Listeria innocua and Listeria monocytogenes strains from dairy plants behave similarly in biofilm sanitizer testing

Five L. innocua and five L. monocytogenes, including persistent and non-persistent isolates collected from Gorgonzola processing plants, were compared regarding their biofilm-forming ability and their biofilm susceptibility to two hydrogen peroxide (HP) based disinfectants in use at the plants. No significant difference in biofilm-forming ability by both species was observed (P>0.05) in crystal violet staining and viable count assays. The susceptibility to HP disinfectants of the L. monocytogenes and L. innocua biofilms was determined. In order to mimic clean and soiled biofilm forming conditions, biofilms were grown, respectively, in 1/10 diluted TSB-YE and in TSB-YE. The results showed no significant differences between species or conditions (P>0.05) regardless of whether the isolates were classified as persistent or non-persistenttb. A hierarchical clustering based on Principal Component Analysis performed on the tested variables, indicated the presence of two major clusters. Persistent and non-persistent isolates from both species were allocated in both clusters, suggesting that they behaved in a similar way in response to the tested conditions. This study showed that biofilms of in-house L. innocua could monitor the effectiveness of HP-based disinfectants. Moreover, biofilms of L. innocua could be used as surrogates of L. monocytogenes in sanitizer-based biofilm eradication trials simulating dairy processing environments, whenever the use of the pathogen is not an option.