Physiological response of Lactobacillus plantarum to salt and nonelectrolyte stress.

In this report, we compared the effects on the growth of Lactobacillus plantarum of raising the medium molarity by high concentrations of KCl or NaCl and iso-osmotic concentrations of nonionic compounds. Analysis of cellular extracts for organic constituents by nuclear magnetic resonance spectroscopy showed that salt-stressed cells do not contain detectable amounts of organic osmolytes, whereas sugar-stressed cells contain sugar (and some sugar-derived) compounds. The cytoplasmic concentrations of lactose and sucrose in growing cells are always similar to the concentrations in the medium. By using the activity of the glycine betaine transport system as a measure of hyperosmotic conditions, we show that, in contrast to KCl and NaCl, high concentrations of sugars (lactose or sucrose) impose only a transient osmotic stress because external and internal sugars equilibrate after some time. Analysis of lactose (and sucrose) uptake also indicates that the corresponding transport systems are neither significantly induced nor activated directly by hyperosmotic conditions. The systems operate by facilitated diffusion and have very high apparent affinity constants for transport (>50 mM for lactose), which explains why low sugar concentrations do not protect against hyperosmotic conditions. We conclude that the more severe growth inhibition by salt stress than by equiosmolal concentrations of sugars reflects the inability of the cells to accumulate K+ (or Na+) to levels high enough to restore turgor as well as deleterious effects of the electrolytes intracellularly.

Study of the Citrate Metabolism of Lactococcus lactis subsp. lactis Biovar Diacetylactis by Means of C Nuclear Magnetic Resonance.

The metabolic fate of citrate and pyruvate in four strains of Lactococcus lactis subsp. lactis biovar diacetylactis has been studied by means of C nuclear magnetic resonance, using as a substrate either [3-C]pyruvic acid or custom-synthesized citric acid that is C labeled either at carbons 2 and 4 or at carbon 3. The fermentations were carried out batchwise in modified M17 broth. For the actual conversions of the C-labeled substrates, cells at the end of their logarithmic growth phase were used to minimize the conversion to lactic acid. A mass balance of the main citric acid metabolites was obtained; the four strains produced from 50 to 70% (on a molar basis) lactic acid from either citrate or pyruvate. The remaining 50 to 30% was converted mainly to either alpha-acetolactic acid (for one strain) or acetoin (for the other three strains). One of the strains produced an exceptionally high concentration of the diacetyl precursor alpha-acetolactic acid. Another strain (SDC6) also produced alpha-acetolactic acid, but this was decarboxylated to acetoin at a high rate. The C nuclear magnetic resonance method confirmed that the biosynthesis of alpha-acetolactic acid occurs via condensation of pyruvate and "active" acetaldehyde. Diacetyl was not found as a direct metabolite of citrate or pyruvate metabolism.