Agar plate

In 1881, Fanny Hesse, who was working as a technician for her husband Walther Hesse in the laboratory of Robert Koch, suggested agar as an effective setting agent, since it had been commonplace in jam making for some time.^[3]

An agar plate being viewed in an electronic colony counter

An agar culture of E. coli colonies

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.^[4]

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 growth a particular subset of those organisms.^[5] 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, while 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 on the basis of 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.^[6]

The plates are incubated for 12 hours up to several days depending on the test that is performed.

Some commonly used agar plate types are:-

Red blood cells 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

Hemolyses of Streptococcus spp. (left) α-hemolysis (S. mitis); (middle) β-hemolysis (S. pyogenes); (right) γ-hemolysis (= nonhemolytic, S. salivarius)

Blood agar plate

Blood agar plates (BAPs) contain mammalian blood (usually sheep or horse), typically at a concentration of 5–10%. BAPs are enriched, differential media 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 will appear green or brown, due to the conversion of hemoglobin to methemoglobin. An example of this would be Streptococcus viridans. γ-Hemolysis (or nonhemolytic) is the term referring to a lack of hemolytic activity.^[7] BAPs also contain meat extract or yeast extract, tryptone, sodium chloride, and agar.^[8]

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 actually contained in the plate.

Horse blood agar

Horse blood agar is a type of blood-enriched microbiological culture media. As it is enriched, it allows the growth of certain fastidious bacteria, and allows indication of haemolytic activity in these bacterial cultures.

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.^[9]

General bacterial media

Four types of agar plate demonstrating differential growth depending on bacterial metabolism

• 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 differentiate 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 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.^[10]
• MacConkey agar is a selective and differential medium used to differentiate between gram-negative bacteria while inhibiting the growth of gram-positive bacteria. The addition of 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.^[11]
• 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.

Fungi (ascomycetes) growing in axenic cultures, each of which is a culture of one selected organism and is free of all other organisms, enabling study of the cultured organism in isolation

Aspergillus niger growing on potato dextrose 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.^[9]

Fungal media

Bottom view of a Sabouraud agar plate with a colony of Trichophyton rubrum var. rodhaini

• Sabouraud agar is used to culture fungi and has a low pH that inhibits the growth of most bacteria; it also contains the antibiotic gentamicin to specifically inhibit the growth of Gram-negative bacteria.
• Hay infusion agar is specific for the culturing of slime moulds (which are not fungi).
• Potato dextrose agar is used to culture certain types of fungi.
• Malt extract agar has a high content of peptone and is acidic. It is essentially used in the isolation of fungal microorganisms.
• MMN (Modified Melin-Norkrans) medium and BAF medium are used for ectomycorrhizal fungi.

Moss media

• Knop agar is used to axenically culture protonema and whole moss plants, such as those of Physcomitrella patens, a model organism.^[12]

Yeast media

• 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.^[13]

the yeast Candida albicans growing both as yeast cells and filamentous cells on YPD agar

Discover more about Types related topics

Escherichia coli

Escherichia coli

Escherichia coli, also known as E. coli, is a Gram-negative, facultative anaerobic, rod-shaped, coliform bacterium of the genus Escherichia that is commonly found in the lower intestine of warm-blooded organisms. Most E. coli strains are harmless, but some serotypes (EPEC, ETEC etc.) can cause serious food poisoning in their hosts, and are occasionally responsible for food contamination incidents that prompt product recalls. Most strains do not cause disease in humans and are part of the normal microbiota of the gut; such strains are harmless or even beneficial to humans (although these strains tend to be less studied than the pathogenic ones). For example, some strains of E. coli benefit their hosts by producing vitamin K2 or by preventing the colonization of the intestine by pathogenic bacteria. These mutually beneficial relationships between E. coli and humans are a type of mutualistic biological relationship — where both the humans and the E. coli are benefitting each other. E. coli is expelled into the environment within fecal matter. The bacterium grows massively in fresh faecal matter under aerobic conditions for three days, but its numbers decline slowly afterwards.

Growth medium

Growth medium

A growth medium or culture medium is a solid, liquid, or semi-solid designed to support the growth of a population of microorganisms or cells via the process of cell proliferation or small plants like the moss Physcomitrella patens. Different types of media are used for growing different types of cells.

Lactose

Lactose

Lactose is a disaccharide sugar synthesized by galactose and glucose subunits and has the molecular formula C12H22O11. Lactose makes up around 2–8% of milk (by mass). The name comes from lac (gen. lactis), the Latin word for milk, plus the suffix -ose used to name sugars. The compound is a white, water-soluble, non-hygroscopic solid with a mildly sweet taste. It is used in the food industry.

Carbon

Carbon

Carbon is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent—its atom making four electrons available to form covalent chemical bonds. It belongs to group 14 of the periodic table. Carbon makes up about 0.025 percent of Earth's crust. Three isotopes occur naturally, 12C and 13C being stable, while 14C is a radionuclide, decaying with a half-life of about 5,730 years. Carbon is one of the few elements known since antiquity.

Metabolism

Metabolism

Metabolism is the set of life-sustaining chemical reactions in organisms. The three main functions of metabolism are: the conversion of the energy in food to energy available to run cellular processes; the conversion of food to building blocks for proteins, lipids, nucleic acids, and some carbohydrates; and the elimination of metabolic wastes. These enzyme-catalyzed reactions allow organisms to grow and reproduce, maintain their structures, and respond to their environments. The word metabolism can also refer to the sum of all chemical reactions that occur in living organisms, including digestion and the transportation of substances into and between different cells, in which case the above described set of reactions within the cells is called intermediary metabolism.

Enzyme

Enzyme

Enzymes are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules known as products. Almost all metabolic processes in the cell need enzyme catalysis in order to occur at rates fast enough to sustain life. Metabolic pathways depend upon enzymes to catalyze individual steps. The study of enzymes is called enzymology and the field of pseudoenzyme analysis recognizes that during evolution, some enzymes have lost the ability to carry out biological catalysis, which is often reflected in their amino acid sequences and unusual 'pseudocatalytic' properties.

Infection

Infection

An infection is the invasion of tissues by pathogens, their multiplication, and the reaction of host tissues to the infectious agent and the toxins they produce. An infectious disease, also known as a transmissible disease or communicable disease, is an illness resulting from an infection.

Staphylococcus

Staphylococcus

Staphylococcus is a genus of Gram-positive bacteria in the family Staphylococcaceae from the order Bacillales. Under the microscope, they appear spherical (cocci), and form in grape-like clusters. Staphylococcus species are facultative anaerobic organisms.

Streptococcus

Streptococcus

Streptococcus is a genus of gram-positive coccus or spherical bacteria that belongs to the family Streptococcaceae, within the order Lactobacillales, in the phylum Bacillota. Cell division in streptococci occurs along a single axis, so as they grow, they tend to form pairs or chains that may appear bent or twisted. This differs from staphylococci, which divide along multiple axes, thereby generating irregular, grape-like clusters of cells. Most streptococci are oxidase-negative and catalase-negative, and many are facultative anaerobes.

Fastidious organism

Fastidious organism

A fastidious organism is any organism that has complex or particular nutritional requirements. In other words, a fastidious organism will only grow when specific nutrients are included in its medium. The more restrictive term fastidious microorganism is used in microbiology to describe microorganisms that will grow only if special nutrients are present in their culture medium. Thus fastidiousness is often practically defined as being difficult to culture, by any method yet tried.

Hemolysis (microbiology)

Hemolysis (microbiology)

Hemolysis is the breakdown of red blood cells. The ability of bacterial colonies to induce hemolysis when grown on blood agar is used to classify certain microorganisms. This is particularly useful in classifying streptococcal species. A substance that causes hemolysis is a hemolysin.

Meat extract

Meat extract

Meat extract is highly concentrated meat stock, usually made from beef or chicken. It is used to add meat flavour in cooking, and to make broth for soups and other liquid-based foods.