The microbiota in pneumonia: From protection to predisposition.

Mucosal surfaces of the upper respiratory tract and gut are physiologically colonized with their own collection of microbes, the microbiota. The normal upper respiratory tract and gut microbiota protects against pneumonia by impeding colonization by potentially pathogenic bacteria and by regulating immune responses. However, antimicrobial therapy and critical care procedures perturb the microbiota, thus compromising its function and predisposing to lung infections (pneumonia). Interindividual variations and age-related alterations in the microbiota also affect vulnerability to pneumonia. We discuss how the healthy microbiota protects against pneumonia and how host factors and medical interventions alter the microbiota, thus influencing susceptibility to pneumonia.