Like other
growth media, the formulations of agar used in plates may be classified as either "defined" or "undefined"; a defined medium is synthesized from individual chemicals required by the organism so the exact molecular composition is known, whereas an undefined medium is made from natural products such as
yeast extract, where the precise composition is unknown. Agar plates may be formulated as either permissive, with the intent of allowing the growth of whatever organisms are present, or restrictive or selective, with the intent of only allowing the growth of a particular subset of those organisms. This may take the form of a nutritional requirement, for instance providing a particular compound such as
lactose as the only source of
carbon and thereby selecting only organisms which can
metabolize that compound, or by including a particular antibiotic or other substance to select only organisms which are
resistant to that substance. This correlates to some degree with defined and undefined media; undefined media, made from natural products and containing an unknown combination of very many organic molecules, is typically more permissive in terms of supplying the needs of a wider variety of organisms. In contrast, defined media can be precisely tailored to select organisms with specific properties. Agar plates may also be indicator plates, in which the organisms are not selected based on growth, but are instead distinguished by a color change in some colonies, typically caused by the action of an
enzyme on some compound added to the medium. The plates are incubated for 12 hours up to several days, depending on the test that is performed. Commonly used types of agar plates include: on an agar plate are used to diagnose
infection. On the left is a positive
Staphylococcus infection, on the right a positive
Streptococcus culture.
Blood agar (left) α-hemolysis (
S. mitis); (middle) β-hemolysis (
S. pyogenes); (right) γ-hemolysis (= non-hemolytic,
S. salivarius)
Blood agar plate Blood agar plates (BAPs) contain mammalian blood (usually sheep or horse), typically at a 5–10% concentration. BAPs are enriched, and differential media is used to isolate
fastidious organisms and detect
hemolytic activity. β-Hemolytic activity will show lysis and complete digestion of red blood cell contents surrounding a colony. Examples include
Streptococcus haemolyticus. α-Hemolysis will only cause partial lysis of the red blood cells (the cell membrane is left intact) and appear green or brown due to the conversion of hemoglobin to methemoglobin. An example of this would be
Streptococcus viridans. γ-Hemolysis (or non-hemolytic) is the term referring to a lack of hemolytic activity. BAPs also contain
meat extract or
yeast extract,
tryptone,
sodium chloride, and agar.
Chocolate agar Chocolate agar is a type of blood agar plate in which the blood cells have been
lysed by heating the cells to 80 °C. It is used for growing fastidious respiratory bacteria, such as
Haemophilus influenzae. Chocolate agar is named for its color, and no
chocolate is contained in the plate.
Thayer–Martin agar Thayer–Martin agar is a chocolate agar designed to isolate
Neisseria gonorrhoeae and
Neisseria meningitidis.
Thiosulfate–citrate–bile salts–sucrose agar Thiosulfate–citrate–bile salts–sucrose agar enhances growth of
Vibrio spp., including
Vibrio cholerae.
General bacterial media •
Bile esculin agar is used for the isolation of
Enterococcus and
group D Streptococcus species. •
CLED agar –
cysteine,
lactose, electrolyte-deficient agar is used to isolate and differentiate urinary tract bacteria, since it inhibits
Proteus species swarming and can distinguish between lactose fermenters and nonfermenters. •
Granada medium is used to isolate and differentiate group B
Streptococcus,
Streptococcus agalactiae from clinical samples. It grows in Granada medium as red colonies, and most of the accompanying bacteria are inhibited. •
Hektoen enteric agar is designed to isolate and recover fecal bacteria of the family
Enterobacteriaceae. It is particularly useful in isolating
Salmonella and
Shigella. •
Lysogeny broth is used to culture
Escherichia coli. •
MacConkey agar is a selective and differential medium used to differentiate between
gram-negative bacteria while inhibiting the growth of
gram-positive bacteria. Adding bile salts and
crystal violet to the agar inhibits the growth of most gram-positive bacteria, making MacConkey agar selective. Lactose and
neutral red are added to differentiate the lactose fermenters, which form pink colonies, from lactose nonfermenters that form clear colonies. An alternative medium,
eosin methylene blue serves a similar purpose. •
Mannitol salt agar is also a selective and differential medium. The
mannitol indicates organisms that ferment mannitol: mannitol fermentation produces
lactic acid, lowering the pH and turning the plate yellow. The salt is to select for
halophiles; organisms that cannot withstand a high salt content are unable to grow well. •
Mueller–Hinton agar contains beef infusion, peptone, and
starch, and is used primarily for antibiotic susceptibility testing. It can be in a form of
blood agar. •
Nutrient agar is usually used for growth of nonfastidious organisms and observation of pigment production. It is safe to use in school science laboratories because it does not selectively grow
pathogenic bacteria. •
Önöz agar allows more rapid bacteriological diagnosis, as
Salmonella and
Shigella colonies can be clearly and reliably differentiated from other Enterobacteriaceae. The yields of
Salmonella from stool samples obtained, when using this medium, are higher than those obtained with LEIFSON agar or
Salmonella–Shigella agar. •
Phenylethyl alcohol agar selects for
Staphylococcus species while inhibiting Gram-negative bacilli (e.g.,
Escherichia coli,
Shigella,
Proteus, etc.). •
R2A agar, a nonspecific medium, imitates water, so is used for water analysis. •
Tryptic (trypticase) soy agar (TSA) is a general-purpose medium produced by enzymatic digestion of
soybean meal and
casein. It is frequently the base medium of other agar types; for example, blood agar plates are made by enriching TSA plates with blood. TSA plates support growth of many semifastidious bacteria, including some species of
Brucella,
Corynebacterium,
Listeria,
Neisseria, and
Vibrio. •
Xylose-
lysine-
deoxycholate agar is used for the culture of
stool samples and contains two indicators. It is formulated to inhibit Gram-positive bacteria, while the growth of Gram-negative
bacilli is encouraged. The colonies of lactose fermenters appear yellow. It is also used to culture possible
Salmonella that may be present in a food sample. Most
Salmonella colonies produce a black centre on it. •
Cetrimide agar is used for the selective isolation of the Gram-negative bacterium
Pseudomonas aeruginosa. • Tinsdale agar contains
potassium tellurite, which can isolate
Corynebacterium diphteriae.
Yeast media '' growing both as yeast cells and
filamentous cells on YPD agar •
YEPD media is often used as a general growth media for yeasts like
Saccharomyces cerevisiae and
Candida albicans • Sporulation medium is medium used when spores have to be formed. It can also be used when working with fungi or bacteria depending on whether or not the strain is capable of forming spores.
Mega Plate • A 2' x 4' petri plate filled with 14L (liters) of seaweed derived agar medium created by Harvard scientists that was used to see how
E. coli evolved to be resistant to antibiotics. The mega plate also helped study more unique concepts of microbiology such as parallel evolution, mutation selection, colonial interference etc. == See also ==