Physiological state of bacteria growing in meat during cold storage: a molecular approach

摘要

Bacteria growing on meat are subjected to specific life conditions (nutrient, pH, salt, temperature) which differ drastically from typical laboratory procedures on synthetic media, so physiological and molecular adjustments are expected. This study was conducted to determine the behaviour of bacteria when transferred from a rich-liquid medium to a solid food matrix as it is the case during microbial process validation. Planktonic Escherichia coli cultured in Brain Heart Infusion (BHI) to exponential and stationary growth phases were inoculated onto aseptically prepared ground beef meat and stored at 5°C for 12 days. No growth was observed during storage and time-course expression of sigma factors (encoded by rpoD, E, H, S) and of genes under their transcriptional control was established by RT-PCR. Overall, gene expression profiles were more stable in stationary phase bacteria. After transfer onto refrigerated meat, rpoD mRNA levels increased significantly in exponential phase bacteria (P = 0.0005) but not in stationary phase ones (P = 0.1). The rpoS mRNA levels were not significantly changed whatever the physiological state of the inoculum (P > 0.1) and a decrease of gadA transcription, controlled by σ~s, were only observed after 12 days of cold storage in exponential phase bacteria (P = 0.03). Surprisingly, activation of σ~H was only observed after more than 1 day of storage. Although the sigma factors expression profiles indicate that bacteria quickly adapt, cells do not seem to perceive a real stress after transfer from a rich medium to the cold meat environment.