Diversity of beta-hemolysins produced by the human opportunistic streptococci

The genus Streptococcus infects a broad range of hosts, including humans. Some species, such as S. pyogenes, S. agalactiae, S. pneumoniae, and S. mutans, are recognized as the major human pathogens, and their pathogenicity toward humans has been investigated. However, many of other streptococcal species have been recognized as opportunistic pathogens in humans, and their clinical importance has been underestimated. In our previous study, the Anginosus group streptococci (AGS) and Mitis group streptococci (MGS) showed clear beta-hemolysis on blood agar, and the factors responsible for the hemolysis were homologs of two types of beta-hemolysins, cholesterol-dependent cytolysin (CDC) and streptolysin S (SLS). In contrast to the regular beta-hemolysins produced by streptococci (typical CDCs and SLSs), genetically, structurally, and functionally atypical beta-hemolysins have been observed in AGS and MGS. These atypical beta-hemolysins are thought to affect and contribute to the pathogenic potential of opportunistic streptococci mainly inhabiting the human oral cavity. In this review, we introduce the diverse characteristics of beta-hemolysin produced by opportunistic streptococci, focusing on the species/strains belonging to AGS and MGS, and discuss their pathogenic potential.