Last updated on June 21st, 2021
Eosin Methylene Blue (EMB) agar is both a selective and differential culture medium. It selectively promotes the growth of Gram-negative bacteria (inhibits Gram-positive bacteria) and aids in the differentiation of lactose fermenter and non-lactose fermenting colonies.
EMB agar, first described by Holt-Harris and Teague, contained lactose and sucrose as source of carbohydrates. Levine further modified the medium by adding peptone and phosphate and removed sucrose from the formula and increased the lactose content. This aided in the differentiation of fecal and non-fecal types of the coliforms and also salmonellae and other non-lactose fermenters from the coliforms.
EMB agar contains sucrose and lactose, utilized as fermentable carbohydrates substrates, which encourage the growth of some gram-negative bacteria, especially fecal and non-fecal coliforms. Differentiation of enteric bacteria is possible due to the presence of the sugars lactose and sucrose in the EMB agar and the ability of certain bacteria to ferment the lactose in the medium.
- Lactose-fermenting gram-negative bacteria acidify the medium, which reduces the pH, and the dye produces a dark purple complex usually associated with a green metallic sheen. This metallic green sheen is an indicator of vigorous lactose and/or sucrose fermentation ability typical of fecal coliforms.
- Organisms that are slow lactose-fermenters, produce less acid, and the colonies appear brown-pink.
- Non-lactose fermenters, increase the pH of the medium by deamination of proteins and produce colorless or light pink colonies.
Eosin Y and methylene blue are pH indicator dyes that combine to form a dark purple precipitate at low pH; they also serve to inhibit the growth of most Gram-positive organisms. Peptic digest of animal tissue serves as a source of carbon, nitrogen, and other essential growth nutrients. Phosphate buffers the medium.
Composition of EMB Agar
The composition of EMB agar and modified EMB agar (Levine EMB) agar differs slightly. Levine modification contains 10g of lactose (twice as in EMB agar) and contains no sucrose.
|Ingredients||EMB agar (gm/L)||Levine EMB agar (gm/L)|
Preparation of EMB agar
- Weigh and suspend 35.96 grams of dehydrated media in 1000 ml distilled water.
- Mix until the suspension is uniform and heat to boiling to dissolve the medium completely.
- Sterilize by autoclaving at 15 lbs pressure (121°C) for 15 minutes.
- Cool to 45-50°C, and with frequent gentle swirling, pour the media into sterile Petri plates.
Note: frequent swirling is recommended to restore the blue color o methylene blue and to suspend the flocculent precipitate if any.
- Label with initials of the name of the medium, date of preparation, and store the plates upside down (lids below) in the refrigerator until use.
|Organism||Colonial appearance on EMB agar|
|Escherichia coli||Colonies are 2-3 mm in diameter, and have greenish metallic sheen in reflected light, dark or even black centre in transmitted light|
|Enterobacter aerogenes||Colonies are 4-6mm in diameter, raised and mucoid, tending to become confluent. |
No metallic sheen, grey-brown centers by transmitted light
|Salmonella and Shigella spp||Translucent and colorless colonies|
|Pseudomonas spp||Colorless irregular colonies|
|Proteus spp||Colorless colonies|
|Gram positive cocci||Partially inhibited or no growth|
|Coagulase-positive staphylococci||Colorless, “pin-point” colonies on modified EMB|
Quality control of EMB agar
Sterility testing can be performed by incubating 3-5% uninoculated plates from each batch at 37°C for 18-24 hours. Any growth on the media should be regarded as contamination and the whole lot should be discarded.
Performance testing of prepared EMB agar plates can be done by inoculating known strains of bacteria into the medium and observing growth and colonial characteristics.
|Organism||Growth and colony characteristics|
|E.coli ATCC 25922||Good growth, blue-black colonies with a green metallic sheen|
|Salmonella choleraesuis subsp. Choleraesuis serotype Typhimurium ATCC 14028||Luxuriant growth, colorless to amber colonies|
|Enterococcus faecalis ATCC 29212||Inhibition (partial)|
|Shigella flexneri ATCC 12022||Moderate to heavy growth, colorless to amber colonies|
Uses of EMB agar
Isolation and differentiation of lactose fermenting and non-lactose fermenting enteric bacilli.
- EMB agar is used in water quality tests to distinguish coliforms and fecal coliforms that signal possible pathogenic microorganism contamination in water samples (presence of E.coli in the river/water sample indicates the possibility of fecal contamination of water so does the presence of other pathogenic enterics).
- EMB media assists in the visual distinction ofEscherichia coli, other nonpathogenic lactose-fermenting enteric gram-negative rods, and the Salmonella and Shigella genera. Escherichia coli colonies grow with a metallic sheen with a dark center. Aerobacter aerogenes colonies have a brown center, and non-lactose-fermenting gram-negative bacteria appear pink.
- EMB agar is also used to differentiate the organisms in the colon-typhoid-dysentery group. For culture of Salmonella and Shigella, selective medium such as MacConkey agar and EMB agar is commonly used.
- Levine EMB Agar can be used for the isolation and identification of Candida albicans from clinical specimens. Addition of 0.1g/L of chlortetracycline hydrochloride after autoclaving makes the medium selective by inhibiting the accompanying bacterial flora. The culture medium then is blue in color. Colonies of Candida albicans appear `spidery’ or `feathery’ after 24 to 48 hours of incubation at 35°C in 10% carbon dioxide. Other Candida species produce smooth yeast-like colonies.
References and further readings
- ASMscience | Eosin-Methylene Blue Agar Plates Protocol. Retrieved June 12, 2020, from https://www.asmscience.org/content/education/protocol/protocol.2869
- Welcome to Microbugz—Eosin Methylene Blue Agar. Retrieved June 12, 2020, from https://www.austincc.edu/microbugz/eosin_methylene_blue_agar.php
- Eosin Methylene Blue Agar (EMB). (2016, April 12). Biology LibreTexts.