Investigating the Antibacterial and Immunomodulatory Properties of Lactobacillus acidophilus Postbiotics

Probiotics are nonpathogenic microorganisms that have been extensively studied for their ability to prevent various infectious, gastrointestinal, and autoimmune diseases. The mechanisms underlying these probiotic effects have not been elucidated. However, we and other researchers have evidence suggesting that probiotic bacteria secrete metabolites that are antimicrobial and anti-inflammatory. As such, we developed a methodology to collect the secreted metabolites from a probiotic bacterium, Lactobacillus acidophilus, and tested this cell free filtrate (CFF) both in vitro and in vivo. Using this CFF, we have demonstrated that L. acidophilus secretes a molecule(s) that has specific bactericidal activity against the opportunistic pathogen, Pseudomonas aeruginosa. Additionally, we have shown that the CFF inhibits P. aeruginosa biofilm formation, an important virulence factor that contributes to P. aeruginosa’s antibiotic resistance. Importantly, our data show that the CFF eradicates 20 h and 48 h established biofilm. We hypothesized that the CFF could modulate pro-inflammatory mediator secretion. To investigate the role of the CFF on tumor necrosis factor-α (TNF-α) and interleukin-8 (IL-8) production, we measured TNF-α and IL-8 production from THP-1 monocytes and macrophages. Interestingly, our data show that the CFF could stimulate or reduce inflammatory mediator release, depending on the concentration of CFF and whether the cells had been prestimulated with LPS. To elucidate the mechanism underlying the immunomodulatory activity of the CFF, we conducted luciferase assays using a NF-κB reporter THP-1 cell line. Here, we provide evidence that the CFF modulates inflammatory mediators at least in part by regulating NF-κB activation. In addition, we tested the effects of the CFF in a mouse model of P. aeruginosa wound infection. The CFF cleared P. aeruginosa bacterial load and reduced plasma inflammatory mediator production by 5 days. To investigate the bioactive molecule within the CFF, we performed sequential fractionation steps and LCMS to identify and characterize the bioactive molecule, which has a similar mass to charge ratio (m/z) as known polyunsaturated fatty acids (PUFAs) and modified fatty acids. Taken together, our findings provide evidence that L. acidophilus secretes a bioactive molecule that has bactericidal, anti-biofilm, and immunomodulatory activities.