Streptococcus pyogenes is an important pathogen that causes a variety of diseases. The most common
infections involve the throat (
pharyngitis) or skin (
impetigo); however, the factors that determine tissue tropism and severity are incompletely understood. The S. pyogenes
NAD(+) glycohydrolase (SPN) is a
virulence factor that has been implicated in contributing to the pathogenesis of severe
infections. However, the role of SPN in determining the bacterium's tissue tropism has not been evaluated. In this report, we examine the sequences of spn and its endogenous inhibitor ifs from a worldwide collection of S. pyogenes strains. Analysis of average pairwise
nucleotide diversity, average number of
nucleotide differences, and ratio of nonsynonymous to synonymous substitutions revealed significant diversity in spn and ifs. Application of established models of molecular evolution shows that SPN is evolving under positive selection and diverging into
NAD(+) glycohydrolase (
NADase)-active and -inactive subtypes. Additionally, the
NADase-inactive SPN subtypes maintain the characteristics of a functional gene while ifs becomes a pseudogene. Thus,
NADase-inactive SPN continues to evolve under functional constraint. Furthermore,
NADase activity did not correlate with invasive disease in our collection but was associated with tissue tropism. The ability to cause
infection at both the pharynx and the skin ("generalist" strains) is correlated with
NADase-active SPN, while the preference for causing
infection at either the throat or the skin ("specialist" strains) is associated with
NADase-inactive SPN. These findings suggest that SPN has a
NADase-independent function and prompt a reevaluation of the role of SPN in streptococcal pathogenesis.