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 asBacillus licheniformishave emerged as promising alternatives, offering new avenues for effective treatment. This study aimed to evaluate the activity oflicheniformisagainst the growth, biofilm formation, motility, and quorum sensing (QS) ofSalmonella typhimurium. Several experiments were conducted: The minimum inhibitory concentration (MIC) ofBacillus licheniformisagainstSalmonella typhimuriumwas determined to be 0.5 mg/mL using the broth microdilution method. The inhibition zone of 100 mg/mL ofB. licheniformisagainstSalmonella typhimuriumwas 19.98 ± 1.38 mm; the time-growth curve showed thatB. licheniformiscan effectively inhibit the growth ofSalmonella typhimurium. In biofilm experiments, at the MIC ofB. licheniformis, the inhibition rate of immature biofilm ofSalmonella typhimuriumwas 86.9%, and it significantly reduced the production of biofilm components (EPS, e-DNA, and extracellular proteases) (p< 0.05). The disruption rate of mature biofilm byB. licheniformisat the MIC was 66.89%, and it significantly decreased the levels of biofilm components (EPS and e-DNA) (p< 0.5). Microscopic observation showed that both the MIC and 1/2 MIC ofB. licheniformiscould reduce the number of bacteria in theSalmonella typhimuriumbiofilm, which was not conducive to the formation and maintenance of the biofilm structure. Swimming/Swarming assays and QS experiments confirmed thatB. licheniformisinhibits the motility ofSalmonella typhimuriumand the secretion of AI-1-type quorum sensing molecules and downregulates the AI-2 quorum sensing system by upregulatinglsrgene expression. These findings suggest thatB. licheniformiscould be a potential antimicrobial agent and biofilm inhibitor.