MacConkey agar is a selective and differential medium.
Ø Used to isolate and differentiate members of the Enterobacteriaceae often referred to as enterics
Ø Also used to isolate some Staphylococcus and Enterococcus
Enteric bacteria are facultatively anaerobic Gram-negative rods.
Ø Divided into those that produce acid from lactose and those that do not
Ø the coliforms ferment lactose
Ø coliforms are nonpathogenic Escherichia coli and Enterobacter aerogenes
Ø the lactose non-fermenting group includes pathogens such as Salmonella typhi and Shigella dysenteriae
The purpose for MacConkey Agar is to give a quick preliminary indication of whether a specimen contains enteric pathogens or not.
It is basically an undefined medium.
Ø Contains nutrients, including lactose, bile salts, neutral red and crystal violet
Ø Bile salts and crystal violet inhibit growth of Gram-positive bacteria
Ø this makes it a selective medium
Ø you can add more bile salts and delete crystal violet to make it less selective for isolation of Staphylococcus and Enterococcus
Ø Neutral red is the pH indicator
Ø colorless above 6.8 and red at less than 6.8
Ø acid from lactose fermentation turns neutral red a red color
Ø The picture at the right shows E. coli (red) on the left of the plate indicating acid production
After incubation, coliform coloniues have turned red and lactose non-fermenters remain colorless
New formulations and alterations to existing media are being constantly made. An example is:
MacConkey II Agar with Sorbitol*
MacConkey II Agar with Sorbitol effectively differentiates the most common serotype of Escherichia coli associated with hemorrhagic colitis (O157:H7) from most other nonpathogenic E. coli. On MacConkey II Agar this strain of E. coli is indistinguishable from other lactose-fermenting E. coli. However, when plated onto MacConkey II Agar with Sorbitol, the O157:H7 strain fails to ferment sorbitol, thus producing colorless colonies, while other E. coli yield sorbitol-positive pink to red colonies.