Bacteria grown in a laboratory environment, like captive animals in a zoo, need to have everything provided for them—food, water, a suitable environment—in order to survive and thrive. Some microbes are not especially choosy in their requirements for growth, while others, such as Treponema pallidum, the causative agent of syphilis, has never been successfully grown in culture, although scientists have been trying to do so for more than 100 years.
Article Summary: Many types of bacterial growth media are used to culture bacteria in the laboratory. Here's a summary of defined, complex, selective and differential media.
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Page last updated: 4/2014
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MacConkey's Agar (MAC)
Bacterial Growth Medium
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SPO VIDEO: Blood Agar (BAP) Microbiology Lab Tutorial
Bauman, R. (2007). Microbiology with Diseases by Taxonomy. Pearson Benjamin Cummings.
Perry, J. and Stanley, J. (1997) Microbiology Dynamics & Diversity. Saunders College Publishing.
Selective Bacterial Growth Media
Selective media contain ingredients that inhibit the growth of certain types of bacteria and/or encourage the growth of others. This type of media is useful in helping to identify unknown bacteria and in encouraging the growth of only the types of bacteria that the microbiologist is interested in cultivating.
Differential Bacterial Growth Media
Differential culture media are formulated to display a color change when the bacteria growing metabolize a certain ingredient. For example MacConkey’s Agar, in addition to being selective, contains the sugar lactose and a pH sensitive dye. When bacteria growing on MAC ferment lactose (metabolize it for food), they generate acidic waste products that trigger the pH sensitive dye to turn the bacteria pink. So, when grown on MAC, colonies of Gram-negative, lactose fermenting bacteria are pink, the intensity of the pink color corresponds to how good the bacteria are at eating lactose. Colonies of Gram-negative non-lactose fermenting bacteria grow in colorless colonies.
Mannitol Salt Agar also contain food (mannitol, a sugar alcohol) and a pH sensitive dye. When the bacteria growing on MSA ferment mannitol, the medium changes from its original pink color to a bright highlighter yellow.
Bacteria that grow on MSA are all halophiles. If halophilic normal flora bacteria are growing on MSA (such as Staphylococcus epidermidis) the medium remains pink. If halophilic pathogens are present (such as Staphylococcus aureus), the medium changes to bright yellow.
Another specialized medium, Blood Agar (BAP) contains sheep’s blood, if bacteria growing on the medium produce exotoxins that hemolyze (cut up) the red blood cells, the medium changes color. BAP medium that has changed from red to transparent 9completely clear) indicates pathogen Staphylococcus pyogenes. BAP bruised or unaffected by bacterial growth present indicate normal flora, non-pathogenic bacteria.
For example MacConkey’s Agar (MAC) is used to cultivate Gram-negative bacteria, by discouraging the growth of Gram positive bacteria through the use of crystal violet dyes and bile salts. Another selective medium, Mannitol Salt Agar (MSA), has a high concentration of sodium chloride, which selects for halophiles (salt-loving bacteria) such as members of the genus Staphylococcus.
SPO VIDEO: Mannitol Salt Agar (MSA) Microbiology Lab Tutorial
Several basic types of media are discussed below. Although their differences are featured, there are several characteristics that all culture media have in common:
Media must be prepared in such a way that it is sterile prior to being inoculated with a bacterial sample, so that when a particular type of bacteria is cultured (cultivated) on that medium, it is the only type of bacteria present.
Growth media must also provide everything the bacterial culture needs to live and grow, including water, nutrients, and the proper pH. Media can be either liquid (nutrient broth) or solid (agar).
Defined Media versus Complex Media
Some media formulations are very specific recipes in which certain ingredients must be present in specific amounts. These defined media (also known as synthetic media) are used to grow bacteria that have very particular needs.
Most clinical cultures do not have such exacting requirements, and can be grown in what is referred to as “complex media”. Complex media are composed of partially digested yeast, beef, soy and additional proteins, in which the exact concentration and composition is unknown. In comparison with defined media, which are good for growing bacteria with very particular needs, complex media can be thought of as a crowd-pleaser, suitable for growing many different types of less fastidious microbes.
In addition to growth media formulations being classified as either defined or complex, there are also media that are designed to do more than just grow bacteria, selective and differential media provide information about the bacteria growing.