MacConkey Agar
MacConkey agar is selective for many Gram-negative rods and differentiates lactose fermenters from non-lactose fermenters.
Core idea
MacConkey growth and lactose reaction are major early clues in enteric and nonfermenter workflows.
Principle
Bile salts and crystal violet inhibit many Gram-positive organisms. Lactose and neutral red show acid production from lactose fermentation.
MacConkey Reading Table
| Observation | Meaning | Examples to consider |
|---|---|---|
| Good growth, pink colonies | Lactose fermentation | E. coli, Klebsiella, Enterobacter group |
| Good growth, colorless colonies | Non-lactose fermentation | Salmonella, Shigella, Proteus, many nonfermenters |
| Poor or no growth | Organism may be inhibited or fastidious | Gram-positive organisms, Haemophilus, anaerobes, special pathogens |
| Growth plus oxidase positive | Not classic Enterobacterales | Pseudomonas, Aeromonas, Vibrio, other oxidase-positive branches |
Basic workflow
- Inoculate the plate from an appropriate specimen or isolate.
- Incubate according to lab procedure.
- Check whether the organism grows.
- Read lactose reaction and colony character.
- Use oxidase and other branch tests to continue the workup.
How to read it
- Pink colonies suggest lactose fermentation.
- Colorless colonies suggest non-lactose fermentation.
- No growth can suggest a Gram-positive, fastidious, or inhibited organism depending on context.
High-yield trapCommon mistakes
- Calling lactose reaction before enough incubation.
- Ignoring oxidase after MacConkey growth.
- Assuming every MacConkey grower is Enterobacterales.
ShortcutStudent shortcut
MacConkey answers two early questions: did it grow, and did it ferment lactose?