Why does macconkey agar turn pink

Therefore, enterics and a select group of other Gram-negative bacteria, notably Pseudomonas are selected for on media containing bile. In addition to enriching for Gram-negative bacteria, MacConkey also wanted to be able to differentiate between types of enteric organisms. Of particular interest was determining whether a colony represented Escherichia coli then Bacillus coli communis or Salmonella enterica serovar Typhi then B. Although definitive identification of these organisms required additional testing, MacConkey used the previous observation by Theodor Escherich for whom the genus Escherichia is named that E.

When bacteria ferment a sugar, the pH of the medium becomes acidic. Of course, acidity cannot be directly observed, so sugar fermentation was traditionally assayed in broth media containing a chemical pH indicator often litmus.

However, if broth-based assays contained more than one organism, as would often be the case for MacConkey's water samples, any fermenting bacteria would drop the pH and obscure presence of non-fermenting organisms. What MacConkey needed was a way to evaluate lactose fermentation on individual colonies on solid media.

To do this, he incorporated lactose directly into the agar. Changes in pH attributable to fermentation were observed by taking advantage of the knowledge that bile acids precipitate in an acidic environment.

In this way, lactose-fermenting colonies were surrounded by a haze of precipitated bile. After the first description of MacConkey agar was published in The Lancet in , use of the medium caught on rapidly amongst those interested in water microbiology. However, other scientists recognized that MacConkey's original recipe had some limitations. Increasing the pH of the media may result in a change in the color of media to yellow as neutral red turns yellow at alkaline pH. Ingredient For ml of media For 1L of media Peptone 1.

As per the necessity of the user, the fermentable sugar lactose can be replaced in the medium by other sugars. The ability of gram negative bacteria to ferment these replacement sugars is analyzed in the same way as is lactose fermentation. Alternatively, the commercially available MacConkey agar media powders can be used. Weigh the mixture of content as prescribed by the manufacturer. Add agar after adjusting the pH of the media. At acidic pH, agar hydrolyzes and results in soft agar plates.

MacConkey Agar plates protocols. Bacterial target sites for biocide action. Save my name, email, and website in this browser for the next time I comment. Am a student of food and technology, working on the microbiology of pap was like been in the dark cos I was just following manual instructions without having a full understanding of my project work but now I know better. Thank you very much. Save my name and email in this browser for the next time I comment.

Principle of MacConkey Agar MacConkey agar is used for the isolation of gram-negative enteric bacteria and the differentiation of lactose fermenting from lactose non-fermenting gram-negative bacteria. It is used in the differentiation of lactose fermenting from lactose non-fermenting gram-negative bacteria. It is used for the isolation of coliforms and intestinal pathogens in water, dairy products and biological specimens.

Preparation of MacConkey Agar Suspend Heat to boiling to dissolve the medium completely. Mix well before pouring into sterile Petri plates. Result Interpretation on MacConkey Agar Lactose fermenting strains grow as red or pink and may be surrounded by a zone of acid precipitated bile.

It is necessary to subculture and carry out confirmation tests for final identification. Some strains may be encountered that grow poorly or fail to grow on this medium. Incubation of MacConkey Agar plates under increased CO2 has been reported to reduce growth and recovery of a number of strains of Gram-negative bacilli. Strongly lactose fermenting bacteria produce sufficient acid which causes precipitation of the bile salts around the growth. It appears as a pink halo surrounding colonies or areas of confluent growth.

Pink halo is not seen around the colonies of weaker lactose fermenting bacteria. Gram-negative bacteria that grow on MacConkey agar but do not ferment lactose appear colorless on the medium and the agar surrounding the bacteria remains relatively transparent.

You can also purchase the ready-made MacConkey agar from the commercial suppliers. Pink-red colonies: Pink-red colonies on MacConkey agar indicate the presence of lactose fermenting bacteria. Examples include Escherichia coli, Klebsiella spp, Citrobacter, Enterobacter , etc. Besides differentiating on the basis of color, colonies on MacConkey agar either lactose fermenting or non-lactose fermenting can further be presumptively identified based on their colonial appearances shape, size, margin, time of growth, etc.

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