Purpose: The objective of this study was to evaluate biofilm formation and sanitizer resistance of L. monocytogenes in mono- and mixed-species biofilms with cultivable indigenous microorganisms (CIM) from fresh salmon under simulated salmon processing environment.
Methods: Biofilms of L. monocytogenes in mono- and mixed-species with CIM from fresh salmon were formed on stainless steel coupons by incubating in diluted TSB (1:20) or salmon juice at 4 and 15°C, respectively. The biofilm density and sanitizer (Ecolab® Whisper™ V, quaternary ammonium compounds-based) resistance of L. monocytogenes were determined on day 1, 4, 7, and 14. Confocal laser scanning microscope (CLSM) was used to characterize the spatial structure of biofilms.
Results: Higher temperature and longer biofilm age improved biofilm formation by L. monocytogenes. Compared to diluted TSB, higher biofilm density of L. monocytogenes was observed in salmon juice. Biofilm density of L. monocytogenes was significantly (P<0.05) lower in mixed-species (6.06 log CFU/cm2) than single-species (5.29 log CFU/cm2) at 15°C from day 4 to day 14, in salmon juice. However, L. monocytogenes biofilm cells in mixed-species were more resistant than those in single-species, with 0.42 and 2.85 log CFU/cm2 log reductions on day 4, respectively. The honeycomb-like cell clusters shielding the bottom layer structure of biofilms shown by CLSM images might explain the enhanced sanitizer resistance of L. monocytogenes in mixed-species.
Significance: This study provides insights into L. monocytogenes biofilm formation under simulated salmon processing environment and highlighs that the presence of indigenous microorganisms could protect L. monocytogenes biofilm cells from sanitizer treatment.