Introduction

Selective media is used to grow microorganisms in many different types of experiments. It is unique in its class because it can encourage the growth of certain microorganisms, but inhibit the growth of others. In other words, these types of media contain a mix of nutrients that allows only specific types of species to survive. This gives it a lot of specific value when conducting experiments on bacterial tolerance, interactions, species-specificity, etc.

Figure 1: Wort, used to make beer, is considered a growth medium. It contains required nutrients for yeast to both survive and produce alcohol under anaerobic conditions (called fermentation). When fermentation is complete, the media and dormant microbes can be consumed as beer.

Fundamental Nutrients

All living things require certain elements for survival and growth; bacterial species are no exception. Therefore, growth media must also contain certain elements to grow bacterial species. These elements may vary by species, but some elements are required for all species. To begin, growth media must contain a carbon source. Carbon is essential for the organic compounds that comprise a living cell. It is estimated to make up half of the dry weight of a typical bacterium. The carbon in media and agar is typically supplied as a combination of glucose and proteins from either plant or meat extracts. In nature, some bacteria can obtain carbon from proteins, carbohydrates, and lipids. These bacteria are called chemoheterotrophs. Other types of bacteria can obtain their carbon from carbon dioxide. These types of bacteria are called chemoautotrophs and photoautotrophs.

Nitrogen, sulfur, and phosphorus are three additional elements that are required for survival and growth of bacteria. A variety of salts and amino acids are added to media to provide these essential elements. Nitrogen and sulfur are required for protein synthesis. Nitrogen and phosphorous are required for DNA and RNA synthesis. It is estimated that nitrogen comprises approximately 14% of a bacterium cell's dry weight, with sulfur and phosphorus combined comprise another approximately 4% of the cell.

Chemically Defined vs. Complex Media

Media can be either chemically defined or chemically complex. Chemically defined media is a media in which the exact chemical composition is known and is made with purified ingredients. Different bacterial species can have different nutritional requirements. Therefore, using different chemically defined mediums can be helpful to grow different microorganisms. Chemically defined media is often used to grow autotrophic bacteria; meaning, bacteria which can produce their own complex organic molecules from simple, inorganic ones.

Complex Media

In contrast, complex media is composed of yeast, plants, and/or animal extracts. Unlike the complete definition of ingredients found in defined media, the complete composition of this type of complex media is not fully known. Carbon, nitrogen, and sulfur are provided when proteins from the extracts are broken down. The extracts also provide vitamins and other essential organic growth factors for bacteria. Complex media is frequently used for the growth of heterotrophic bacteria; meaning, bacteria which require a source of organic carbon to produce their organic molecules.

Media Inhibitors

Selective media contains nutrients required for optimal growth of a selected bacteria species and also inhibitor element(s) to make the media selective. Inhibitors are targeted at the specific types or groups of bacteria that a microbiologist does not want to grow. Inhibitors can function in a variety of ways, but they typically are used to attack the structure or function of the unwanted species. For example, an inhibitor might be used to block DNA synthesis and/or protein expression. They may also be used to decrease membrane stability and/or permeability.

Differential Media

Differential media contains compounds that allow microbiologists to distinguish amongst various microorganisms growing on the same culture plate. Bacterial types can be distinguished (differentiated) by virtue of colony appearance (e.g., color) or by a functional effect on the media (e.g., hemolysis by Gram-positive bacteria on blood agar plates). Media can be either selective, differential, or both selective and differential.

Figure 2: Bacillus sphaericus is used to control mosquito populations as a microbial larvicidal insecticide. B. sphaericus is a naturally occurring bacterium that mosquito larva ingest. The bacteria then produce a toxin that disrupts the gut and kills the larva, resulting in a reduced adult mosquito population.

Clinical and Environmental Use

Selective media is used by microbiologists to help identify types of bacteria in a sample. Clinical microbiologists use selective media to eliminate bacteria that may normally reside in or on the body. They may also use selective media to help detect pathogenic bacteria that can cause disease.

Environmental microbiologists may use selective media to test for the presence of coliform bacteria in water samples. Coliform bacteria often indicates that a water sample has been exposed to fecal contamination. In general, media can be made to be selective by the addition of certain dyes (crystal violet in MacConkey's agar; methylene blue in EMB), by high salt (7% NaCl in MSA) in normal media, or by manipulating the pH of the media

Figure 3: A wooden house frame damaged by termites. Termites harbor cellulose degrading bacteria in their digestive systems that allows the termites to use wood cellulose as an energy source. Cellulose comprises approximately 50% of the total biomass of wood.

Although these different media make bacterial identification and investigation much easier, there are still many obstacles to overcome. For example, one potential stumbling block when selecting for Gram-positive cocci (spheres) from a mixed sample is the fact that Gram-negative microorganisms can block the Gram-positive cell growth.