Culture Media in Microbiology
The science of microbiology involves studying organisms too small to see with the naked eye, such as bacteria. Typically, these microorganisms are present at levels in samples too low for most testing procedures. Therefore, a microbiologist places a sample in growth media and incubates it for a certain time to allow the organisms to reproduce to testable levels. As the microbiologist wishes to study only the organisms present in a sample, most media are sterilized in an autoclave although some are simply heated on a hotplate to kill contaminants.
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Types
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Culture media in microbiology can be solid, semisolid or liquid when an analyst is finished preparing them. For practicality, most labs buy culture media in a powder form that an analyst then mixes with water. Alternatively, a lab can buy prepared sterile media ready for use.
The solid and semisolid media generally contain a substance called agar, derived from marine algae. The agar is not a food source for the microorganisms but is present to solidify the media into a jelly-like solid form. Solid or semi-solid media is also known simply as "agar." Analysts dispense agar into petri dishes (or agar plates) or pour small amounts into a test tube. Sometimes an analyst places the tube at an angle for the agar to soldify, known as a "slant."
Liquid media are called broths, and, usually, microbiologists keep broth in test tubes or glass bottles.
Complex Media
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Sometimes a microbiologist wants to get a general idea of the amount and type of microorganisms present in a sample. Therefore, he uses media that allow lots of organisms to grow. This type of media is known as complex media since it contains lots of different vitamins, minerals, proteins and carbohydrates. Complex media generally come from plant or animal sources, which have lots of different organic components, instead of from factories, which simply mix known substances together.
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Selective Media
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Selective media contain known amounts of particular substances. Every microbe has nutrient preferences and is able to grow only in certain circumstances. A culture medium with a specific set of characteristics will encourage only some microbes to grow and prevent others from growing. Therefore, a microbiologist will be able to see whether his sample contains microbes of particular interest. Selective media may have a higher than usual salt concentration or use blood as an ingredient for some pathogenic bacteria to grow on, for example.
Examples
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Tryptic soy agar (TSA), which uses nutrients from the soy plant, is an example of a common complex solid medium. A liquid version is tryptic soy broth. These media allow a wide range of microbes to grow. A selective agar example is MacConkey agar, which contains lactose and allows microbes such as gastrointestinal pathogens to grow. MacConkey broth is a liquid medium alternative. Sometimes a complex medium can, with the addition of a particular substance, perform as a selective medium. For example, TSA with sheep's blood allows a microbiologist to identify the presence of pathogenic bacteria that break down blood.
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