Sorbitol-MacConkey agar
Encyclopedia
Sorbitol MacConkey agar is a variant of traditional MacConkey agar
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. This is important because gut bacteria, such as Escherichia coli
, can typically ferment lactose, while important gut pathogens, such as Salmonella enterica
and most shigellas are unable to ferment lactose. Shigella sonnei
can ferment lactose, but only after prolonged incubation, so it is referred to as a late-lactose fermenter.
During fermentation of the sugar, acid is formed and the pH of the medium drops, changing the color of the pH indicator. Different formulations use different indicators; neutral red
is often used. For example, lactose fermenters turn a deep red when this pH indicator is used. Those bacteria unable to ferment lactose, often referred to as nonlactose fermenters, or NLFs for short, use the peptone in the medium. This releases ammonia, which raises the pH of the medium. Although some authors refer to NLFs as being colourless, in reality they turn neutral red a buffish color.
E. coli O157:H7 differs from most other strains of E. coli in being unable to ferment sorbitol
. In sorbitol MacConkey agar, lactose is replaced by sorbitol. Most strains of E. coli ferment sorbitol to produce acid: E. coli O157:H7 can not ferment sorbitol, so this strain uses peptone to grow. This raises the pH
of the medium, allowing the O157:H7 strain to be differentiated from other E. coli strains through the action of the pH indicator
in the medium.
MacConkey agar
MacConkey agar is a culture medium designed to grow Gram-negative bacteria and stain them for lactose fermentation.-Contents:It contains bile salts MacConkey agar is a culture medium designed to grow Gram-negative bacteria and stain them for lactose fermentation.-Contents:It contains bile salts...
used in the detection of E. coli O157:H7. Traditionally, MacConkey agar has been used to distinguish those bacteria that ferment lactose
Lactose
Lactose is a disaccharide sugar that is found most notably in milk and is formed from galactose and glucose. Lactose makes up around 2~8% of milk , although the amount varies among species and individuals. It is extracted from sweet or sour whey. The name comes from or , the Latin word for milk,...
from those that do not. This is important because gut bacteria, such as Escherichia coli
Escherichia coli
Escherichia coli is a Gram-negative, rod-shaped bacterium that is commonly found in the lower intestine of warm-blooded organisms . Most E. coli strains are harmless, but some serotypes can cause serious food poisoning in humans, and are occasionally responsible for product recalls...
, can typically ferment lactose, while important gut pathogens, such as Salmonella enterica
Salmonella enterica
Salmonella enterica is a rod-shaped flagellated, facultative anaerobic, Gram-negative bacterium, and a member of the genus Salmonella.- Epidemiology :...
and most shigellas are unable to ferment lactose. Shigella sonnei
Shigella sonnei
Shigella sonnei is a species of Shigella. Together with Shigella flexneri, it is responsible for 90% of shigellosis. Shigella sonnei is named for the Danish bacteriologist Carl Olaf Sonne....
can ferment lactose, but only after prolonged incubation, so it is referred to as a late-lactose fermenter.
During fermentation of the sugar, acid is formed and the pH of the medium drops, changing the color of the pH indicator. Different formulations use different indicators; neutral red
Neutral red
Neutral Red 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 staining methods. Together with Janus Green B, it is used to stain embryonal tissues and supravital...
is often used. For example, lactose fermenters turn a deep red when this pH indicator is used. Those bacteria unable to ferment lactose, often referred to as nonlactose fermenters, or NLFs for short, use the peptone in the medium. This releases ammonia, which raises the pH of the medium. Although some authors refer to NLFs as being colourless, in reality they turn neutral red a buffish color.
E. coli O157:H7 differs from most other strains of E. coli in being unable to ferment sorbitol
Sorbitol
Sorbitol, also known as glucitol, Sorbogem® and Sorbo®, is a sugar alcohol that the human body metabolizes slowly. It can be obtained by reduction of glucose, changing the aldehyde group to a hydroxyl group. Sorbitol is found in apples, pears, peaches, and prunes...
. In sorbitol MacConkey agar, lactose is replaced by sorbitol. Most strains of E. coli ferment sorbitol to produce acid: E. coli O157:H7 can not ferment sorbitol, so this strain uses peptone to grow. This raises the pH
PH
In chemistry, pH is a measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH close to 7.0 at . Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline...
of the medium, allowing the O157:H7 strain to be differentiated from other E. coli strains through the action of the pH indicator
PH indicator
A pH indicator is a halochromic chemical compound that is added in small amounts to a solution so that the pH of the solution can be determined visually. Hence a pH indicator is a chemical detector for hydronium ions or hydrogen ions in the Arrhenius model. Normally, the indicator causes the...
in the medium.