Streptococci are part of the normal flora in humans and animals.
· They are nonmotile, nonsporing, spherical or ovoid cocci, and have hyaluronic acid capsules.
· They are catalase negative by which they are distinguished from staphylococci.
· They are relatively fastidious bacteria requiring enriched medium, such as blood agar for their growth.
The streptococci based on their oxygen requirement are classified into aerobes and obligate anaerobes. Obligate anaerobes are designated as peptostreptococci.. The aerobes and facultative anaerobes are further classified as follows:
The aerobes and facultative anaerobes are further classified on the basis of their hemolytic properties on blood agar. Brown (1919) classified these aerobic streptococci into three groups on the basis of their growth in 5% horse blood agar.
Alpha-hemolytic streptococci: These cocci produce colo-nies surrounded by a narrow zone (greenish zone) of hemoly-sis with persistence of some partially lysed RBCs. The greenish discoloration is due to the formation of a reduced product of hemoglobin. Alpha-hemolytic streptococci are known as viridans streptococci. These are found as commensals in the upper respiratory tract of humans, and these may cause oppor-tunistic infections. Streptococcus salivarius is an important opportunistic pathogen belonging to this group (Streptococcuspneumoniae also belongs to alpha-hemolytic group).
Beta-hemolytic streptococci: These cocci produce a well-defined, clear, colorless zone of hemolysis (2–4 mm wide) around the colonies. RBCs in the zone of hemolysis are completely lysed. This lysis is due to the liberation of enzymes streptolysin O (SLO) and streptolysin S (SLS). The term hemolytic streptococci is applicable only to beta-hemolytic streptococci. Most of the pathogenic streptococci belong to this group, and among them Streptococcus pyogenes is the most important one.
Gamma-hemolytic streptococci: These streptococci do notproduce any hemolysis or discoloration on blood agar. These nonhemolytic streptococci are generally found as commen-sals. Streptococcus faecalis (Enterococcus faecalis) belongs to this group.
Lancefield classification is a serological classification of the beta-hemolytic streptococci. This serological classification is based on the detection of group-specific carbohydrate antigen (C antigen) on the cell wall of the streptococci.
The beta-hemolytic streptococci are classified into 21 serological groups known as Lancefield groups, desig-nated from A to W (with exception of I and J). The major-ity of hemolytic streptococci that cause human infections belong to group A. Group A streptococci are also known as S. pyogenes, while group B streptococci are known as Strepto-coccus agalactiae.
Based on the M, T, and R protein antigens present on the cell surface, S. pyogenes have been further classified into 80 sero-types. This classification is known as Griffith typing. M protein is the most important type-specific antigen. This serotyping is important for epidemiological studies.