MacConkey agar is a selective and differential culture medium for

bacteria Bacteria (; : bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of Prokaryote, prokaryotic microorganisms. Typically a few micr ...

. It is designed to selectively isolate

gram-negative Gram-negative bacteria are bacteria that, unlike gram-positive bacteria, do not retain the crystal violet stain used in the Gram staining method of bacterial differentiation. Their defining characteristic is that their cell envelope consists ...

and enteric (normally found in the intestinal tract) bacteria and differentiate them based on lactose fermentation. Lactose fermenters turn red or pink on MacConkey agar, and nonfermenters do not change color. The media inhibits growth of

gram-positive In bacteriology, gram-positive bacteria are bacteria that give a positive result in the Gram stain test, which is traditionally used to quickly classify bacteria into two broad categories according to their type of cell wall. The Gram stain is ...

organisms with

crystal violet Crystal violet or gentian violet, also known as methyl violet 10B or hexamethyl pararosaniline chloride, is a triphenylmethane, triarylmethane dye used as a histological stain and in Gram staining, Gram's method of classifying bacteria. Crystal ...

and

bile salts Bile acids are steroid acids found predominantly in the bile of mammals and other vertebrates. Diverse bile acids are synthesized in the liver in peroxisomes. Bile acids are conjugated with taurine or glycine residues to give anions called bile ...

, allowing for the selection and isolation of

gram-negative bacteria Gram-negative bacteria are bacteria that, unlike gram-positive bacteria, do not retain the Crystal violet, crystal violet stain used in the Gram staining method of bacterial differentiation. Their defining characteristic is that their cell envelo ...

. The media detects lactose fermentation by enteric bacteria with the pH indicator

neutral red Neutral red (toluylene red, Basic Red 5, or C.I. 50040) is a eurhodin dye used for staining in histology. It stains lysosomes red. It is used as a general stain in histology, as a counterstain in combination with other dyes, and for many stain ...

It contains

(to inhibit most

bacteria),

dye (which also inhibits certain gram-positive bacteria), and

dye (which turns pink if the microbes are fermenting

lactose Lactose is a disaccharide composed of galactose and glucose and has the molecular formula C12H22O11. Lactose makes up around 2–8% of milk (by mass). The name comes from (Genitive case, gen. ), the Latin word for milk, plus the suffix ''-o ...

). Composition: * Peptone – 17 g * Proteose peptone – 3 g *

Lactose Lactose is a disaccharide composed of galactose and glucose and has the molecular formula C12H22O11. Lactose makes up around 2–8% of milk (by mass). The name comes from (Genitive case, gen. ), the Latin word for milk, plus the suffix ''-o ...

– 10 g *

Bile salts Bile acids are steroid acids found predominantly in the bile of mammals and other vertebrates. Diverse bile acids are synthesized in the liver in peroxisomes. Bile acids are conjugated with taurine or glycine residues to give anions called bile ...

– 1.5 g *

Sodium chloride Sodium chloride , commonly known as Salt#Edible salt, edible salt, is an ionic compound with the chemical formula NaCl, representing a 1:1 ratio of sodium and chloride ions. It is transparent or translucent, brittle, hygroscopic, and occurs a ...

– 5 g *

Neutral red Neutral red (toluylene red, Basic Red 5, or C.I. 50040) is a eurhodin dye used for staining in histology. It stains lysosomes red. It is used as a general stain in histology, as a counterstain in combination with other dyes, and for many stain ...

– 0.03 g *

Crystal violet Crystal violet or gentian violet, also known as methyl violet 10B or hexamethyl pararosaniline chloride, is a triphenylmethane, triarylmethane dye used as a histological stain and in Gram staining, Gram's method of classifying bacteria. Crystal ...

– 0.001 g *

Agar Agar ( or ), or agar-agar, is a jelly-like substance consisting of polysaccharides obtained from the cell walls of some species of red algae, primarily from " ogonori" and " tengusa". As found in nature, agar is a mixture of two components, t ...

– 13.5 g *

Water Water is an inorganic compound with the chemical formula . It is a transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance. It is the main constituent of Earth's hydrosphere and the fluids of all known liv ...

– add to make 1 litre; adjust pH to 7.1 +/− 0.2 * Sodium taurocholate There are many variations of MacConkey agar depending on the need. If the spreading or swarming of ''Proteus'' species is ''not'' required, sodium chloride is omitted.

at a concentration of 0.0001% (0.001 g per litre) is included when needing to check if gram-positive bacteria are inhibited. MacConkey with sorbitol is used to isolate ''E. coli'' O157, an enteric pathogen.

History

The medium was developed by Alfred Theodore MacConkey while working as a

bacteriologist A bacteriologist is a microbiologist, or similarly trained professional, in bacteriology— a subdivision of microbiology that studies bacteria, typically Pathogenic bacteria, pathogenic ones. Bacteriologists are interested in studying and learnin ...

for the Royal Commission on Sewage Disposal.

Uses

Using neutral red pH indicator, the agar distinguishes those gram-negative bacteria that can ferment the sugar lactose (Lac+) from those that cannot (Lac-). This medium is also known as an "indicator medium" and a "low selective medium". Presence of bile salts inhibits swarming by ''Proteus'' species.

Lac positive

By utilizing the lactose available in the medium, Lac+ bacteria such as ''

Escherichia coli ''Escherichia coli'' ( )Wells, J. C. (2000) Longman Pronunciation Dictionary. Harlow ngland Pearson Education Ltd. is a gram-negative, facultative anaerobic, rod-shaped, coliform bacterium of the genus '' Escherichia'' that is commonly fo ...

'', ''

Enterobacter ''Enterobacter'' is a genus of common Gram-negative, Facultative anaerobic organism, facultatively anaerobic, bacillus (shape), rod-shaped, non-spore-forming bacteria of the family Enterobacteriaceae. Cultures are found in soil, water, sewage, ...

'' and ''

Klebsiella ''Klebsiella'' is a genus of Gram-negative, oxidase-negative, rod-shaped bacteria with a prominent polysaccharide-based capsule. ''Klebsiella'' is named after German-Swiss microbiologist Edwin Klebs (1834–1913). Carl Friedlander described ' ...

'' will produce

acid An acid is a molecule or ion capable of either donating a proton (i.e. Hydron, hydrogen cation, H+), known as a Brønsted–Lowry acid–base theory, Brønsted–Lowry acid, or forming a covalent bond with an electron pair, known as a Lewis ...

, which lowers the pH of the agar below 6.8 and results in the appearance of pink

colonies A colony is a territory subject to a form of foreign rule, which rules the territory and its indigenous peoples separated from the foreign rulers, the colonizer, and their '' metropole'' (or "mother country"). This separated rule was often or ...

. The bile salts precipitate in the immediate neighborhood of the colony, causing the medium surrounding the colony to become hazy.

Lac negative

Organisms unable to ferment lactose will form normal-colored (i.e., un-dyed) colonies. The medium may also turn yellow. Examples of non-lactose fermenting bacteria include ''

Salmonella ''Salmonella'' is a genus of bacillus (shape), rod-shaped, (bacillus) Gram-negative bacteria of the family Enterobacteriaceae. The two known species of ''Salmonella'' are ''Salmonella enterica'' and ''Salmonella bongori''. ''S. enterica'' ...

'', ''Proteus'', and ''

Shigella ''Shigella'' is a genus of bacteria that is Gram negative, facultatively anaerobic, non–spore-forming, nonmotile, rod shaped, and is genetically nested within ''Escherichia''. The genus is named after Kiyoshi Shiga, who discovered it in 1 ...

'' spp.

Slow

Some organisms ferment lactose slowly or weakly, and are sometimes put in their own category. These include ''

Serratia ''Serratia'' is a genus of Gram-negative, facultatively anaerobic, rod-shaped bacteria of the family Enterobacteriaceae. They are typically 1–5 μm in length, do not produce spores, and can be found in water, soil, plants, and animals. Some mem ...

'' and ''

Citrobacter ''Citrobacter'' is a genus of Gram-negative, aerobic, rod-shaped coliform bacteria of the Enterobacteriaceae family. ''Citrobacter'' spp. cause opportunistic infections (including urinary tract infections, gastroenteritis, and bacteremia). ...

''.

Mucoid colonies

Some organisms, especially ''Klebsiella'' and ''Enterobacter'', produce mucoid colonies which appear very moist and sticky and slimy. This phenomenon happens because the organism is producing a capsule, which is predominantly made from the lactose sugar in the agar.

Variant

A variant, sorbitol-MacConkey agar, (with the addition of additional selective agents) can assist in the isolation and differentiation of enterohemorrhagic ''E. coli''

serotype A serotype or serovar is a distinct variation within a species of bacteria or virus or among immune cells of different individuals. These microorganisms, viruses, or Cell (biology), cells are classified together based on their shared reactivity ...

O157:H7, by the presence of colorless circular colonies that are non-sorbitol fermenting.

References

{{Growth media Biochemistry detection reactions Microbiological media Cell culture media

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