Streptococci 

Streptococci are Gram positive cocci those arrange in chains or pairs. Streptococci found in normal flora of humans and other animals as well. Some species of streptococci are human pathogens. Among them Streptococcus pyogenes are main, which causes pyogenic infections, with a characteristic tendency to spread.

Classification

There are several way to classify streptococci, but in medical term it is decided in two groups, aerobic and facultative anaerobic streptococci are classified on the bases of their nature towards hemolytic. (i) alpha hemolytic streptococci and (ii) Beta hemolytic streptococci.

Alpha hemolytic streptococci produce a greenish discoloration with partial hemolysis around the colonies. The alpha streptococci are normal commensalism in the throat, but may cause opportunist infections rarely. Pneumococcus is also an alpha hemolytic streptococcus.

Beta hemolytic streptococci produce a sharply defined, clear, colourless zone of hemolysis 2 – 4 mm wide. Within which red cells are completely lyses. The term ‘hemolytic streptococci’ strictly applicable only to beta hemolytic streptococci. Most pathogenic streptococci belong to this group.Gamma or non hemolytic streptococci produce no change in the medium and so are sometimes referred to as indifferent streptococci. They include the fecal streptococci and related species. They are also called as enterococcus group.