Eosin methylene blue
Encyclopedia
Eosin methylene blue is a slightly selective stain
for Gram-negative bacteria
. It is a blend of two stains, eosin
and methylene blue
in the ratio of 6:1. A common application of this stain is in the preparation of EMB agar, a differential microbiological medium, which slightly inhibits the growth of Gram-positive bacteria
and provides a color indicator distinguishing between organisms that ferment lactose
(e.g., E. coli") and those that do not (e.g., Salmonella, Shigella). Organisms that ferment lactose
display "nucleated colonies" -- colonies with dark centers.
This medium is important in medical laboratories by distinguishing pathogenic microbes in a short period of time.
On EMB if E. coli
is grown it will give a distinctive metallic green sheen (due to the metachromatic properties of the dyes, E. coli movement using flagella, and strong acid end-products of fermentation). Some species of Citrobacter and Enterobacter will also react this way to EMB.
Some Gram Positive bacteria are able to grow on EMB agar such as fecal streptococci and staphylococci.
EMB contains the following ingredients:
Pancreatic digest of gelatin,
Lactose,
Dipotassium phosphate,
Eosin Y (dye),
Methylene blue (dye),
Agar
There are also EMB agars that do not contain lactose
Staining (biology)
Staining is an auxiliary technique used in microscopy to enhance contrast in the microscopic image. Stains and dyes are frequently used in biology and medicine to highlight structures in biological tissues for viewing, often with the aid of different microscopes...
for Gram-negative bacteria
Gram-negative
Gram-negative bacteria are bacteria that do not retain crystal violet dye in the Gram staining protocol. In a Gram stain test, a counterstain is added after the crystal violet, coloring all Gram-negative bacteria with a red or pink color...
. It is a blend of two stains, eosin
Eosin
Eosin is a fluorescent red dye resulting from the action of bromine on fluorescein. It can be used to stain cytoplasm, collagen and muscle fibers for examination under the microscope. Structures that stain readily with eosin are termed eosinophilic....
and methylene blue
Methylene blue
Methylene blue is a heterocyclic aromatic chemical compound with the molecular formula C16H18N3SCl. It has many uses in a range of different fields, such as biology and chemistry. At room temperature it appears as a solid, odorless, dark green powder, that yields a blue solution when dissolved in...
in the ratio of 6:1. A common application of this stain is in the preparation of EMB agar, a differential microbiological medium, which slightly inhibits the growth of Gram-positive bacteria
Gram-positive
Gram-positive bacteria are those that are stained dark blue or violet by Gram staining. This is in contrast to Gram-negative bacteria, which cannot retain the crystal violet stain, instead taking up the counterstain and appearing red or pink...
and provides a color indicator distinguishing between organisms that ferment lactose
Lactose
Lactose is a disaccharide sugar that is found most notably in milk and is formed from galactose and glucose. Lactose makes up around 2~8% of milk , although the amount varies among species and individuals. It is extracted from sweet or sour whey. The name comes from or , the Latin word for milk,...
(e.g., E. coli") and those that do not (e.g., Salmonella, Shigella). Organisms that ferment lactose
Lactose
Lactose is a disaccharide sugar that is found most notably in milk and is formed from galactose and glucose. Lactose makes up around 2~8% of milk , although the amount varies among species and individuals. It is extracted from sweet or sour whey. The name comes from or , the Latin word for milk,...
display "nucleated colonies" -- colonies with dark centers.
This medium is important in medical laboratories by distinguishing pathogenic microbes in a short period of time.
- Lactose fermentation produces acids, which lower the pH. This encourages dye absorption by the colonies, which are now coloured purple-black.
- Lactose non-fermenters may increase the pH by deamination of proteins. This ensures that the dye is not absorbed. The colonies will be colorless.
On EMB if E. coli
Escherichia coli
Escherichia coli is a Gram-negative, rod-shaped bacterium that is commonly found in the lower intestine of warm-blooded organisms . Most E. coli strains are harmless, but some serotypes can cause serious food poisoning in humans, and are occasionally responsible for product recalls...
is grown it will give a distinctive metallic green sheen (due to the metachromatic properties of the dyes, E. coli movement using flagella, and strong acid end-products of fermentation). Some species of Citrobacter and Enterobacter will also react this way to EMB.
Some Gram Positive bacteria are able to grow on EMB agar such as fecal streptococci and staphylococci.
EMB contains the following ingredients:
Pancreatic digest of gelatin,
Lactose,
Dipotassium phosphate,
Eosin Y (dye),
Methylene blue (dye),
Agar
There are also EMB agars that do not contain lactose