The quintessential bacterial characterization procedure is the Gram stain. When Gram's iodine and crystal violet are added to bacteria, they form a large purple complex inside of the cells. When rinsed with a red solution, the complex is washed out of gram-negative bacteria, making those cells appear red. In contrast, the cell walls of Gram-positive bacteria are dense enough to lock in the dark purple complex.
Microbiologists can detect patients who have been exposed to diseases like HIV. Patient blood serum is added to plastic wells or membranes coated with HIV proteins. If the patient has antibodies to HIV, they will bind to the HIV proteins. A second reaction is performed, which produces a color change only if human antibodies are present.
Microbiologists use electrophoresis to separate DNA fragments by size. DNA samples are placed in a gel and subjected to electrical charges. DNA is negatively charged, so it migrates toward the positively charged end of the gel. The smaller the DNA fragment, the further it will migrate.