SAARS: Streptococcus spp. (microbiology)

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:

• Alpha-hemolytic streptococci cause an incomplete destruction of the red cells within the sheep blood agar, producing a typical dark green discoloration of the media surrounding or directly underneath the colony, reflecting the presence of biliverdin and other heme compounds. The pneumococcus and viridans-type streptococci are typically alpha-hemolytic, as are some enterococci.

• Beta-hemolytic streptococci completely destroy the red cells in the sheep blood agar, resulting in transparency of the agar. Streptolysin O is an oxygen-labile hemolysin found in streptococci, especially in Group A S. pyogenes and, in smaller amounts, in Group G and in some Group C streptococci. The oxygen-stable streptolysin S is present in about 98% of Group A S. pyogenes isolates. The zone of beta-hemolysis caused by Group A streptococci is therefore enhanced if the culture is incubated anaerobically. Both Group A and Group B streptococci are beta-hemolytic, as are several other streptococci of less clinical relavance.

• Non-hemolytic streptococci are most often found as commensals in respiratory specimens, although they can occur in mixed cultures or as agents causing UTI's. Also, Group D streptococci and enterococci can be alpha-, beta-, or non-hemolytic.

For further information, see the review of clinical identification of Gram-positive cocci.