Many bacteria, yeasts and molds that occur in nature can also be grown in the laboratory. In addition to water, the culture media used for this purpose must contain the nutrients the microorganisms need for growth, in particular
The most commonly used growth media for microorganisms are liquid media or nutrient broths containing nutrients. In such broths, microorganisms usually grow exponentially until growth is limited by either insufficient nutrient availability or the accumulation of growth-inhibiting substances.
Solid media, on the other hand, allow the growth and isolation of individual microbial colonies on their surface. Each colony originates from a single colony forming unit (CFU), usually a single bacterium. Therefore, all the cells in a colony are assumed to be of the same strain and species. Solid culture media are prepared by adding 1 to 2% agar, a mixture of agarose and agaropectin of algal origin that very few bacteria can decompose (liquefy). This media is autoclaved, cooled down to about 45 °C, and subsequently poured into Petri dishes where they cool and solidify. The gelatinous mass supplies water and nutrients to the microorganisms that are inoculated onto the agar surface.
Semisolid media are prepared with lower agar concentrations of 0.2 to 0.5%. They have a soft, custard-like consistency and are used to cultivate microaerophilic bacteria or determine bacterial motility by cultivation in stab tubes and must be handled in an upright position. Semisolid media can be used to distinguish between typhoid and colon bacilli.
Ready-to-use media are a convenient option for microbiology laboratories. The liquid media are readily sterilized and delivered in plastic bags, tubes and bottles of various sizes, whereas the solid media come readily poured into Petri dishes and slant tubes or in larger bottles. For agar media in bottles, re-melting and cooling down to 44-47 °C is necessary before pouring the agar plates.
Dehydrated culture media, on the other hand, are commercially available in long-time stable form (up to 5 years), either powders or granules, that contain all the ingredients of the growth media, except water. In the microbiological laboratory, these dehydrated media must be dissolved in water and autoclaved before use.
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Selective media allow only growth to a certain spectrum of organisms, for example, all Enterobacteriaceae or all bacteria from the genus Salmonella. Selectivity is achieved by composing the media to be optimal for the growth of the desired microorganisms and/or unfavorable for other organisms. This can include adding antibiotics, high salt concentrations, low pH or inhibitors like for example bile salts. Selective media in microbiology are particularly useful where the desired organism is only one among many in the inoculum. An example of selective media is mannitol salt agar. It contains a high concentration of sodium chloride that inhibits the growth of most organisms but permits staphylococci to grow.
Enriched media, on the other hand, are semi-selective: they contain supplements such as blood, serum, and egg yolk to enhance the growth of fastidious bacteria among the undemanding ones. While enriched media usually come in agar plates, enrichment media are for liquid cultivation. These media also favor the growth of a particular microorganism over others in a sample. Enrichment is often performed to have enough material of an organism available for subsequent detection, isolation and/or identification.
General-purpose media (also known as basic or complex media) are routine culture media that microbiology laboratories use to cultivate a broad spectrum of microorganisms. They contain complex organic ingredients such as yeast, animal, or plant material that are not chemically precisely determined, so they vary slightly in composition from batch to batch. These media are used to culture non-fastidious microorganisms that do not need extra growth nutrients. Commonly used general-purpose media in the microbiological laboratory include nutrient agar and broth, peptone water, as well as tryptic soy broth and agar.
Synthetic media, on the other hand, are wholly composed of substances that are chemically known. These media are used to study the physiology, metabolism, and nutritional requirements of certain microorganisms.
Differential media (or indicator media) contain substances that lead colonies of certain organisms to take on a distinctive appearance. These media thus allow the microbiologist to differentiate various kinds of microorganisms on the same agar plate. An example of differential media is blood agar. If blood agar is inoculated with a mixed culture of bacteria, some of these may produce enzymes that hemolyze the contained red blood cells, forming a hemolytic zone around colonies. This helps to distinguish between hemolytic and nonhemolytic bacteria. There are also differential media that contain pH indicators, which change color if, for example, a bacterial species produces acid during growth that lowers the pH of the medium.
Anaerobic bacteria tolerate little or no oxygen, so their media may have to be reduced by physical or chemical means. Boiling the media and adding reducing agents such as sodium thioglycolate helps to rid these media of dissolved oxygen. A typical way to keep oxygen out during incubation is to seal the media with sterile liquid paraffin. Anaerobic culture media can also be used to study the anaerobic metabolism of facultative anaerobes, i.e., microorganisms that grow aerobically if oxygen is present but switch to fermentation in the absence of oxygen.
Specimens or samples containing microorganisms often must be transported between microbiology labs or stored away to be used again weeks, months, or even years later. Therefore, the chosen media must maintain the viability of the microorganisms. During transportation of microbial samples, the concentrations of organisms must not change, which is achieved by preventing them from multiplying. Therefore, most transport media lack the usual ingredients of culture media and contain only substances that do not promote microbial growth, for example, buffers and salts. During longer storage periods, bacteria are periodically sub-cultured onto freshly prepared culture media. For long-term preservation, it is possible to lyophilize (freeze-dry) most microbes.
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