Listeria spp. and Listeria monocytogenes in ham processing plants: occurrence and assessment of biofilm formation capacity
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Listeria monocytogenes can cause food borne Listeriosis, which is considered a serious public health risk. This pathogen has the ability to form biofilms on food-processing surfaces, potentially leading to food product contamination. The objective of this study was to assess the occurrence of Listeria spp. and Listeria monocytogenes in ham food processing plants during different production phases. And, furthermore, to examine the ability of the obtained L. monocytogenes strains to form biofilms under different circumstances. A Listeria spp. strain, a Coccus strain and 3 L. monocytogenes strains were grown as monocultures and binary cultures on polystyrene microtiter plates. All plates were made in duplicate and incubated at temperatures of 25°C and 37°C for 40 hours. Cell turbidity was measured at different time intervals to examine differences in growth rate. Biofilm formation capacity was indirectly quantified by staining with 1% crystal violet, and measuring the destaining solution absorbance. No correlation between cellular growth rate and biofilm formation was found. Both monocultures and binary cultures showed significantly higher biofilm formation capacity at a temperature of 25°. This is probably due to phenotypic modifications, which are influenced by temperature. Binary cultures consisting of L. monocytogenes and a Coccus strain did not enter a stationary phase of growth after 20 hours of incubation at 37°C. This suggests that the growth L. monocytogenes can be influenced by other non-Listeria strains. These results indicate that Listeria spp. and L. monocytogenes are present in ham processing plants and able to form biofilms. The biofilm formation capacity is influenced by culture composition and temperature. To increase the validity of these results, more plants should be sampled. For founded statements about a health risk regarding the obtained L. monocytogenes strains, the presence of virulence factors should be examined by multiplex PCR.