scientist tests for
Salmonella '' bacteria The most common growth media for microorganisms are nutrient broths (liquid nutrient medium) or
lysogeny broth medium. Liquid media are often mixed with
agar and poured via a sterile
media dispenser into
Petri dishes to solidify. These
agar plates provide a solid medium on which microbes may be cultured. They remain solid, as very few bacteria are able to decompose agar (the exception being some species in the genera:
Cytophaga,
Flavobacterium,
Bacillus,
Pseudomonas, and
Alcaligenes). Bacteria grown in liquid cultures often form
colloidal
suspensions. The difference between growth media used for cell culture and those used for microbiological culture is that cells derived from whole organisms and grown in culture often cannot grow without the addition of, for instance,
hormones or
growth factors which usually occur
in vivo. In the case of animal cells, this difficulty is often addressed by the addition of
blood serum or a synthetic serum replacement to the medium. In the case of microorganisms, no such limitations exist, as they are often
unicellular organisms. One other major difference is that animal cells in culture are often grown on a flat surface to which they attach, and the medium is provided in a liquid form, which covers the cells. In contrast, bacteria such as
Escherichia coli may be grown on solid or in liquid media. An important distinction between growth media types is that of
chemically defined versus undefined media. 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. Minimal media can also be used to select for or against
recombinants or
exconjugants. Minimal medium typically contains: • a carbon source, which may be a sugar such as glucose, or a less energy-rich source such as
succinate • various salts, which may vary among bacteria species and growing conditions; these generally provide essential elements such as
magnesium,
nitrogen,
phosphorus, and
sulfur to allow the bacteria to synthesize
protein and
nucleic acids • water Supplementary minimal media are minimal media that also contains a single selected agent, usually an amino acid or a sugar. This supplementation allows for the culturing of specific lines of
auxotrophic recombinants.
Selective media ''
culture; on the left is a positive Streptococcus'' culture. 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 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. Examples of selective media: •
Eosin methylene blue contains dyes that are toxic for Gram-positive bacteria. It is the selective and differential medium for coliforms. •
YM (
yeast extract agar) has a low
pH, deterring bacterial growth. • MEA (
malt extract agar) has a low
pH, deterring bacterial growth. •
MacConkey agar is for
Gram-negative bacteria. •
Hektoen enteric agar is selective for Gram-negative bacteria. •
HIS-selective medium is a type cell culture medium that lacks the amino acid histidine. •
Mannitol salt agar is selective for gram-positive bacteria and differential for mannitol. •
Xylose lysine deoxycholate is selective for Gram-negative bacteria. •
Buffered charcoal yeast extract agar is selective for certain gram-negative bacteria, especially
Legionella pneumophila. •
Baird-Parker agar is for gram-positive
staphylococci. •
Sabouraud agar is selective to certain fungi due to its low pH (5.6) and high glucose concentration (3–4%). • DRBC (dichloran rose Bengal chloramphenicol agar) is a selective medium for the enumeration of moulds and yeasts in foods. Dichloran and rose Bengal restrict the growth of mould colonies, preventing overgrowth of luxuriant species and assisting accurate counting of colonies. •
MMN (Modified Melin-Norkrans) medium and
BAF medium are used for
ectomycorrhizal fungi. •
Columbia Nalidixic Acid (CNA) agar contains antibiotics (nalidixic acid and colistin) that inhibit Gram-negative organisms, aiding in the selective isolation of
Gram-positive bacteria.
Differential media s (UTIs). Differential or indicator media distinguish one microorganism type from another growing on the same medium. 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. These media are used for the detection of microorganisms and by molecular biologists to detect recombinant strains of bacteria. Examples of differential media: •
Blood agar (used in
strep tests) contains bovine heart blood that becomes transparent in the presence of β-hemolytic organisms such as
Streptococcus pyogenes and
Staphylococcus aureus. •
Eosin methylene blue is differential for lactose fermentation. •
Granada medium is selective and differential for
Streptococcus agalactiae (group B streptococcus) which grows as distinctive red colonies in this medium. •
MacConkey agar is differential for lactose fermentation. •
Mannitol salt agar is differential for mannitol fermentation. •
X-gal plates are differential for
lac operon mutants.
Transport media Transport media should fulfill these 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. Examples of transport media: •
Thioglycolate broth is for strict
anaerobes. • Stuart transport medium is a non-nutrient soft agar gel containing a reducing agent to prevent oxidation, and charcoal to neutralize. • Certain bacterial inhibitors are used for gonococci, and buffered glycerol saline for enteric bacilli. • Venkataraman Ramakrishna (VR) medium is used for
V. cholerae. == Enriched media ==