MacConkey agar is a selective and differential

culture Culture () is an umbrella term which encompasses the social behavior, institutions, and norms found in human societies, as well as the knowledge, beliefs, arts, laws, customs, capabilities, and habits of the individuals in these groups.Tyl ...

medium Medium may refer to: Science and technology Aviation *Medium bomber, a class of war plane *Tecma Medium, a French hang glider design Communication * Media (communication), tools used to store and deliver information or data * Medium of ...

for

bacteria Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria were among ...

. It is designed to selectively isolate

Gram-negative Gram-negative bacteria are bacteria that do not retain the crystal violet stain used in the Gram staining method of bacterial differentiation. They are characterized by their cell envelopes, which are composed of a thin peptidoglycan cell wall ...

and enteric (normally found in the intestinal tract) bacteria and differentiate them based on

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 ' (gen. '), the Latin word for milk, plus the suffix '' - ...

fermentation Fermentation is a metabolic process that produces chemical changes in organic substrates through the action of enzymes. In biochemistry, it is narrowly defined as the extraction of energy from carbohydrates in the absence of oxygen. In food ...

. 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. Gram-positive bacte ...

organisms with

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

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. Bile acids are conjugated with taurine or glycine residues to give anions called bile salts. Primary b ...

, allowing for the selection and isolation of

gram-negative bacteria Gram-negative bacteria are bacteria that do not retain the crystal violet stain used in the Gram staining method of bacterial differentiation. They are characterized by their cell envelopes, which are composed of a thin peptidoglycan cell wall ...

. 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 stai ...

It contains

(to inhibit most

bacteria),

dye (which also inhibits certain Gram-positive bacteria), and neutral red dye (which turns pink if the microbes are fermenting

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

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 ' (gen. '), the Latin word for milk, plus the suffix '' - ...

– 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. Bile acids are conjugated with taurine or glycine residues to give anions called bile salts. Primary b ...

– 1.5 g *

Sodium chloride Sodium chloride , commonly known as salt (although sea salt also contains other chemical salts), is an ionic compound with the chemical formula NaCl, representing a 1:1 ratio of sodium and chloride ions. With molar masses of 22.99 and 35.45 g ...

– 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 stai ...

– 0.03 g *

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

– 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 (''Gracilaria'') and "tengusa" (''Gelidiaceae''). As found in nature, agar is ...

– 13.5 g *

Water Water (chemical formula ) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts as a ...

– add to make 1 litre; adjust pH to 7.1 +/− 0.2 *

Sodium taurocholate Taurocholic acid, known also as cholaic acid, cholyltaurine, or acidum cholatauricum, is a deliquescent yellowish crystalline bile acid involved in the emulsification of fats. It occurs as a sodium salt in the bile of mammals. It is a conjuga ...

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 Alfred Theodore MacConkey (1861-17 May 1931) was the British bacteriologist who developed MacConkey's agar, a selective medium that is used in the diagnosis of enteric pathogens. He was born McConkey but appears to have spelled his name "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 {{Use dmy dates, date=April 2022 The Royal Commission on Sewage Disposal was established by the British government in 1898 to report on: (1) What method or methods of treating and disposing of sewage (including any liquid from any factory, or manu ...

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. also known as ''E. coli'' (), is a Gram-negative, facultative anaerobic, rod-shaped, coliform bacterium of the genus ''Escher ...

'', ''

Enterobacter ''Enterobacter'' is a genus of common Gram-negative, facultatively anaerobic, rod-shaped, non-spore-forming bacteria of the family Enterobacteriaceae. It is the type genus of the order Enterobacterales. Several strains of these bacteria are ...

'' and ''

Klebsiella ''Klebsiella'' is a genus of Gram-negative, oxidase-negative, rod-shaped bacteria with a prominent polysaccharide-based capsule. ''Klebsiella'' species are found everywhere in nature. This is thought to be due to distinct sublineages developi ...

'' will produce

acid In computer science, ACID ( atomicity, consistency, isolation, durability) is a set of properties of database transactions intended to guarantee data validity despite errors, power failures, and other mishaps. In the context of databases, a sequ ...

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

colonies In modern parlance, a colony is a territory subject to a form of foreign rule. Though dominated by the foreign colonizers, colonies remain separate from the administration of the original country of the colonizers, the '' metropolitan state'' ...

. 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 will remain yellow. Examples of non-lactose fermenting bacteria are ''

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

'', ''Proteus'' species, ''

Yersinia ''Yersinia'' is a genus of bacteria in the family Yersiniaceae. ''Yersinia'' species are Gram-negative, coccobacilli bacteria, a few micrometers long and fractions of a micrometer in diameter, and are facultative anaerobes. Some members of ''Yer ...

'', ''

Pseudomonas aeruginosa ''Pseudomonas aeruginosa'' is a common encapsulated, gram-negative, aerobic–facultatively anaerobic, rod-shaped bacterium that can cause disease in plants and animals, including humans. A species of considerable medical importance, ''P. aerugi ...

'' and ''

Shigella ''Shigella'' is a genus of bacteria that is Gram-negative, facultative anaerobic, non-spore-forming, nonmotile, rod-shaped, and genetically closely related to '' E. coli''. The genus is named after Kiyoshi Shiga, who first discovered it in 1 ...

''.

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 Yersiniaceae. According to the List of Prokaryotic names with Standing Nomenclature (LPSN), there are currently 19 species of ''Serratia'' that a ...

'' and ''

Citrobacter ''Citrobacter'' is a genus of Gram-negative coliform bacteria in the family Enterobacteriaceae. The species ''C. amalonaticus'', ''C. koseri'', and ''C. freundii'' can use citrate as a sole carbon source. ''Citrobacter'' species are differentia ...

''.

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 cells are classified together based on their surface antigens, allowing the epi ...

''E. coli'' 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

Contents

History

Uses

Lac positive

Lac negative

Slow

Mucoid colonies

Variant

See also

References