MacConkey agar (MAC) is both selective and differential. It selects for Gram-negative bacteria by inhibiting Gram-positive organisms, and it differentiates among those Gram-negative bacteria based on whether they can ferment lactose. This dual function makes it one of the most commonly used media in microbiology labs.
How MAC Works as a Selective Medium
Two ingredients in MacConkey agar do the selecting: crystal violet dye and bile salts. Both of these substances are toxic to most Gram-positive bacteria, preventing them from growing on the plate. Gram-negative bacteria, however, have an outer membrane that protects them from these compounds, so they grow normally.
This means that if you streak a mixed sample onto MAC, only the Gram-negative organisms will form visible colonies. Gram-positive species like Staphylococcus or Streptococcus are effectively filtered out, giving you a cleaner view of the Gram-negative population in the specimen.
How MAC Works as a Differential Medium
Once the Gram-positive bacteria are eliminated, MAC still has another job: sorting the surviving Gram-negative bacteria into two groups based on lactose fermentation. It does this with two key ingredients working together.
Lactose is included in the agar as a fermentable sugar. Bacteria that can break down lactose produce acidic byproducts, which lower the pH of the medium around their colonies. Neutral red, a pH indicator embedded in the agar, responds to that acid by turning bright pink when the pH drops below 6.8. So lactose-fermenting bacteria like E. coli grow as pink or red colonies, while non-fermenters like Salmonella and Shigella remain colorless or translucent.
This color difference is visible to the naked eye and gives lab workers an immediate, preliminary identification without needing additional biochemical tests.
What the Colony Colors Tell You
Pink or red colonies indicate lactose fermenters. Common examples include E. coli, Klebsiella, and Enterobacter. Strong fermenters may even produce a surrounding zone of precipitated bile, making the colonies look darker or more opaque.
Colorless or pale colonies indicate non-lactose fermenters. This group includes Salmonella, Shigella, and Proteus. Spotting these colorless colonies on a MAC plate is often the first clue that a pathogenic non-fermenter might be present in a stool or urine sample, prompting further testing.
Sorbitol MacConkey Agar: A Modified Version
A notable variant called sorbitol MacConkey agar (SMAC) replaces lactose with sorbitol. This swap targets one specific pathogen: E. coli O157:H7, the strain responsible for hemorrhagic colitis. Unlike most E. coli strains, O157:H7 does not ferment sorbitol. On standard MAC, its colonies look pink and blend in with normal gut flora. On SMAC, it grows as colorless colonies while the rest of the fecal bacteria ferment sorbitol and turn pink.
In stool cultures, SMAC allows heavy, nearly pure-culture growth of E. coli O157:H7 as colorless colonies, making it easy to pick out from the background flora. On regular MacConkey agar, the same pathogen is essentially invisible among the other lactose-fermenting E. coli. This is a good example of how tweaking the sugar in a differential medium can shift its diagnostic target entirely.
Why the Distinction Matters
Understanding that MAC is both selective and differential (not one or the other) helps clarify how a single plate can do two things at once. The selective function narrows the field by killing off Gram-positive bacteria. The differential function then sorts what’s left by metabolic ability, using color as a visual readout. These are independent mechanisms driven by different ingredients in the same medium, which is why the correct answer on any exam or practical is “both.”