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mnoll Group Title

why is seletive media useful?

  • 2 years ago
  • 2 years ago

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  1. naf Group Title
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    Selective media are used for the growth of only selected microorganisms. For example, if a microorganism is resistant to a certain antibiotic, such as ampicillin or tetracycline, then that antibiotic can be added to the medium in order to prevent other cells, which do not possess the resistance, from growing. Media lacking an amino acid such as proline in conjunction with E. coli unable to synthesize it were commonly used by geneticists before the emergence of genomics to map bacterial chromosomes. Selective growth media are also used in cell culture to ensure the survival or proliferation of cells with certain properties, such as antibiotic resistance or the ability to synthesize a certain metabolite. Normally, the presence of a specific gene or an allele of a gene confers upon the cell the ability to grow in the selective medium. In such cases, the gene is termed a marker. Selective growth media for eukaryotic cells commonly contain neomycin to select cells that have been successfully transfected with a plasmid carrying the neomycin resistance gene as a marker. Gancyclovir is an exception to the rule as it is used to specifically kill cells that carry its respective marker, the Herpes simplex virus thymidine kinase (HSV TK). Four types of agar plates demonstrating differential growth depending on bacterial metabolism. Some examples of selective media include: eosin-methylene blue agar (EMB) that contains methylene blue – toxic to Gram-positive bacteria, allowing only the growth of Gram negative bacteria YM (yeast and mold) which has a low pH, deterring bacterial growth blood agar (used in strep tests), which contains bovine heart blood that becomes transparent in the presence of hemolytic Streptococcus MacConkey agar for Gram-negative bacteria Hektoen enteric agar (HE) which is selective for Gram-negative bacteria mannitol salt agar (MSA) which is selective for Gram-positive bacteria and differential for mannitol Terrific Broth (TB) is used with glycerol in cultivating recombinant strains of Escherichia coli. xylose lysine desoxyscholate (XLD), which is selective for Gram-negative bacteria buffered charcoal yeast extract agar, which is selective for certain gram-negative bacteria, especially Legionella pneumophila

    • 2 years ago
  2. biochic_pc Group Title
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    The easiest answer to your question would be to say that selective media is useful because it allows only certain organisms to grow by interfering with the growth of some organisms while allowing others to grow normally. So, a media which is selective for gram negative cells, such as EMB or MacConkey would inhibit growth of gram positive cells through the use of a selective toxin. In the case of the EMB plate, methylene blue is toxic to gram positive cells so only the gram negative cells will grow. In the case of the Mac Conkey plate, the bile salts in the media are toxic to the gram positive cells allowing only gram negative cells to grow. So, not only do we know what kind of conditions our organism like to grow in, we also know something about the structural composition of our cells, which helps us identify our organism. If we are working with eukaryotic cells, selective media might look a little different since there are no major structural differences to select for only one kind of bacteria, as with peptidoglycan in bacteria. In these cases, antibiotic sensitivity is frequently used to select particular strains of cells. In these cases, a particular strain of cell is usually genetically modified to be either resistant to an antibiotic and the presence of that antibiotic in the growth media will inhibit any cells which are not resistant, thereby helping to weed-out cells with are not of interest to the researcher.

    • 2 years ago
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