Osmoprotection of Escherichia call by peptone is mediated by the uptake and accumulation of free proline but not of proline-containing peptides

The effect of meat peptone type I (Sigma) on the growth of Escherichia coli cells under hyperosmotic stress has been investigated. Peptone is a comple x mixture of peptides with a small content of free amino acids, which resem bles nutrients found in natural environments. Our data showed that peptone enhances the growth off. coli cells in high-osmolarity medium to Levels hig her than those achieved with the main compatible solute In bacteria, glycin e betaine. The mechanism of osmoprotection by peptone comprises the uptake and accumulation of the compatible solute, proline. The main role of the pe ptides contained in peptone is the provision of nutrients rather than the i ntracellular accumulation of osmolytes. In contrast to Listeria monocytogen es (M. R. Amezaga, I. Davidson, D. McLaggan, A. Verheul, T. Abee, and I. R. Booth, Microbiology 141:41-19, 1995), E. coli does not accumulate exogenou s peptides for osmoprotection and peptides containing proline do not lead t o the accumulation of proline as a compatible solute. In late-logarithmic-p hase cultures of E. coli growing at high osmolarity plus peptone, proline b ecomes the limiting factor for growth, and the intracellular pools of proli ne are not maintained. This is a consequence of the low concentration of fr ee proline in peptone, the catabolism of proline by E. coli, and the inabil ity of E. coli to utilize proline-containing peptides as a source of compat ible solutes. Our data highlight the role that natural components in food s uch as peptides play in undermining food preservation regimes, such as high osmolarity, and also that the specific mechanisms of osmoprotection by the se compounds differ according to the organism,