Evaluation of growtho growth interface of Listeria monocytogenes growing on stainless steel surfaces, detached from biofilms or in suspension, in response to pH and NaCl.

摘要

The present study aimed to describe the growtho growth interface of Listeria monocytogenes at three potential states of growth in industrial environments, namely attached, (Att), detached (Det) from a biofilm, or in a planktonic state (suspended; Plan). A 3-strain composite of L. monocytogenes cells was left to colonize stainless steel (SS) surfaces in tryptic soy broth supplemented with 0.6% yeast extract (TSBYE) at 20 degrees C for 72h and then transferred to TSBYE at 30 different pH and NaCl concentrations, which were renewed every two days during incubation at 10 degrees C. Survival of attached population was observed at optimal conditions (pH 7.2, a_w 0.996), whereas at 4.5-8.0% salt and/or pH <6.0, reduction of attached population on SS surfaces was observed. PFGE patterns showed that 91% of the cells colonizing the SS coupons after 30days, at any pH and a_w conditions, belonged to a single strain. Furthermore, the change in the probability of a single cell to initiate growth (P_in) over time, as well as the number of cells needed (CN) for growth initiation of planktonically growing Plan and Det L. monocytogenes cells were evaluated based on MPN Tables. An ordinary logistic regression model was also used to describe the growtho growth interface of varying inoculation levels (from <10 to 104CFU/ml) of Plan and Det cells in response to pH and a_w. Although both cell types demonstrated similar growth limits at populations of 102-104CFU/ml, higher numbers of Det than Plan cells were needed (CN) in order to initiate growth at low a_w and pH. Individual Plan cells reached higher maximum levels of probability of growth initiation (P_max) and had shorter times to reach P_max/2 (t_au), compared to their Det counterparts. Data on growth potential of cells in suspension, attached or detached status, may assist in ranking the risk from different sources of contamination. In addition, they may establish the link between the behavior of L. monocytogenes in foods and its origin from the processing plant. The latter link is important component of biotraceability.