Evaluation of growth/no growth interface of Listeria monocytogenes growing on stainless steel surfaces, detached from biofilms or in suspension, in response to pH and NaCl

The present study aimed to describe the growth/no 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 °C for 72 h and then transferred to TSBYE at 30 different pH and NaCl concentrations, which were renewed every two days during incubation at 10 °C. Survival of attached population was observed at optimal conditions (pH 7.2, aw 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 30 days, at any pH and aw conditions, belonged to a single strain. Furthermore, the change in the probability of a single cell to initiate growth (Pin) 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 growth/no growth interface of varying inoculation levels (from < 10 to 104 CFU/ml) of Plan and Det cells in response to pH and aw. Although both cell types demonstrated similar growth limits at populations of 102–104 CFU/ml, higher numbers of Det than Plan cells were needed (CN) in order to initiate growth at low aw and pH. Individual Plan cells reached higher maximum levels of probability of growth initiation (Pmax) and had shorter times to reach Pmax / 2 (tau), 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.

Research Highlights► Attached cells of L. monocytogenes require conditions closer to the optimal aw and pH levels for multiplication than planktonically grown cells. ► Strains from dairy factory seem to dominate attached populations on surfaces. ► Initial population levels on surfaces or in a growth media markedly affect the subsequent growth initiation in response to pH and aw. ► Planktonically growing cells show higher probability of growth initiation and require lower number of cells to initiate growth than attached cells.