Duration of Heat Stress Effect on Invasiveness ofL. monocytogenesStrains

Abstract

During food production and food conservation, as well as the passage through the human gastrointestinal (GI) tract, L. monocytogenes is exposed to many adverse conditions which may elicit a stress response. As a result the pathogen may become more resistant to other unpropitious factors and may change its virulence. It has been shown that low and high temperature, salt, low pH, and high pressure affect the invasion capacity of L. monocytogenes. However, there is a scarcity of data on the duration of the stress effect on bacterial biology, including invasiveness. The aim of this work was to determine the period during which L. monocytogenes invasiveness remains altered under optimal conditions following exposure of bacteria to mild heat shock stress. Ten L. monocytogenes strains were exposed to heat shock at 54°C for 20 minutes. Then both heat-treated and nontreated control bacteria were incubated under optimal growth conditions, 37°C, for up to 72 hours and the invasion capacity was tested. Additionally, the expression of virulence and stress response genes was investigated in 2 strains. We found that heat stress exposure significantly decreases the invasiveness of all tested strains. However, during incubation at 37°C the invasion capacity of heat-treated strains recovered to the level of nontreated controls. The observed effect was strain-dependent and lasted from less than 24 hours to 72 hours. The invasiveness of 6 out of the 10 nontreated strains decreased during incubation at 37°C. The expression of inlAB correlated with the increase of invasiveness but the decrease of invasiveness did not correlate with changes of the level of these transcripts. Conclusions. The effect of heat stress on L. monocytogenes invasiveness is strain-dependent and was transient, lasting up to 72 hours.

Effect of Temperatures Used in Food Storage on Duration of Heat Stress Induced Invasiveness of L. monocytogenes

“…The heat exposure significantly affected the survival and the invasiveness of bacteria. This supports our earlier study [17]. In the present study, the invasiveness of bacteria changed along with the incubation time and was temperature-dependent.…”

“…This is the average temperature which the pathogen may experience during reheating of food products or in undercooked food [22]. This temperature allows the reduction of bacteria number, but the number of survived bacteria is still high enough to determine changes of the invasiveness in response to stress [17].…”

“…The invasion ability of non-treated strains significantly decreased during a storage at 20 °C, whereas at 5 °C they remained at the similar level. Our earlier study showed that in 60% of non-treated strains invasiveness was reduced along with the incubation at 37 °C [17]. The observed reduction of invasiveness in the non-treated strains might be due to gradual nutrients and oxygen depletion which is a stress factor for bacteria (Rees et al, 1995) [31].…”

“…Nonetheless, the effect of stress on bacterial virulence is presumably transient. Our recent study has revealed that the heat-induced invasiveness change lasts on average 32 h in L. monocytogenes strains when bacteria are kept at optimal growth temperature (37 °C) after stress exposure [17]. There is practically no data on the duration of stress-induced alterations of L. monocytogenes invasiveness at temperatures used in the food storage.…”

Ascorbic Acid Changes Growth of Food-Borne Pathogens in the Early Stage of Biofilm Formation

“…In the food industry, however, room temperature (25 • C) and lower are applied. L. monocytogenes is the bacterium well-known for its ability to adapt to wide range of temperatures [9,10] and form a biofilm even at temperatures as low as 0 • C [11]. The ability of S. aureus and E. coli to colonize the surfaces and form a biofilm at low temperatures on various surfaces also may contribute to bacterial survival in the food industry environment, increasing the risk of food cross-contamination [12].…”