Guide to the main agars
Download the free guide to the main agars used in the laboratory: composition, purpose, colony appearance and incubation conditions — ready to check at the bench.
Culture media covered
Each medium covers what it's for, what to expect on the plate, and how to incubate it.
- Blood agar
- Enriched medium for general growth and haemolysis reading (alpha, beta and gamma) when screening Gram-positive cocci.
- Chocolate agar
- Heated blood that releases factors X and V — essential for fastidious Haemophilus and Neisseria species.
- MacConkey agar
- Selective for Gram-negative organisms and differential for lactose fermentation: pink colonies versus colourless ones.
- EMB agar (eosin methylene blue)
- Differentiates Enterobacterales by strong lactose fermentation, with the typical metallic-green sheen of Escherichia coli.
- SS and Hektoen agar
- Selective media for Salmonella and Shigella in stool culture, indicating H₂S production.
- Mannitol salt agar
- Selective through high NaCl concentration and differential for mannitol fermentation when screening for Staphylococcus aureus.
- Mueller-Hinton agar
- Standardised medium for disk-diffusion antibiogram testing and zone-diameter reading according to BrCAST.
- Sabouraud dextrose agar
- Acidic pH and high glucose content for isolating yeasts and filamentous fungi.
- CLED and chromogenic agars
- Used in urine culture: they prevent Proteus swarming and give a presumptive identification through colony colour.
Keep going at the bench
After plating, the next step is interpretation. Use the BrCAST antibiogram calculator for zone diameters, the CFU/mL and dilution calculators for counting, or go deeper in the Practical Microbiology e-book.