Biochemical tests are used to detect the presence or absence of gram-negative or gram-positive bacteria. These tests are commonly used in the fields of science and medicine, particularly biology. Biochemical tests are performed using a variety of methods and experimental techniques; the method of testing varies depending on whether researchers are testing for positive or negative bacteria.
The Mannitol Salt Agar test (MSA) is widely used by biologists and chemists to test for the presence of gram-positive bacteria, such as Staph and Strep species. This test is classified as selective and differential: the main differential ingredient in the MSA is the sugar mannitol; this substance is used to attract organisms (gram-positive bacteria) that feed on mannitol as a food source. Upon consuming the substance, these bacteria produce acidic byproducts, also called fermentation, which ultimately lower the pH of the surrounding media. The varying degrees of acidity produced by the media are used as pH indicators, which range in color from red to orange and pale yellow. According to the University of Wyoming, oxidase tests are used to determine the presence of gram-negative bacteria, specifically the bacteria that contain the enzyme oxidase. In this test, positive results occur when electrons are oxidized by the cytochrome oxidase, which turns dark purple in color.