The streptococci are facultative anaerobes which produce a small gray colony after 24 hour incubation at 35°C on sheep blood agar. Unlike the staphylococci, streptococcal colonies grown under anaerobic conditions are larger than those grown aerobically. Also, the streptococci are catalase-negative, while the staphylococci are not (see the discussion concerning weak positive catalase reactions exhibited by enterococci). Microscopically, Gram-positive cocci occurring in chains or pairs with individual cells being somewhat elongated can be presumed to be streptococci or enterococci, and the pneumococcus itself has a distinctive microscopic morphology occurring as lancet-shaped pairs.
Historically, clinical identification of the streptococci has been guided by two factors: Lancefield serogrouping and hemolysis pattern on sheep blood agar.
The Lancefield classification scheme is based on cell wall carbohydrate constitution. In clinical medicine the most important streptococci classified within the Lancefield scheme are Group A S. pyogenes, Group B S. agalactiae, and Group D Streptococcus spp., not to be confused with Enterococcus spp., which also posess the Group D antigen.
Three hemolytic patterns are commonly reported:
For further information, see the review of clinical identification of Gram-positive cocci.