The Genus Streptococcus – Part II: New Species and Pathogens in the “Miscellaneous” Streptococci and “Viridans” Streptococci Groups

In the second part of this two-part series on an update on the streptococci, new species and emergent human pathogens in the “viridans streptococci” and the “miscellaneous streptococci” groups are discussed. Among the “miscellaneous streptococci,” the most important organism in human infectious diseases is Streptococcus suis. This organism is primarily an agent of disease in swine and other animals and has now emerged as a significant human pathogen, causing bacteremia and meningitis, particularly among populations in Asia and the Far East. While S. suis capsular serotype 2 is the most frequently isolated agent, infections caused by other serotypes are increasingly being reported. In addition, disease caused by S. suis is now being described more frequently in North America in individuals with significant occupational exposure to swine. Among the viridans streptococci, several new species in the mitis-sanguinis group have been described as human pathogens, including Streptococcus pseudopneumoniae, Streptococcus oligofermentans, and Streptococcus tigurinus. Other newly reported viridans group species either comprise part of the human oral microbiome or have been isolated from animals. This article presents information on these bacterial agents, including characteristics for the recognition and identification of the more important clinical isolates in these streptococcal groups.