Laboratory classification of streptococci

What it is

Streptococci are gram positive cocci in chains and are catalase negative. Within that group they are told apart first by hemolysis and then by one further test.

How it is diagnosed

Hemolysis divides the group in three, and each arm has its own second test.

• Alpha, partial hemolysis. The second test is optochin sensitivity. Optochin sensitive is S. pneumoniae, which is encapsulated. Not sensitive is the viridans strep, which are unencapsulated: S. mutans and S. mitis.
• Beta, complete hemolysis. The second test is bacitracin sensitivity. Bacitracin sensitive is group A strep, S. pyogenes. Not sensitive is group B strep, S. agalactiae.
• Gamma, no hemolysis. The second test is growth in 6.5% NaCl. Growth is the enterococci, S. faecium and S. faecalis. No growth is S. bovis.