Cell-free supernatants of Bacillus subtilis and Bacillus polyfermenticus inhibit Listeria monocytogenes biofilm formation

Listeria monocytogenes is the most common foodborne pathogenic bacterium. L. monocytogenes can make biofilms on living or inanimate surfaces, helping it resist antimicrobial agents. This study aimed to evaluate the anti-biofilm effect of cell-free supernatants (CFS) of Bacillus subtilis KU43, B. subtilis KU201, and B. polyfermenticus KU3 against biofilm-forming L. monocytogenes. The minimum inhibitory concentrations (MIC) of CFS against L. monocytogenes were 12.5, 25, and 50% (v/v). All CFS inhibited and degraded L. monocytogenes biofilms at 1/2 MIC. In addition, the cell-surface characteristics of L. monocytogenes, including hydrophobicity, auto-aggregation, and adhesion ability, as well as exopolysaccharide production, were negatively impacted by CFS treatment. Among these Bacillus species, B. subtilis KU43 showed the most potent anti-biofilm activity. CFS of B. subtilis KU43 downregulated the expression of biofilm-associated genes that encode flagella, virulence factors, and quorum sensing in CFS-treated L. monocytogenes compared to those of the control. Scanning electron microscopy verified the anti-biofilm effect of CFS against biofilm-forming L. monocytogenes. Hence, these outcomes suggest that CFS of Bacillus species, especially B. subtilis KU43, has effective anti-biofilm activity against the foodborne pathogen L. monocytogenes.