Comparison of food-associated Listeria monocytogenes growth phenotypes across sporadic, transient, and persistent strains

This work focuses on the persistence of Listeria monocytogenes strains in food production facilities and their growth phenotypes. Therefore, 26 strains of L. monocytogenes were selected from a food production facility that manufactures ready-to-eat products, which are prone to contamination with Listeria. This is a serious issue, as ingestion and subsequent infection with L. monocytogenes can lead to listeriosis, which is an infectious disease that can cause a variety of health problems. To ensure that the used samples do in fact contain L. monocytogenes, all samples were tested using selective RAPID'L.mono agar and qPCR according to Rossmanith et al. One sample showed contradictory results, with a negative RAPID'L.mono agar and positive qPCR for L. monocytogenes. This strain was further investigated using IAP PCR according to Bubert et al., selective ALOA agar and the AquaSpark™ chemiluminescence indicating enzyme activity. The results showed that this strain was indeed found to be a L. monocytogenes strain with reduced phospholipase enzyme activity. For the main experiment, the selected strains were divided into three categories: ‘persistent’, ‘non-persistent’ and ‘transient’, and growth curves were created at enrichment temperature (37°C) and processing temperature (4°C) in TSB-Y culture medium. The website dashing-growth-curves.ethz.ch was first used to analyze the growth curves for both temperatures, but no usable results could be obtained for the 4°C growth curves because the calculation models did not fit the growth curve well. Therefore, the 4°C growth curves were analyzed and compared graphically using Excel, while the 37°C growth curves were analyzed using the Gompertz tight model and manually on the Dashing Growth Curves website. All strains showed good growth under both enrichment and processing conditions, but no major difference in growth between ‘persistent’, ‘non-persistent’ and ‘transient’ strains could be observed under both temperatures.