agar

(noun)

A gelatinous material obtained from the marine algae, used as a bacterial culture medium, in electrophoresis and as a food additive.

Examples of agar in the following topics:

  • Pure Culture

    • For the purpose of gelling the microbial culture, the medium of agarose gel (agar) is used.
    • Agar is a gelatinous substance derived from seaweed.
    • A cheap substitute for agar is guar gum, which can be used for the isolation and maintenance of thermophiles .
    • Microbiological cultures can be grown in petri dishes of differing sizes that have a thin layer of agar-based growth medium.
    • Describe how pure microbial cultures can be grown in agar-based growth medium

  • Viable Cell Counting

    • Typical media include Plate count agar for a general count or MacConkey agar to count gram-negative bacteria such as E. coli.
    • The initial analysis is done by mixing serial dilutions of the sample in liquid nutrient agar which is then poured into bottles.
    • The bottles are then sealed and laid on their sides to produce a sloping agar surface.
    • In the pour plate method a diluted bacterial sample is mixed with melted agar and then that mixture is poured into a petri dish.
    • Urine cultured on Oxoid Brilliance UTI Agar plate. 1uL of urine spread onto the agar surface.

  • Selective and Differential Media

    • Mannitol salt agar (MSA) which is selective for Gram-positive bacteria and differential for mannitol.
    • 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.
    • Blood agar (used in strep tests), which contains bovine heart blood that becomes transparent in the presence of hemolytic.
    • MacConkey (MCK), which is differential for lactose fermentationmannitol salt agar (MSA), which is differential for mannitol fermentation.

  • Culture Media

    • These are often mixed with agar and poured 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.
    • Many microbes can also be grown in liquid cultures comprised of liquid nutrient media without agar.
    • Red blood cells are used to make an agar plate.

  • Enrichment and Isolation

    • The most common growth media for microorganisms are nutrient broths and agar plates; specialized media are required for some microorganisms.
    • Blood agar is an enriched medium in which nutritionally-rich whole blood supplements the basic nutrients.
    • Chocolate agar is enriched with heat-treated blood (40-45°C), which turns brown and gives the medium the color for which it is named.
    • The agar triple-sugar iron (TSI) is one of the culture media used for the differentiation of most enterobacteria.
    • The agar triple-sugar iron is one of the culture media used for the differentiation of most enterobacteria.

  • Detecting Acid and Gas Production

    • Sorbitol MacConkey agar is a variant of the traditional MacConkey commonly used in the detection of E. coli O157:H7 .
    • Traditionally, MacConkey agar has been used to distinguish those bacteria that ferment lactose from those that do not.
    • In sorbitol MacConkey agar, lactose is replaced by sorbitol.
    • Sorbitol fermenting non pathogenic commensal bacteria from faeces growing on Cefixime Tellurite Sorbitol MacConkey Agar.

  • Hydrophilic and Hydrophobic Colloids

    • For example, agar is a reversible hydrocolloid of seaweed extract; it can exist in a gel or liquid state and can alternate between states with either heating or cooling.

  • Diagnosing Microbial Diseases

    • The streak plate method is a way to physically separate the microbial population, and is done by spreading the inoculate back and forth with an inoculating loop over the solid agar plate.
    • It is established that most pathogenic bacteria can be grown on nutrient agar, and the addition or subtraction of specific nutrients can aid in further identification.
    • This represents four nutrient agar plates with various bacterial species represented.

  • Kirby-Bauer Disk Susceptibility Test

    • Then, a known quantity of bacteria are grown overnight on agar (solid growth media) plates in the presence of a thin wafer that contains a known amount of a relevant antibiotic.
    • In Kirby–Bauer testing, discs containing antibiotics are placed on agar where bacteria are growing, and the antibiotics diffuse out into the agar.

  • Precipitation Reactions

    • These can be performed in semisolid media such as agar or agarose, or non-gel support media such as cellulose acetate.
    • Precipitation methods include double immunodiffusion (qualitative gel technique that determines the relationship between antigen and antibody), radial immunodiffusion (semi-quantitation of proteins by gel diffusion using antibody incorporated in agar), and electroimmunodiffusion (variation of the double immunodiffusion method reaction that uses an electric current to enhance the mobility of the reactants toward each other).