Sunday, September 11, 2011

Types of Bacteriological media

Various artificial culture media are employed in the diagnostic laboratory to facilitate the growth of medically important bacteria to be detectable by the naked eye. These media are essentially recipes prepared from digests of animal or plant protein supplemented with nutrients such as glucose , yeast extract , serum or blood to meet the metabolic requirements of the organisms. Some organisms, termed fastidious organisms, require specialized environments due to complex nutritional requirements. Viruses, for example, are obligate intracellular parasites and require a growth medium containing living cells ( Madigan and Martinko, 2005).
Bacteriologic media can be prepared in solid (agar), semisolid and liquid (broth) forms to which bacteria or clinical specimens may be added directly. There are two major types of growth media: those used for cell culture, which use specific cell types derived from plants or animals, and microbiologic culture, which are used for growing microorganisms, such as bacteria or yeast ( Ryan and Ray, 2004).
Bacteriology media may be classified as nutrient ,minimal , enrichment, selective , differential or transport media according to their main function in the laboratory ( Loboffe et al .,2005).

Nutrient media are designed to provide the growth requirements of most nonfastidious bacteria without giving growth advantage to any particular organism (e.g. Mueller-Hinton medium). Minimal media are those that contain the minimum nutrients possible for colony growth, generally without the presence of amino acids, and are often used by microbiologists and geneticists to grow "wild type" microorganisms (Park ., 2008).
Enrichment media contain specific nutrients which enhance the growth of a particular bacterial pathogen from a mixture of organisms on the basis of nutrient specificity (e.g. chocolate agar, blood agar ).Selective media contain dyes, bile salts, acids , alcohols or antimicrobial agents that are inhibitory to all organisms except those being sought (e.g. Mannitol salt agar )( Schauer and Cynthia, 2009).
Differential media distinguish one microorganism type from another growing on the same media. This type of media uses the biochemical characteristics of a microorganism growing in the presence of specific nutrients or indicators (such as neutral red, phenol red, eosin y, or methylene blue) added to the medium to visibly indicate the defining characteristics of a microorganism. This type of media is used for the detection of microorganisms and by molecular biologists to detect recombinant strains of bacteria. ( e.g. MacConkey sorbitol agar ) ( Stratton et al ., 2010)
Transport media should fulfill the following criteria:temporary storage of specimens being transported to the laboratory for cultivation, maintain the viability of all organisms in the specimen without altering their concentration, contain only buffers and salt, lack of carbon, nitrogen, and organic growth factors so as to prevent microbial multiplication, transport media used in the isolation of anaerobes must be free of molecular oxygen. (e.g. Thioglycolate broth for strict anaerobes.) ( Schauer and Cynthia, 2009)

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