Colony Morphology on MacConkey Agar

MacConkey Agar- Composition, Principle, Uses, Preparation and Colony  Morphology

Besides differentiating on the basis of colour, colonies on MacConkey medium can further be presumptively identified based on their colonial appearances (shape, size, margin, time of growth, etc). Some of them are enlisted below:

OrganismTypeColony characteristics
Escherichia coliLactose fermenterE. coli gives flat, dry, pink, non-mucoid colonies with a surrounding darker pink area of precipitated bile salts.
Klebsiella sppLactose fermenterColonies typically appear large, mucoid, and pink, with pink-red pigment usually diffusing into the surrounding agar
Citrobacter sppLate lactose fermenterAppear as non-lactose fermenter (NLF) up to 24 hours; however, after 48 hours colonies are light pink.
Enterobacter sppLactose fermenterPink, mucoid colonies but smaller than Klebsiella spp.
Serratia sppLate lactose fermenterS. marcescens may be red-pigmented, especially if the plate is left at 25°C
Proteus sppNon-Lactose FermenterPale colonies with swarming, characteristic foul smell.
Shigella sppNon-Lactose Fermenter except S.sonnei which is a late lactose fermenterPale colonies,1-2 mm, flat colonies with jagged edges
Providencia sppNon-Lactose FermenterColonies are colourless, flat, 2-3 mm in diameter, and do not swarm.
Salmonella sppNon-Lactose FermenterColourless colonies, convex, 2-3 mm with a serrated margin.
Pseudomonas sppNon-Lactose FermenterColourless, flat, smooth colonies, 2-3 mm in diameter with greenish to brownish pigmentation.
Yersinia sppNon-Lactose FermenterColonies may be colourless to peach
Gram-positive bacteriaNo growth