5. NASOPHARYNGEAL BACTERIAL COMMUNITY AS A SHIELD AGAINST RESPIRATORY INFECTIONS
Nasopharyngeal bacteria may counteract bacterial or viral infection. In infant mice, nasal priming with viable Corynebacterium pseudodiphtheriticum , a Gram-positive bacterium member of the normal microbiota of the upper respiratory tract, improved their resistance to primary RSV infection and secondary pneumococcal pneumonia [52]. The protection against RSV infection and pneumococcal superinfection was related to modulation of Toll-like receptor 3 activation in the respiratory tract leading to enhanced production of the protective cytokines TNF‐α, IL‐6, IFN‐γ and IFN-β, but also of IL-10 that could contribute to restrict the inflammatory reaction and ultimately to limit tissue injury. IFN-γ producing CD3+CD4+ cells were also detected in the lungs of treated infant mice, viral load was significantly lower and lung tissue damage markers (LDH and albumin concentrations) in bronchoalveolar lavage were decrease in RSV-infected mice. Priming with viable Corynebacterium pseudodiphtheriticumalso reduced lung bacterial cell counts and prevented the dissemination of S. pneumoniae into the blood [52]. Thus, a possible perspective use of these results could be a seasonal application of a nasal probiotic spray to boost respiratory innate immunity in immunocompetent subjects. Bearing in mind the “gut-lung axis” interaction, one could consider orally given probiotics or bacterial derived products as alternative options; a fascinating proposal whose theoretical efficacy needs eventually to be demonstrated by further studies [53]. A possible approach can be to assess bacterial lysate in germ free mice models to determine the effect on immune maturation and the capacity to mimic the microbiota effect in respiratory virus infection.