Lactic acid bacteria as potential probiotics for the pharyngeal mucosa

The probiotic approach represents an alternative strategy in the prevention and treatment of infections in those body sites where the microbiota plays a role in the maintenance of physiological homeostasis. In this context, we evaluated in vitro the potential abilities of oral, probiotic and dairy bacteria in controlling Streptococcus pyogenes infections at the pharyngeal level. Initially, we analysed the bacterial adhesion on FaDu hypopharyngeal carcinoma cells and the ability to antagonize S. pyogenes on FaDu layer and HaCat keratinocytes. Due to their promising adhesive and antagonistic features, we studied the oral isolate Streptococcus salivarius ST3 and the dairy strain Lactobacillus helveticus MIMLh5 also through immunological in vitro experiments. We performed the quantification of several cytokines and the measurement of NF-κB activation in FaDu cells. Strain ST3 significantly increased NF-κB activation at baseline, while it had an opposite effect when FaDu cells were co-stimulated with IL-1β. In addition, ST3 biased cytokines production at baseline (reduction of IL-1β and TNFα, increase of MIP-1β and GM-CSF ) and after IL-1β-induction(reduction on IL-6 and IL-8). MIMLh5 efficiently reduced the induction of IL-6, IL-8 and TNFα in a dose-dependent manner. After stimulating with IL-1β, active NF-κB was still markedly lowered. Nevertheless, we observed an increased secretion of IL-6, IFN-γ and GM-CSF under these conditions. In addition, MIMLh5 increased the GM-CSF/G-CSF ratio. This is compatible with a switch of the immune response towards a TH1 pathway, as supported by our observation that MIMLh5, once in contact with bone marrow derived dendritic cells, triggered the secretion of TNFα and IL-2. In conclusion, we propose ST3 and MIMLh5 as a potential probiotic bacteria for the human pharynx, with promising antagonistic and immunomodulatory properties.