<i>Streptococcus mitis</i>: walking the line between commensalism and pathogenesis

<jats:title>Summary</jats:title><jats:p> <jats:italic>Streptococcus mitis</jats:italic> is a viridans streptococcus and a normal commensal of the human oropharynx. However, <jats:italic>S. mitis</jats:italic> can escape from this niche and cause a variety of infectious complications including infective endocarditis, bacteraemia and septicaemia. It uses a variety of strategies to effectively colonize the human oropharynx. These include expression of adhesins, immunoglobulin A proteases and toxins, and modulation of the host immune system. These various colonization factors allow <jats:italic>S. mitis</jats:italic> to compete for space and nutrients in the face of its more pathogenic oropharyngeal microbial neighbours. However, it is likely that in vulnerable immune‐compromised patients <jats:italic>S. mitis</jats:italic> will use the same colonization and immune modulation factors as virulence factors promoting its opportunistic pathogenesis. The recent publication of a complete genome sequence for <jats:italic>S. mitis</jats:italic> strain B6 will allow researchers to thoroughly investigate which genes are involved in <jats:italic>S. mitis</jats:italic> host colonization and pathogenesis. Moreover, it will help to give insight into where <jats:italic>S. mitis</jats:italic> fits in the complicated oral microbiome. This review will discuss the current knowledge of <jats:italic>S. mitis</jats:italic> factors involved in host colonization, their potential role in virulence and what needs to be done to fully understand how a an oral commensal successfully transitions to a virulent pathogen.</jats:p>