The influence of pH on heat stress response by probiotic Lactobacillus plantarum LP-Onlly

Abstract

The influence of pH on the heat stress response of a human-derived probiotic strain of Lactobacillus plantarum, LP-Onlly, was investigated. The heat resistance of mid-log- and stationary-phase cells that were heat-adapted (45°C) in cultures that were or were not pH adjusted (pH 4.3–7.0) was evaluated at a lethal temperature of 55°C. The results demonstrated that the heat resistance of heat-adapted stationary-phase cells increased sharply as the pH for heat adaptation in cultures was adjusted from pH 4.3 (non-adjusted pH) to pH 7.0 step by step (pH 5.0, 5.5, 6.0, 6.5, and 7.0); however, the mid-log-phase cells did not show the similar pH-related trend. The optimal adjusted pH for heat adaptation in cultures was found to be 6.5, with stationary-phase cells that were heat-adapted at pH 6.5 exhibiting a 3.4-log-cycle increase in heat resistance and 6-fold greater storage stability than controls. Real-time reverse transcription PCR analysis of five major heat shock protein genes (dnak, dnaJ, grpE, groES, and groEL) revealed that the heat-inductive expression of these genes after heat adaptation was up-regulated in stationary-phase cells when the pH in cultures for heat adaptation was adjusted from 4.3 (non-adjusted pH) to 6.5. Nevertheless, the transcription levels of these genes after heat adaptation was down-regulated in mid-log-phase cells when the pH in cultures for heat adaptation was shifted from 6.0 (non-adjusted pH) to 6.5.