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Streptococci are gram positive cocci arranged in chains. They are widely distributed in nature, some are commensals in the body.


Streptococci are heterogenous group and are classified according to their O2 requirements into obligate anaerobes (peptostreptococci) and facultative anaerobes.

Facultative anaerobes can survive in the presence of O2. They are subdivided according to their action on red cells in blood agar into:

Beta hemolytic streptococci produce complete hemolysis and a clear zone around the colonies e.g. streptococcus pyogenes.

Alpha hemolytic streptococci produce partial hemolysis with a zone of greenish discoloration e.g. streptococcus viridans.

Non-hemolytic or gamma streptococci which cause no hemolysis or change in RBCs e.g. streptococcus faecalis (enterococci).

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