In dairy processing environments, many bacterial species form biofilms on surfaces and equipment. Among them, Listeria monocytogenes and Pseudomonas spp. could be present in mixed-species biofilms with increased resistance to disinfectants. 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 to produce the blue pigment. The biofilm-forming capability

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... f single- and mixed-cultures was evaluated on polystyrene (PS) and stainless steel (SS) surfaces at 12 °C for 168 h. Biofilm biomass was assessed by crystal violet staining, the planktonic and sessile cells were quantified in terms of Colony Forming Unit (CFU), and the carbohydrates were quantified by the anthrone method. The biofilms were also observed through Confocal Laser Scanning Microscopy (CLSM) analysis. Results showed that only P. fluorescens was able to form biofilms on PS. In dual-species biofilms at the end of the incubation time, a lower biomass compared to P. fluorescens mono-species was observed. On the SS, the biofilm cell population of L. monocytogenes was higher in the dual-species than in the mono-species, particularly after 48 h. Carbohydrates in the dual-species system were higher than those of the mono-species and were also revealed at 168 h. The production of blue pigment by P. fluorescens in the Ricotta-based model system was revealed in both single cultures and co-cultures and was confirmed by the CLSM results, showing agglomeration, probably linked to the blue pigment. Our study suggests that the interactions between the two species can influence biofilm formation, but not the capability of P. fluorescens to produce blue pigment.