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Short answer
MacConkey agar (often searched as “agar mac conkey” or abbreviated MAC) is a selective and differential culture medium used to isolate and tell apart coliform organisms and other enteric bacteria. Bile salts and crystal violet suppress most gram-positive bacteria, while lactose plus the pH indicator neutral red shows which organisms ferment lactose: they form pink to red colonies, and non-fermenters stay colourless.
Definition and scope
One manufacturer’s technical data sheet (HiMedia M081) states the intended use as “the selective isolation and differentiation of coliform organisms and other enteric pathogens from clinical and non clinical samples”, and describes it as the earliest selective and differential medium for cultivating enteric microorganisms. The same sheet lists clinical, dairy, food, water, pharmaceutical and industrial sample types, notes that USP recommends the medium for microbial limit tests, and says the M081 formulation corresponds with the one recommended by APHA for direct plating of water for coliform bacilli.
The CDC REACH culture media job aid places MacConkey agar in the class of media that are both selective and differential, alongside Hektoen, SS, XLD, TCBS and similar plates. It defines selective media as those containing substances such as dyes, acids, alcohols or antibiotics that encourage some microbes while inhibiting others, and differential media as those designed to show visible differences between organisms based on biochemical or metabolic activity, for example a colour change due to acid production. For the broader picture of plate formats, see our guide to the agar plate.
How it works in practice
What each ingredient does
The HiMedia M081 formula lists peptones and tryptone as nutrient sources, lactose as the fermentable carbohydrate, bile salts and crystal violet as the selective agents, sodium chloride for osmotic balance, neutral red as the pH indicator and agar as the gelling agent. The sheet gives the final pH as 7.1 ± 0.2 at 25 °C; the exact quantities differ between manufacturers, so treat any single formula as one maker’s recipe rather than a universal one.
Why colonies turn pink or stay colourless
According to the same sheet, lactose-fermenting strains grow as red or pink colonies and may be surrounded by a zone of acid-precipitated bile. The red colour comes from acid produced from lactose, absorption of neutral red, and the dye’s colour change when the medium falls below pH 6.8. Lactose non-fermenting strains, such as Shigella and Salmonella, are colourless and transparent and typically leave the medium’s appearance unchanged.
Why it is called “slightly” selective
The manufacturer describes the medium as slightly selective and differential because the bile salt concentration is low compared with other enteric plating media. The selective action is attributed to crystal violet and bile salts, which inhibit most gram-positive species. This is why the CDC job aid simply classes it as selective while a data sheet qualifies the degree.
Reference expectations: quality-control organisms
Lot release testing against defined strains gives a practical picture of what the medium should show. The table below is taken from the HiMedia M081 certificate-of-analysis organisms (incubation 30–35 °C, 18–72 hours).
| Organism (strain) | Expected growth | Colony appearance |
|---|---|---|
| Escherichia coli (ATCC 8739) | Luxuriant | Pink-red, with bile precipitate |
| Klebsiella aerogenes (ATCC 13048) | Luxuriant | Pink to red |
| Salmonella Typhimurium (ATCC 14028) | Growth | Colourless |
| Shigella flexneri (ATCC 12022) | Fair to good (30–40% recovery) | Colourless |
| Enterococcus faecalis (ATCC 29212) | None to poor (≤10%) | Colourless to pale pink |
| Staphylococcus aureus (ATCC 25923) | Inhibited (0%) | No growth |
The same sheet also lists other Salmonella serovars and Proteus hauseri as growing with colourless colonies, and Staphylococcus epidermidis as inhibited.
Types and related media
The CDC job aid lists a sorbitol variant, Mac-Sorbitol (SMAC), which it labels as the selective and differential plate for E. coli O157. Both MAC and SMAC sit in the same CDC class as Hektoen, SS, XLD and TCBS, so the label “selective and differential” describes a group of plates, not MacConkey alone.
Product names also vary on the bottle. The M081 sheet names its version “MacConkey Agar with 0.15% bile salts, crystal violet and NaCl”, which signals the bile salt level and the presence of crystal violet and sodium chloride. Compare those three items when you read a label, because they change how selective the plate is.
Where it matters for labs, clinics and supply decisions
- Intended use statement. The M081 sheet is marked “In Vitro diagnostic use. For professional use only.” Check the intended-use wording of any product before using it for a purpose it does not list.
- Lot documentation. The QC organism table above comes from the manufacturer’s documentation; the sheet says users should validate performance for organisms that are not on the certificate of analysis.
- Storage. The sheet specifies dehydrated powder stored tightly closed at 10–30 °C because it is hygroscopic and lumps, and prepared medium at 20–30 °C. Other makers publish their own ranges, so follow the sheet that came with the product.
- Plate surface. The sheet says the agar surface must be dry when inoculated.
For related background on the discipline behind these plates, see what is bacteriology, what is microbiology and what is a Petri dish.
Common misunderstandings
- A pink colony is not an identification. The manufacturer states that further biochemical and serological testing must be carried out for confirmation. Lactose fermentation separates groups; it does not name a species.
- Colourless colonies are not an absence of growth. Salmonella and Shigella are among the lactose non-fermenters that appear colourless and transparent, so they are easy to overlook next to pink colonies.
- It does not exclude every gram-positive organism. The inhibition applies to most gram-positive species. The QC data show Enterococcus faecalis with none-to-poor growth, not a guaranteed zero, whereas S. aureus is inhibited.
- Not every selective and differential plate is MacConkey. The CDC class includes several other media with different indicators and targets.
FAQ
Is MacConkey agar selective or differential?
Both. Bile salts and crystal violet make it selective against most gram-positive bacteria, and lactose with neutral red makes it differential for lactose fermentation.
Why are the colonies pink?
Lactose fermenters produce acid, the pH falls below 6.8, and neutral red takes up and changes colour, giving pink to red colonies.
What does MacConkey agar inhibit?
Most gram-positive bacteria, through the action of bile salts and crystal violet. It is described as slightly selective because bile salt levels are low compared with other enteric media.
What colour are Salmonella and Shigella on MacConkey agar?
Colourless and transparent, because they do not ferment lactose.
Sources
- HiMedia Laboratories, Technical Data Sheet M081, MacConkey Agar with 0.15% bile salts, CV and NaCl: himedialabs.com/media/TD/M081.pdf (checked 2026-10-29).
- CDC REACH, Basic Culture Media and Isolation Techniques Table: reach.cdc.gov culture media table (PDF) (checked 2026-10-29).
