Differential biofilm formation and chemical disinfection resistance of sessile cells of Listeria monocytogenes strains under mono-species and dual-species conditions with Salmonella enterica

ABSTRACT

This study aimed to investigate the possible influence of bacterial intra- and inter-species interactions on the ability of Listeria monocytogenes and Salmonella enterica to develop mixed culture biofilms on an abiotic substratum, as well as on the subsequent resistance of sessile cells to chemical disinfection. Initially, three strains from each species were selected and left to attach and form biofilms on stainless steel (SS) coupons incubated at 15°C for 144 h, in periodically renewable Tryptone Soy Broth (TSB), under either monoculture or mixed culture (mono-/dual-species) conditions. Following biofilm formation, mixed culture sessile communities were subjected to 6 min disinfection treatments with: (i) benzalkonium chloride (50 ppm), (ii) sodium hypochlorite (10 ppm), (iii) peracetic acid (10 ppm), and (iv) mixture of hydrogen peroxide (5 ppm)/peracetic acid (5 ppm). Results revealed that both species reached similar biofilm counts (ca. 10⁵ CFU cm⁻²) and that, in general, interspecies interactions did not have any significant effect neither at the biofilm forming ability (as this was assessed by agar plating enumeration of the mechanically detached biofilm bacteria), nor at the antimicrobial resistance of each individual species. Interestingly, pulsed field gel electrophoresis (PFGE) analysis clearly showed that the three L. monocytogenes strains did not contribute to the same level neither at the formation of the mixed culture sessile communities (mono-/dual-species), nor at their antimicrobial recalcitrance. Additionally, the simultaneous existence, inside the biofilm structure, of S. enterica cells seemed to influence the occurrence and resistance pattern of L. monocytogenes strains. To sum, this study highlights the impact of microbial interactions taking place inside a mixed culture sessile community on both its population dynamics and disinfection resistance.