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 important URT pathogens such as Moraxella catarrhalis which linked to otitis media, sinusitis and chronic obstructive pulmonary disease. However, a direct interaction between these microbes has not been explored in detail. Our results, now published in ‘Beneficial Microbes’ demonstrate that many of the Lactobacillus strains tested, exhibit antipathogenic activities against M. catarrhalis using agar-based assays, time course analysis, biofilm assays and MIC testing. Lactic acid was shown to be a key metabolite in these activities. In addition, cell line assays for adhesion competition and immunomodulation were used to substantiate the inhibitory effect of lactobacilli against M. catarrhalis. The well-documented strain Lactobacillus rhamnosus GG was shown to decrease the adhesion of M. catarrhalis to Calu-3 nasopharyngeal cells and to inhibit inflammation markers which were activated by M. catarrhalis.
This study suggests that several lactobacilli and their key metabolite lactic acid are possible candidates for probiotic therapeutic interventions against URT infections.
The closely related species of the Lactobacillus casei group (L. casei, L. paracasei and L. rhamnosus) are extensively studied because of their applications in food fermentations and as probiotics. Our results, now published in mSystems, show that many strains in this group are incorrectly classified. Surprisingly many bacteria classified as L. casei are misclassified and should be relabeled as L. paracasei instead. We found that reclassifying them to their most closely related type strain improves the functional predictive power of their taxonomy.
In addition, our findings may spark increased interest in the L. casei species. We found that after reclassification, only 10 genomes remain classified as L. casei. Moreover, these strains show very interesting properties. First, they all appear to be catalase positive. This suggests that they have an increased oxidative stress resistance. Second, we isolated a L. casei strain, AMBR2, from the human upper respiratory tract and discovered that it harbors one or even two large, glycosylated putative surface adhesins. This might inspire further exploration of this strain as a potential probiotic organism.