Effect of sub-lethal heating and growth temperature on expression of the ribosomal RNA rrnB P2 promoter during the lag phase of Pseudomonas fluorescens

Current models for the lag phase of food-borne pathogens are limited by our poor understanding of the physiological changes taking place as bacterial cells prepare for exponential growth. In a previous paper in this series, a strain of Pseudomonas fluorescens containing the Tn7-luxCDABE gene cassette regulated by the rRNA promoter rrnB P2 was used to measure the influence of starvation on the lag phase duration (LPDOD) and growth rate (ROD). rrnB P2 promoter activity increased exponentially during the lag phase, and was characterized by lag (LPDExp) and rate (RExp) parameters. In the present study, this work was expanded to include the influence of growth temperature (10 to 30 °C) and exposure to sub-lethal heating at 47 °C. With these additional datasets, the LPDExp was often more pronounced than had been noted with starvation, so the original exponential association model (EXP) was compared to logistic and Gompertz (GOM) models. Based on root mean square error, the GOM model gave the better fit for some of the sub-lethal heating and growth temperature datasets; however, the EXP model was assessed as best overall. Increased growth temperature and decreased time of sub-lethal heating produced significant decreases in LPDOD and LPDExp and increases in ROD and RExp. The results suggest that different stressors have differential effects on gene expression and subsequent growth.