MacConkey Agar

MacConkey agar is a selective and differential medium.

   Used to isolate and differentiate members of the Enterobacteriaceae often referred to as enterics

   Also used to isolate some Staphylococcus and Enterococcus

Enteric bacteria are facultatively anaerobic Gram-negative rods.

   Divided into those that produce acid from lactose and those that do not

   the coliforms ferment lactose

   coliforms are nonpathogenic Escherichia coli and Enterobacter aerogenes

   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.

   Contains nutrients, including lactose, bile salts, neutral red and crystal violet

   Bile salts and crystal violet inhibit growth of Gram-positive bacteria

   this makes it a selective medium

   you can add more bile salts and delete crystal violet to make it less selective for isolation of Staphylococcus and Enterococcus

   Neutral red is the pH indicator

   colorless above 6.8 and red at less than 6.8

   acid from lactose fermentation turns neutral red a red color

   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.