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Question: Why do fermenters produce black colonies on EMB Agar?

EMB agar is characterized by the presence of a combination of the two dyes eosin and methylene blue in the ratio of 6:1. Gram-negative bacteria that ferment the lactose produce acid which lowers the pH. This encourages dye absorption by the colonies and turns the colonies dark purple as the acid acts upon the dyes.

What color colonies does a strong fermenter form on an EMB plate?

EMB contents Strong acidity produces a deep purple colony with a green metallic sheen, whereas less acidity may produce a brown-pink coloration of colony. Nonlactose fermenters appear as translucent or pink. Colonies of lactose fermenters will appear very dark purple, or have dark purple centers.

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What does EMB agar indicate by colony color?

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.

Why is E. coli green on EMB?

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.

Which color is produced by Enterococcus cloacae EMB agar?

Enterobacter aerogenes colonies are a pink/buff color with darker centers.

Why is EMB medium differential?

Selective for Gram negative Organisms Eosin–Methylene Blue (EMB) Agar is a differential medium for the detection of Gram negative enteric bacteria. The growth of Gram positive bacteria is generally inhibited on EMB agar because of the toxicity of methlyene blue dye.

Which of the following colors can be indicative of lactose fermentation on an EMB plate?

On EMB agar, isolated colonies of lactose-fermenting bacteria appear brown to blue-black in color. Escherichia coli grow as large, blue-black colonies that often possess a green metallic sheen (Fig. 3).

What component of EMB makes it selective?

Eosin Methylene Blue (EMB) Agar: is a selective AND a differential medium. The selective components are the eosin and methylene blue dyes that inhibit the growth of Gram-positive bacteria and permit the growth of Gram-negative enteric rods.

Why EMB has both selective and differential properties?

EMB contains the dyes eosin and methylene blue that inhibit the growth of gram-positve bacteria. Therefore, EMB is selective for gram-negatives. In addition, the gram-negatives that grow can be differentiated based on their ability to ferment lactose.

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What are the roles of eosin Y and methylene blue in EMB agar?

Eosin Y and methylene are responsible for color changes made by lactose fermenters. It inhibits the growth of most gram positive bacteria. The roles of these two dyes are to stop the growth of gram positive organisms and react to lactose fermenters.

What color is E. coli on an EMB plate?

Escherichia coli often produces a metallic green sheen on EMB.

What color will E. coli exhibit when grown on an EMB agar plate explain your answer?

Escherichia coli (E. coli) will give a “ metallic green sheen” color on EMB (eosin methylene blue) agar.

Why do Escherichia coli give metallic green sheen on eosin methylene blue agar?

The distinctive metallic sheen produced by E. coli on this medium is due to acid production resulting in an amide bonding between the eosin and methylene blue, other coliforms do not produce enough acid to cause this reaction. Eosin inhibits most Gram-positive organisms. The prepared medium is sensitive to light.

What does Levine EMB test for?

Levine EMB Agar is a slightly selective and differential medium for the isolation, cultivation and differentiation of gram-negative enteric microorganisms isolated from both clinical and nonclinical specimens. It is widely used for the examination of materials of sanitary importance for the presence of coliforms.

What colors would you expect to see on an EMB plate containing E coli?

What colors would you expect to see on an EMB plate containing E coli? The EMB plate itself is red in color while in the presence of E coli, the growing colonies will take on a distinctive metallic green sheen.

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Does Enterobacter grow on EMB agar?

Eosin methylene blue (EMB) agar is selective for gram negative bacteria. Enterobacter and Klebsiella form large mucoid colonies that are pink to purple because of their lactose fermentation. the organism growing on EMB agar, which you may contrast with Fig 3.

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