Influence of Microbiota on Viral Infections

Most pathogens gain access to the host through surfaces of the body that are exposed to the surrounding environment and rife with resident microorganisms, termed microbiota. Microbiota play an integral role in modulating host health. One significant benefit of the microbiota is that they provide protection against incoming bacterial pathogens [1]. Commensals make their immediate environment inhospitable to many pathogens by producing biosurfactants, by competing for sites of attachment and nutrients, and by excreting metabolites with antimicrobial effects [1]. Furthermore, the presence of commensals promotes maturation of secondary lymphoid organs in the intestine, which are the first line of defense in the intestinal mucosa [2]. Therefore, when a pathogen infiltrates the host, it is not entering a sterile environment, but one that has been shaped by a dynamic commensal community. Although many interactions between bacterial pathogens and the microbiota have been characterized [1], little is known about the interplay between viral pathogens and the natural flora of the host. Are viral pathogens blind to the commensal microbes surrounding them? Judging from recent publications, this appears not to be the case. There is strong evidence that the microbiota can either protect the host from virally induced disease or promote viral propagation/transmission, through direct or indirect mechanisms.