When it comes to dehydrated culture media, within the designations of liquid versus gel and defined versus undefined, there are other specific qualities that media can offer. For example, differential media will have an indicator such as a dye that allows researchers to differentiate certain chemical reactions that occur during growth.
Minimal media contain only the minimum nutrients necessary for the growth of a particular type of cells. Minimal media usually do not contain amino acids and are sometimes used to grow so-called “wild type” microorganisms, or those microorganisms that do not have acquired resistance to a certain agent being tested (such as an antibiotic). Minimal media usually contain a carbon source like glucose, various salts that provide nitrogen, magnesium, sulfur, and phosphorous that allow bacteria to create protein and nucleic acid, and water.
Dehydrated culture media can be made selective, meaning that they are made specifically to ensure survival of cells that have certain properties like antibiotic resistance. As an example, a microorganism resistant to an antibiotic like ampicillin or tetracycline can be added to a media culture to keep other cells (like non-antibiotic resistant cells) from growing. One example of selective growth media include substances like eosin-methylene blue agar that allows growth of only Gram negative bacteria.
Gram negative are bacteria that reject crystal violet dye when washed in a decolorizing solution. E coli and salmonella are two examples of Gram negative bacteria. Many Gram negative bacteria have outer membrane structures containing lipids that are very toxic, causing an infected person to have a high temperature and possibly causing endotoxic shock. That is why it is vital to distinguish Gram negative bacteria from other bacteria in cases of infection.