Abstract
The genus Streptococcus continues to have a profound impact on humans. Recent studies, however, are also demonstrating that some streptococci, including several newly identified species, are primarily associated with various non-human host animals. Currently, approximately 120 streptococcal species have been formally validated. Among them, there are several medically significant species associated with humans, including overt pathogens and pathobionts, some being human specific and others zoonotically acquired. On the other hand, some other streptococcal species are characterized as commensals or have been utilized for industrial purposes. Over time, the taxonomic groupings within the genus have been progressively adjusted as phylogenetic techniques have advanced. Nevertheless, the species that interact with humans largely fall into six main groupings: pyogenic, anginosus, mitis, salivarius, bovis/equinus, and mutans. Additionally, there is a group of species that are phylogenetically undefined or unique. In this chapter, we have loosely characterized selected members of the streptococci based on their functionality for humans, categorizing them as aggressors, opportunists, workers, or protectors. The vast number of streptococcal species that have been shown either to play a role as modulators of human health or that produce potentially commercially valuable molecules and food products suggests that considerable ongoing research will likely continue to focus on exploring the benefits of members within this genus for human well-being.