The streptococci are a very heterogeneous group of bacteria.
Some members are a part of our normal flora while others are potent pathogens.
The primary pathogens are S. pyogenes and S. pneumoniae but other species can be opportunistic. For example, S. agalactiae can produce severe neonatal disease including meningitis, pneumonia and bacteremia in infants aged 7 days up to 3 months. E. faecalis may be implicated in endocarditis and urinary tract infections. S. mutans is an important contributor to dental caries. The streptococci are generally delineated into groups according to the Lancefield method.