Physiological adaptation of Escherichia coli after transfer onto refrigerated ground meat and other solid matrices: A molecular approach

Bacteria on meat are subjected to specific living conditions that differ drastically from typical laboratory procedures in synthetic media. This study was undertaken to determine the behavior of bacteria when transferred from a rich-liquid medium to solid matrices, as is the case during microbial process validation. Escherichia coli cultured in Brain–Heart Infusion (BHI) broth to different growth phases were inoculated in ground beef (GB) and stored at 5 °C for 12 days or spread onto BHI agar and cooked meat medium (CMM), and incubated at 37 °C for several hours. We monitored cell densities and the expression of σ factors and genes under their control over time. The initial growth phase of the inoculum influenced growth resumption after transfer onto BHI agar and CMM. Whatever the solid matrix, bacteria adapted to their new environment and did not perceive stress immediately after inoculation. During this period, the σE and σH regulons were not activated and rpoD mRNA levels adjusted quickly. The rpoS and gadA mRNA levels did not increase after inoculation on solid surfaces and displayed normal growth-dependent modifications. After transfer onto GB, dnaK and groEL gene expression was affected more by the low temperature than by the composition of a meat environment.

► Bacteria in a biofilm behave similarly to stationary phase cells with respect to σS activity.
► The physiological behavior of bacteria growing on meat has not yet been elucidated at a molecular level.
► The expression of σ factors was used to follow stress and physiological changes of bacteria growing on meat.
► Planktonic bacteria did not perceive stress on transfer to solid matrices.
► Cells inoculated in meat were more affected by low temperature than the meat environment.