Effect of starvation on expression of the ribosomal RNA rrnB P2 promoter during the lag phase of Pseudomonas fluorescens

Mathematical modelling of food-borne pathogen survival and growth is an important and expanding area of food microbiology. Effective models have been developed for growth rate as influenced by the environment; however, reliable models which describe the lag phase prior to exponential growth are more difficult to obtain. In order to improve our understanding of the physiological changes that take place in the microbial cell during this adaptation period, the effect of starvation on the expression of a gene for ribosomal RNA (rRNA) synthesis-an important step in preparing the cells for growth-was examined. A strain of Pseudomonas fluorescens containing the Tn7-luxCDABE gene cassette regulated by the rRNA promoter rrnB P2 was used as a model system. Growth was measured as optical density at 600 nm (OD600), and fitting was achieved with a two-phase linear model to obtain the parameters growth rate (ROD) and lag phase duration (LPDOD). The increase in bioluminescence (measured as natural log [ln] relative light units per unit OD600) after inoculation of stationary phase cells into fresh tryptic soy broth (TSB) followed an exponential association model, with lag (LPDExp) and rate (RExp) parameters. Starvation of cells in either spent TSB or in MOPS buffer resulted in time-dependent linear increases in both lag parameters and, in the case of TSB, a decrease in the RExp parameter. The results show that models can be developed for expression of genes during the lag phase, which will improve our ability to make accurate predictions of food-borne pathogen growth. Crown Copyright © 2006.