Multifactorial inhibition of lactobacilli against the respiratory tract pathogen Moraxella catarrhalis

Probiotics, mainly lactic acid bacteria (LAB), are widely focused on gastrointestinal applications. However, recent microbiome studies indicate that LAB can be endogenous members of other human body sites such as the upper respiratory tract (URT). Interestingly, DNA-based microbiome research suggests an inverse correlation between the presence of LAB and the occurrence of potential pathogens, such as Moraxella catarrhalis, an important URT pathogen linked to otitis media, sinusitis and chronic obstructive pulmonary disease. However, a direct interaction between these microbes has not been explored in detail. This study investigated the direct antipathogenic effects of Lactobacillus species, including several well-documented probiotic strains, on M. catarrhalis using agar-based assays, time course analysis, biofilm assays and minimal inhibitory concentration (MIC) testing. These assays were performed using spent culture supernatans (SCS) at two pHs (4.3 and 7) and D- and/or L-lactic acid at three pHs (2, 4 and 7). In addition, cell line assays for adhesion competition and immunomodulation were used to substantiate the inhibitory effect of lactobacilli against M. catarrhalis. A proportion of Lactobacillus strains, including the model probiotic Lactobacillus rhamnosus GG, showed a strong and direct activity against M. catarrhalis. Screening of the activity of the SCS after different treatments demonstrated that lactic acid has an important antimicrobial activity against this pathogen - at least in vitro - with mean MIC values for D- and L-lactic acid varying between 0.5 and 27 g/l depending on the pH. Furthermore, L. rhamnosus GG also decreased the adhesion of M. catarrhalis to human airway epithelial Calu-3 cells with more than 50%, and the expression of mucin MUC5AC, pro-inflammatory cytokines interleukin (IL)-8, IL-1β, and tumor necrosis factor-α at least 1.2 fold. This study suggests that several lactobacilli and their key metabolite lactic acid are possible candidates for probiotic therapeutic interventions against URT infections.