Eosin methylene blue (EMB) agar is a culture medium used in microbiology labs to isolate and identify gram-negative bacteria, particularly E. coli and other coliform organisms. It works by combining two functions in a single plate: it blocks the growth of gram-positive bacteria (selective), and it produces distinct colony colors depending on how each organism ferments lactose (differential). This makes it one of the most practical tools for quickly sorting bacterial species in clinical, food safety, and water quality testing.
What EMB Agar Contains
The standard Levine formulation, used by the FDA for food safety testing, contains five components per liter of distilled water: 10 g of peptone (a protein source that feeds bacterial growth), 10 g of lactose (the sugar being tested for fermentation), 0.4 g of eosin Y, and 0.065 g of methylene blue. The two dyes are present in small amounts, but they do the heavy lifting in terms of making the medium both selective and differential.
Peptone provides the nitrogen and amino acids bacteria need to grow. Lactose is the key ingredient for differentiation: bacteria that can break it down produce acid, and that acid triggers visible color changes through the dyes. The whole mixture is solidified with agar into plates ready for inoculation.
How It Blocks Gram-Positive Bacteria
Methylene blue is the ingredient responsible for EMB agar’s selective properties. Even in the small concentration present in the medium, it effectively inhibits most gram-positive bacteria. This means organisms like Staphylococcus and Streptococcus species generally won’t grow on the plate, leaving only gram-negative bacteria behind. The result is a much cleaner starting point for identification, since you’re already narrowing the field to a specific category of organisms before you even read the colony colors.
Reading Colony Colors
The real power of EMB agar is what happens after gram-negative bacteria grow. The colonies display different colors and textures depending on how aggressively (or whether) each species ferments lactose. When bacteria ferment lactose, they produce acid, and that acid causes the eosin and methylene blue dyes to be absorbed into the colonies, creating visible color changes.
Here’s what different organisms look like on an EMB plate:
- E. coli: Produces a distinctive metallic green sheen, the hallmark finding on EMB agar. This happens because E. coli is such a vigorous lactose fermenter that it drives the local pH very low, causing the dyes to precipitate on the colony surface and reflect light. Colonies also appear purple at their centers. No other common organism produces this exact appearance, making it a quick presumptive identification.
- Enterobacter aerogenes: Grows as brown, dark-centered, mucoid (slimy-looking) colonies. It ferments lactose and produces acid, but not as aggressively as E. coli, so it lacks the metallic green sheen.
- Klebsiella pneumoniae: Similar to Enterobacter with brown, dark-centered, mucoid colonies, though typically smaller in size. Also a lactose fermenter with acid production.
- Salmonella: Grows as colorless colonies because it cannot ferment lactose. The absence of acid means the dyes are not absorbed.
- Pseudomonas aeruginosa: Grows well but produces pinkish colonies with no evidence of sugar fermentation or acid production.
- Proteus vulgaris: Produces pink colonies, indicating it does not ferment lactose, though some mild acid production occurs.
The color spectrum, from metallic green to dark purple to brown to pink to colorless, essentially tells you how much acid each organism is generating from lactose. Strong fermenters sit at one end, non-fermenters at the other.
Why the Metallic Green Sheen Matters
The green metallic sheen produced by E. coli on EMB agar is one of the most recognizable results in basic microbiology. It occurs because E. coli ferments lactose so rapidly and completely that it generates a large amount of acid in the immediate environment around each colony. At that low pH, the eosin and methylene blue dyes form a complex that precipitates directly onto the colony surface. This surface layer of dye crystals reflects light in a way that creates the metallic appearance. Other lactose fermenters like Enterobacter and Klebsiella produce acid too, but not quickly enough or in high enough concentrations to trigger this effect. That difference is what makes EMB agar so useful for distinguishing E. coli from other coliforms.
Where EMB Agar Is Used
EMB agar has been a standard tool since Levine first developed it in 1918 for water analysis. Today it’s used across several fields. In water quality testing, it’s recommended by the American Public Health Association’s Standard Methods for the Examination of Water and Wastewater. The United States Pharmacopeia also includes it in microbiological testing standards. These aren’t niche applications: any time a water utility, food manufacturer, or pharmaceutical company needs to check for fecal contamination, EMB agar is one of the go-to media for detecting E. coli and other coliforms.
In clinical microbiology labs, EMB plates help identify gram-negative pathogens from patient samples, particularly urinary tract infections where E. coli is the most common cause. The plate gives a fast preliminary answer while more definitive testing (biochemical panels or automated identification systems) runs in parallel.
Beyond bacteria, Levine’s EMB agar formulation also sees use in the rapid identification of Candida albicans, a common yeast responsible for fungal infections. This makes the medium more versatile than its name might suggest.
Two Formulations to Know
You’ll sometimes see references to “Levine EMB” and “Holt-Harris and Teague EMB.” The Levine formulation is the simpler and more widely used version. It contains only lactose as its fermentable sugar. The Holt-Harris and Teague version includes both lactose and sucrose, which gives it slightly broader differential capability since some organisms ferment sucrose but not lactose. The Levine version remains the standard for most water and food testing applications, and it’s the formulation specified by the FDA.
Limitations
EMB agar gives presumptive identifications, not definitive ones. A metallic green sheen strongly suggests E. coli, but confirmation requires additional biochemical testing. Some gram-positive organisms can occasionally show limited growth on EMB plates, especially if the medium isn’t prepared correctly or if organisms are present in very high numbers. The medium also won’t distinguish between pathogenic and non-pathogenic strains of the same species. An E. coli colony with a green sheen could be a harmless gut commensal or a dangerous strain like O157:H7, and EMB agar alone can’t tell you which.
Colony colors can also shift if plates are read too early or too late after incubation, so timing matters for accurate interpretation. Despite these caveats, EMB agar remains one of the most efficient screening tools for sorting gram-negative bacteria by their metabolic behavior in a single step.