Comparison of heat stress responses of immobilized and planktonic Salmonella enterica serovar Typhimurium.

Salmonellae are often present as immobilized cells in food products in the form of micro-colonies. Despite this, most research into Salmonella physiology has been performed with bacteria grown as planktonic cultures. In the current study, we compared the transcriptome of planktonic and immobilized Salmonella enterica subsp. enterica serovar Typhimurium (S. Typhimurium) subjected to 30 min of heat stress at 45 °C. The expression of 538 genes was differently regulated between the two conditions after 30 min. Heat stress of an immobilized culture induced expression of flagella and virulence genes compared to the non-heat stressed immobilized bacteria. Immobilized heat stressed S. Typhimurium was more invasive in HeLa cells than the non-heat stressed controls, whereas the heat stress caused planktonic bacteria to be less invasive. The decrease in invasion of heat stressed planktonic cultures returned to non-heat stressed levels 30 min post treatment, whereas the increased invasion of HeLa cells of the heat stressed immobilized cultures still remained 30 min after the heat stress was terminated. This study shows that immobilized bacteria respond to heat stress differently than planktonic bacteria.

A third mode of surface-associated growth: immobilization of Salmonella enterica serovar Typhimurium modulates the RpoS-directed transcriptional programme.

Although the growth of bacteria has been studied for more than a century, it is only in recent decades that surface-associated growth has received attention. In addition to the well-characterized biofilm and swarming lifestyles, bacteria can also develop as micro-colonies supported by structured environments in both food products and the GI tract. This immobilized mode of growth has not been widely studied. To develop our understanding of the effects of immobilization upon a food-borne bacterial pathogen, we used the IFR Gel Cassette model. The transcriptional programme and metabolomic profile of Salmonella enterica serovar Typhimurium ST4/74 were compared during planktonic and immobilized growth, and a number of immobilization-specific characteristics were identified. Immobilized S.Typhimurium did not express motility and chemotaxis genes, and electron microscopy revealed the absence of flagella. The expression of RpoS-dependent genes and the level of RpoS protein were increased in immobilized bacteria, compared with planktonic growth. Immobilized growth prevented the induction of SPI1, SPI4 and SPI5 gene expression, likely mediated by the FliZ transcriptional regulator. Using an epithelial cell-based assay, we showed that immobilized S.Typhimurium was significantly less invasive than planktonic bacteria, and we suggest that S.Typhimurium grown in immobilized environments are less virulent than planktonic bacteria. Our findings identify immobilization as a third type of surface-associated growth that is distinct from the biofilm and swarming lifestyles of Salmonella.