Almost years later, MacConkey agar remains ubiquitous in clinical laboratories, where it is used routinely to select for non-fastidious Gram-negative organisms in wound, urine, stool, and blood cultures.
Despite foundational changes in microbiology practice, including automation, molecular genetics, and mass spectrometry, it seems likely that MacConkey's medium will continue to be used in the foreseeable future. The above represents the views of the author and does not necessarily reflect the opinion of the American Society for Microbiology. Login Toggle navigation Search. MacConkey Agar Is Selective for Non-fastidious Gram-negative Organisms Therefore, MacConkey needed a way to limit this background of environmental flora and allow only his organisms of interest to grow.
MacConkey Agar Differentiates Lactose fermenters and Non-fermenters In addition to enriching for Gram-negative bacteria, MacConkey also wanted to be able to differentiate between types of enteric organisms. Pure deoxycholic acid replaced the mixture of glycholic acid and taurocholic acid originally used by MacConkey.
Panel A shows Escherichia coli, a lactose fermenter. The white color surrounding the colony represents precipitation of bile. Panel B shows Klebsiella pneumoniae. Although this organism also ferments lactose, it does not produce sufficient acid to precipiate bile and looks like a non-fermenter on this medium.
Panel C shows Pseudomonas aeruginosa, a lactose non-fermenter. Source: Courtesy K. Share This. Author: Kenneth K. Smith, Ph. Kenneth K. MacConkey agar contains four key ingredients lactose, bile salts, crystal violet, and neutral red that make it a selective and differential media. Bile salts and crystal violet act as selective agents that inhibit the growth of Gram-positive organisms, and proliferate the selective growth of gram-negative bacteria.
Lactose acts as a source of carbohydrate. Lactose-fermenting bacteria produce pink-red colonies, after fermenting the lactose to acids and dropping the pH of the indicator neutral red present in the medium. Other ingredients such as enzymatic digest of gelatin, casein, and animal tissue provide nitrogen, vitamins, minerals, and amino acids essential for growth.
Sodium chloride provides osmotic balance and supplies essential electrolytes for transport. Agar is incorporated as the solidifying agent. Gram-negative enteric bacteria that grow on MacConkey agar are differentiated by their ability to ferment lactose.
If the lactose is fermented by the bacteria, the production of the acid drops the pH of the media. 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. A because it is able to grow in this medium. Dear Sir, I salute you for the nice explanations in Microbiology you regularly upload in Facebook.
Pls continue your service to Microbiology. If MAC is for non-fastidious Gram-negative bacilli, why would Enterococcus and Staphylococcus give a reaction on this medium? Just curious. I am samiksha working as a microbiologist in ferm we are using macconkeys agar for water pathogen testing some times we found some submerged precipitation like something but not grow when subculture, in macconkeys agar plate after 60 to 72 hrs of incubation at 30 to 35 degrees incubation.
Please suggest. Asante sana thank you much for your post. For this am thankful. This is remarkably helpful. Thank you so much. 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.
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