Biochemical tests for identifying bacteria generally break down into two categories: gram positive tests and gram negative tests. Many tests exist in these two general categories; gram positive tests include catalase, Mannitol Salt Agar, or MSA, Blood Agar Plates, or BAP, and CAMP tests, while tests for gram negative bacteria include oxidase, sugar, methyl red, nitrate broth and motility agar.
These tests employ various methods and mediums for identifying bacteria. Some, like MSA, test for the presence of certain bacteria. This test employs selective and differential tactics for isolating and viewing bacteria. Scientists often use the MSA test to identify bacteria capable of living in salty environments, such as Streptococcus. The MSA tests uses sugar — mannitol — as a control substance; some Streptococcus species ferment mannitol while others do not, helping scientists distinguish among species.
The BAP test also helps scientists identify different species of bacteria in the same family, including Streptococcus. Scientists use sheep blood as a control medium to determine the extent to which organisms perform hemolysis, in turn distinguishing species.
Kliger's Iron Agar, or KIA, a gram negative test, uses differential media to isolate fermenting versus non-fermenting bacteria in the E. coli family. Upon performing fermentation, bacteria change the color of dyes in tubes or beakers, letting scientists correctly identify the specific strains.