Sorbitol-MacConkey agar is a variant of traditional
MacConkey agar
MacConkey agar is a selective and differential culture medium for bacteria. It is designed to selectively isolate Gram-negative and enteric (normally found in the intestinal tract) bacteria and differentiate them based on lactose fermentation. ...
used in the detection of ''
E. coli'' O157:H7.
Traditionally, MacConkey agar has been used to distinguish those bacteria that ferment
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 '' - ...
from those that do not. This is important because gut 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 ...
'', can typically ferment lactose, while important gut pathogens, such as ''
Salmonella enterica
''Salmonella enterica'' (formerly ''Salmonella choleraesuis'') is a rod-headed, flagellate, facultative anaerobic, Gram-negative bacterium and a species of the genus ''Salmonella''. A number of its serovars are serious human pathogens.
Epidemi ...
'' and most shigellas are unable to ferment lactose. ''
Shigella sonnei
''Shigella sonnei'' is a species of ''Shigella''. Together with ''Shigella flexneri'', it is responsible for 90% of shigellosis cases. ''Shigella sonnei'' is named for the Danish bacteriologist Carl Olaf Sonne. It is a Gram-negative, rod-shaped ...
'' 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 (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 staini ...
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 (), less commonly known as glucitol (), is a sugar alcohol with a sweet taste which the human body metabolizes slowly. It can be obtained by reduction of glucose, which changes the converted aldehyde group (−CHO) to a primary alcohol g ...
. In sorbitol-MacConkey agar, lactose is replaced by sorbitol. Non-pathogenic strains of ''E. coli'' ferment sorbitol to produce acid: Pathogenic ''E. coli'' cannot ferment sorbitol, so this strain uses peptone to grow. This raises the
pH of the medium, allowing the pathogenic strain to be differentiated from other non-pathogenic ''E.coli'' strains through the action of the
pH indicator
A pH indicator is a halochromic chemical compound added in small amounts to a solution so the pH (acidity or basicity) of the solution can be determined visually or spectroscopically by changes in absorption and/or emission properties. Hence, ...
in the medium.
References
{{Growth media
Microbiological media