EMB Agar : Principle, Composition, Uses and Interpretation

EMB agar is a commonly used culture media in microbiology laboratories. It is a useful tool for identifying and isolating enteric pathogens, as well as for screening water samples for fecal contamination. In this article, we explore its principle, its preparation, its interpretation and its different uses.

Ⅰ. Overview

Ⅱ. Preparation and composition

Suspend the components, dehydrated powder of EMB Agar, in water (36 grams in 1000 ml of purified/distilled water).

The medium is boiled for a few seconds until the ingredients are completely dissolved. Sterilize in an autoclave at 15 lbs (121°C) pressure for 15 minutes. avoid overheating.

Cool to 45-50°C and stir the medium to oxidize the methylene blue (i.e. restore its blue color) and suspending the flocculant precipitate. Pour into Petri dishes.

Ⅲ. Principle of EMB Agar

Selectivity : Eosin Methylene Blue Agar contains two dyes, eosin and methylene blue, which inhibit the growth of gram-positive bacteria. These dyes have an inhibitory effect on the growth of gram-positive organisms while allowing the growth of gram-negative bacteria.

Differential : EMB agar differentiates between different types of gram-negative bacteria based on their ability to ferment lactose. Lactose-fermenting bacteria produce acid during lactose fermentation, which leads to a color change on the agar.

• Strong lactose fermenters: E. coli is a strong lactose fermenter and produces colonies with a distinct green metallic sheen on EMB agar.
• Weak lactose fermenters : Some other lactose-fermenting bacteria (slow-fermenting lactose) may produce brown-pink colonies.
• Non-lactose fermenters : such as Shigella and Salmonella, do not produce acid and form clear, colorless or amber colonies that are easily distinguished from coliforms.

The phosphate buffers the medium.

Note: The distinctive metallic sheen produced by E. coli on this culture media is due to acid production resulting in an amide bond between eosin and methylene blue, other coliforms not producing enough acid to cause this reaction.

Ⅳ. Results and Interpretation

The interpretation of EMB agar involves observing the colony morphology and color changes of bacterial growth on the medium (In general, after 24 hours at 37°C aerobically). Here are the general interpretations:

First, you observe whether there is any growth of colonies on the EMB agar. If there is no growth, it could indicate :

• The absence of bacteria.
• Presence of gram-positive bacteria (which are inhibited by the selective components of the medium)
• The presence of bacteria that are difficult to grow on this medium (bacteria with special needs).

If there is growth on the EMB agar, you then examine the color of the colonies:

• Strong lactose fermenters, like E. coli, typically produce colonies with a intense green metallic sheen.
• The pink color indicates a lesser degree of lactose fermentation compared to strong fermenters.
• Colorless colonies on EMB agar typically indicate non-lactose fermenting bacteria.

Note : Further confirmatory tests or identification methods should be employed to accurately identify specific bacterial species.