Isolating bacteria in the form of pure cultures is very important in order to obtain individual well distanced colonies which would assist in carrying out further identification and microbiological tests on the bacteria of interest. Producing these bacterial pure cultures can be controlled by preventing or promoting the growth of other microorganisms that can be found in the surrounding area within the agar plate using some types of supplemented growth media. Usually pH indicators are included within the media so as to indicate the growth of the bacterial colony of interest and make a distinction between them and the other bacteria growing on the same media.
Whilst media such as nutrient agar are used to support the growth of a variety of microorganisms, various other media have been particularly designed for the isolation and identification of specific microorganisms. The different types of media used to isolate, identify and grow the bacteria of interest, are selective media, differential media and enriched media.
The concept of selective media is usually a simple one. Some of the ingredients in the medium favor the growth of the targeted bacteria. Other components of the medium such as antibiotics, salt and surfactants can inhibit the growth of competing bacteria. In addition to this the selective media are known to be not entirely selective media. So this makes it sometimes difficult to confirm the identity of suspect pathogens isolated on selective media which then has to be confirmed by biochemical or genetic methods (Montville and Matthews, 2008). Selective media such as bismuth sulphite medium preferentially support the growth of particular bacteria. The bismuth ion inhibits the growth of Gram-positive organisms as well as many Gram-negative types; this medium is used for the isolation of the pathogenic bacterium Salmonella typhi, one of the few organisms that can tolerate the bismuth (Hogg, 2005).
[...] The different types of media used to isolate, identify and grow the bacteria of interest, are selective media, differential media and enriched media. The concept of selective media is usually a simple one. Some of the ingredients in the medium favor the growth of the targeted bacteria. Other components of the medium such as antibiotics, salt and surfactants can inhibit the growth of competing bacteria. In addition to this the selective media are known to be not entirely selective media. [...]
[...] DISCUSSION MacConkey agar is a selective agar which only allows some bacteria to grow on it. It inhibits the growth of some Gram positive bacteria. Conversely, the nutrient agar allows the growth of many different types of bacteria. The nutrient agar was inoculated before the MacConkey agar 1 and 3 in order to prevent the transfer of inhibitory substances such as the bile salts, from the MacConkey agar to the nutrient agar and therefore, if the MacConkey agar was inoculated first, the growth of Gram positive bacteria on the nutrient agar would be inhibited. [...]
[...] Microbial nutrition and cultivation.In Essential microbiology, pp. 87-89.West Sussex: John Wiley & Sons Ltd. Montville, T.J., Matthews, K.R. (2008). Factors that influence microbes in food. In Food microbiology: an Introduction, 2ndedn, p Washington, DC: American Society for Microbiology Press. [...]
[...] OBJECTIVES This experiment aimed at understanding the theory behind the application of selective, enriched and differential media in identifying and growing bacteria. Furthermore, a few examples of these different types of media were used to isolate and differentiate some species of bacteria. RESULTS The cellular morphology of the different types of bacteria in the mixed broth culture was examined after a heat-fixed smear of the mixture was stained using the Gram staining technique. The bacteria in the smear were observed under a light microscope at 100 magnification. [...]
[...] In the MacConkey agar there were three colonies, of which two were red and the other beige. In the MacConkey agar there were 2 colonies, one of which was red and the other beige. The colonies had different morphology of different shapes and different morphology. Table Cellular morphology of bacteria in the colorless, red, and beige colonies on the nutrient agar and the MacConkey agar 1 and 3 (MAC 1 & MAC and the names of the predicted bacteria from the final gram stain results under 100 magnification. [...]
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