed intracellularly by peptidases. iii) As a consequence of ii), a significant increase in the pool of free amino acids occurs. Osmoadaptation in L. monocytogenes iv) We have also demonstrated that depending on the nature of the constituent amino acids, some peptides are not fully hydrolysed which leads to the accumulation of an intracellular peptide pool in L. monocytogenes. v) Proline, glycine and hydroxyproline are the amino acids preferentially accumulated as free amino acids or as part of peptides. vi) The intracellular accumulation of free amino acids and peptides is positively correlated to an increase in the external osmolarity and has an important role in the osmoadaptation of L. monocytogenes.