Growth and growth media

The ability of all organisms, including microorganisms , to grow and divide is the fundamental underpinning of their continued existence. In the laboratory, the nutrients needed for growth are supplied in the form of growth media.

Microorganisms such as bacteria , yeast , and algae grow by increasing in size, replicating their genetic material and other internal factors such as proteins, manufacturing the required additional cell wall material to enclose the new cell, and finally dividing to form two so-called daughter cells. Vital materials are required by most microbes, including carbon, hydrogen, nitrogen, phosphorus, calcium, cobalt, magnesium, and manganese. Often these elements must be supplied in the growth media, because the microbes cannot manufacture them. Once supplied, however, such elements form the building blocks upon which the microorganism can construct some (but not all) amino aids, proteins, and even the DNA (deoxyribonucleic acid ) and ribonucleic acid (RNA ) genetic materials.

Provision of the necessary elements is not sufficient to permit growth, however. The level of oxygen, moisture content, and temperature are examples of other factors that must be adjusted to permit the growth of the target microbe. For example, Escherichia coli requires a temperature of around 37° C [98.6° F], to match the temperature of its normal intestinal habitat. Refrigeration temperature, which does support the growth of some bacteria, does not support the growth of Escherichia coli.

The nature of the growth requirements of bacteria has been used as a means of grouping bacteria together. Photoautotrophic bacteria are those that use sunlight as a source of energy. Heterotrophic bacteria utilize organic carbon as a type of fuel for growth, while lithotrophic bacteria use inorganic carbon sources. As a final example, autotrophic bacteria are those bacteria that use carbon dioxide as the only source of carbon. Similarly, bacteria can be grouped according to their growth requirements with respect to temperature, salinity, the hydrogen ion concentration (also known as the pH ), and oxygen.

In the laboratory, growth media often requires the presence of a few growth factors that the bacteria or other microbe cannot make themselves. For example, depending on the particular bacteria being grown, certain amino acids may need to be supplied. Likewise, the inclusion of compounds known as purines and pyrimidines may be necessary for the manufacture of DNA or RNA. Lastly, some vitamins may need to be added, which allow some enzymes to function. Sometimes bacteria can become altered so that the daughter cell has specific nutritional requirements that the parent cell does not. The mutant strain is referred to as an auxotroph. For example, a strain of Escherichia coli that requires the amino acid tryptophane for growth is a tryptophan auxotroph and is designated as Escherichia coli trp. Auxotrophs can be very useful as markers, or indicators of the success of an experimental procedure.

Growth of microorganisms can occur in a liquid growth medium, which is termed a broth, or on a solid medium. Agar and agarose are two examples of solid growth media. Often a broth can be supplemented with the solidifying agent to form agar . Growth media can be very nonspecific with respect to nutrients. For example, Brain-Hear Infusion (BHI) broth or agar is a blended mixture of animal brain and heart. A medium such as BHI is also referred to as a complex medium. Other media contains defined amounts of specific components. This type of medium is also called a minimal medium.

Growth media can also be tailored to favor the growth of one or a few types of bacteria over the many other types of bacteria that would develop on a nonselective medium, or to provide an enriched environment for those bacteria that would other wise grow poorly or very slowly. An example of a medium that is both selective and enriched is that used to grow bacteria in the genus Halococcus. These bacteria require very high concentrations of salt. The high sodium chloride concentration of the medium is lethal for all other types of bacteria.

In conventional broth and agar cultures, the growth of microorganisms is uncontrolled. The cells grow as fast as possible for as long as possible. However, growth of bacteria is possible such that the rate of growth and division can be controlled. Devices that accomplish this are the chemostat and the turbidostat. Comparison of the chemical make-up of the same type of microbe under relatively fast growing or slow growing conditions can be very useful, particularly because in infections, bacterial growth can be slower than in the laboratory environment.

See also Agar and agarose; Blood agar, hemolysis, and hemolytic reactions; Colony and colony formation; Synchronous growth