In dairy processing environments, many bacterial species adhere and form biofilms on surfaces and equipment, leading to foodborne illness and food spoilage. Among them, Listeria monocytogenes and Pseudomonas spp. could be present in mixed-species biofilms with increased resistance to disinfectants and antimicrobials. This study aimed to evaluate the interactions between L. monocytogenes and P. fluorescens in dual-species biofilms simulating dairy processing conditions, as well as the capability of P. fluorescens in co-culture to produce the blue pigment in a cheese model system. The biofilm-forming capability of single- and mixed-cultures were evaluated on polystyrene (PS) and stainless steel (SS) surfaces at 12°C for 168 h. Biofilm biomass was assessed by crystal violet staining, and the planktonic and sessile cells were quantified in terms of CFU. The biofilms were also observed through Confocal Laser Scanning Microscopy (CLSM) analysis.

Results showed that only P. fluorescens was able to form biofilm on PS. Moreover, in dual-species biofilm at the end of the incubation time (168 h at 12°C), a lower biomass compared to P. fluorescens mono-species was observed on PS. On SS, the biofilm cell population of L. monocytogenes was higher in the dual-species than in mono-species, particularly at 48 h of incubation. The production of blue pigment by P. fluorescens in a cheese model system was revealed both in single- and co-culture after 72 h of incubation (12°C), confirmed also by CLSM results, showing agglomeration, probably linked to the blue pigment.

Our study highlights that the interactions between the two species can influence biofilm formation (biomass and sessile cells) but not the capability of P. fluorescens to produce blue pigment.