The Effects of Bacillus licheniformis on the Growth, Biofilm, Motility and Quorum Sensing of Salmonella typhimurium

With 80% of bacterial infections occurring as biofilms, biofilm-related infections have evolved into a critical public health concern. Probiotics such as <i>Bacillus licheniformis</i> have emerged as promising alternatives, offering new avenues for effective treatment. This study aimed to evaluate the activity of <i>licheniformis</i> against the growth, biofilm formation, motility, and quorum sensing (QS) of <i>Salmonella typhimurium</i>. Several experiments were conducted: The minimum inhibitory concentration (MIC) of <i>Bacillus licheniformis</i> against <i>Salmonella typhimurium</i> was determined to be 0.5 mg/mL using the broth microdilution method. The inhibition zone of 100 mg/mL of <i>B. licheniformis</i> against <i>Salmonella typhimurium</i> was 19.98 ± 1.38 mm; the time-growth curve showed that <i>B. licheniformis</i> can effectively inhibit the growth of <i>Salmonella typhimurium</i>. In biofilm experiments, at the MIC of <i>B. licheniformis</i>, the inhibition rate of immature biofilm of <i>Salmonella typhimurium</i> was 86.9%, and it significantly reduced the production of biofilm components (EPS, e-DNA, and extracellular proteases) (<i>p</i> < 0.05). The disruption rate of mature biofilm by <i>B. licheniformis</i> at the MIC was 66.89%, and it significantly decreased the levels of biofilm components (EPS and e-DNA) (<i>p</i> < 0.5). Microscopic observation showed that both the MIC and 1/2 MIC of <i>B. licheniformis</i> could reduce the number of bacteria in the <i>Salmonella typhimurium</i> biofilm, which was not conducive to the formation and maintenance of the biofilm structure. Swimming/Swarming assays and QS experiments confirmed that <i>B. licheniformis</i> inhibits the motility of <i>Salmonella typhimurium</i> and the secretion of AI-1-type quorum sensing molecules and downregulates the AI-2 quorum sensing system by upregulating <i>lsr</i> gene expression. These findings suggest that <i>B. licheniformis</i> could be a potential antimicrobial agent and biofilm inhibitor.