Staphylococcus and Streptococcus Tests
Coagulase Test
Coagulase is an enzyme capable of converting fibrinogen to fibrin, causing visible clot formation in a suitable analytical system. A positive reaction is shown by the formation of a clot or gel. Staphylococcus aureus bacteria are coagulase-positive.
Glucose Fermentation Test
This test can differentiate the genus Micrococcus from the genus Staphylococcus because the latter gives a positive test result.
Mannitol Fermentation Test (1%)
Strains of S. aureus, in contrast to those of S. epidermidis, possess the ability to ferment mannitol with acid production. The turning of the medium from its initial color to yellow indicates a positive reaction by acidification.
DNase Test
This test is useful in identifying strains of S. aureus, which produces the enzyme DNase that cleaves DNA. The culture medium will undergo a change of color around the colony, indicating hydrolysis of DNA.
Family Streptococcaceae
Gram-positive cocci are grouped in pairs or chains. In solid culture media, they form small, translucent colonies.
Genera of Clinical Importance
Different genera can be classified according to their immunological, hemolytic, and/or biochemical properties. Hemolytic characteristics should be determined in blood agar cultures.
Hemolytic Properties
The type of hemolysis produced on blood agar is useful for the identification of Streptococcus. The medium used should be free of sugars. Incubation in an atmosphere of CO2 can inhibit the production of hemolysis.
Depending on the type of hemolysis, they are classified as:
- Alpha-hemolytic: Partial hemolysis by alteration of hemoglobin or methemoglobin.
- Beta-hemolytic: Produces total hemolysis of red blood cells by the action of hemolysins (Streptolysins S and O).
- Gamma-hemolytic: Those that do not produce any hemolysis.
Biochemical Properties
They are classified on the basis of biochemical reactions that can distinguish the following species:
- Streptococcus pyogenes
- Streptococcus viridans
- Streptococcus pneumoniae