MacConkey (Mac) Agar
Principle:
• A differential plating medium recommended for use in
the isolation and differentiation of lactose-fermenting
organisms from lactose non-fermenting gram negative
enteric bacteria.
• Selectivity of the medium is due to the presence of crystal
violet and bile salts which markedly to completely inhibit
the growth of gram positive organisms.
Purpose:
• Organisms capable of fermenting lactose produce a
localized pH drop which, followed by the absorption of
neutral red, imparts a red/pink/purple color to the colony.
A zone of precipitated bile may also be present due to this localized drop in pH.
• Organisms that do not ferment lactose remain colorless and translucent. They are easily detected by
transmitted light and appear as colorless colonies against a red background.
Test Procedure:
1. Inoculate the organism directly onto the surface of a MacConkey agar plate and streak for isolation.
2. Incubate inoculated plate aerobically at 35-37°C.
3. Examine for growth after 18-24 hours of incubation.
Interpretations:
• Good to excellent growth, red/pink/purple colonies with bile precipitate indicative of Escherichia
coli.
• Good to excellent growth, red/pink/purple colonies without bile precipitate indicative of
Enterobacter aerogenes.
• Good to excellent, colorless colonies without bile precipitate indicative of Proteus vulgaris,
Salmonella typhimurium, and Shigella spp.