exercise 26 KTTK: Eosin methylene blue agar (Micro. lab)

Eosin methylene blue (EMB) agar is

complex, enriched, selective, and differential medium

complex

• Chemically undefined
• Due to digest of gelatin which provides nitrogen and organic carbon

selective

due to inhibition of most Gram-positive bacteria by
• Eosin Y
• Methylene blue

differential

• Organisms able to ferment lactose produce acidic end products
Escherichia coli and Enterobacter aerogenes ferment lactose
Proteus, Shigella, and Salmonella do not ferment lactose
• Aggressive lactose fermenters interact with Eosin Y and methylene blue
Produce dark purple to black colonies
E. coli usually has distinctive green metallic sheen
• Less aggressive lactose fermenters
Produce pink to dark purple colonies
Enterobacter or Klebsiella species would be examples
• Non-fermenters retain normal color or take on medium’s coloration

EMB agar contains

pancreatic digest of gelatin, lactose, dipotassium phosphate, methylene blue,

eosin Y, sucrose, distilled/deionoized water

the purposes of the dyes eosin Y and methylene blue are

render the media selective and allow us to differentiate lactose fermenters from non-fermenters. Because the dyes inhibit the growth of most Gram-positive organisms (some Enterococcus and Staphylococcus species are exceptions), these media are selective for Gram-negative bacteria.

why (application) eosin methylene blue agar is used

• Used for isolation of fecal coliforms
• Can either be streaked for isolation or used in Membrane Filter Technique

Know how Escherichia coli should have grown on the EMB agar

dark blue black colonies with a distinctive metallic green sheen

providencia stuartii

appears colorless or pale colored colonies and non lactose fermenter

gram negative bacterium

enterobacter aerogenes

a large pinkish to purple mucoid colony with a dark center but without metallic sheen. weakly ferments lactose. gram negative

staphylococcus epidermidis

little to no growth, gram positive, non lactose fermenter

Eosin methylene blue agar results and interpretations

Role of methylene blue in EMB agar

It is important for differentiating lactose fermenters from non-fermenters.

It has a role in inhibiting gram positive bacteria from growing

Roles can be tied to eosin Y in EMB agar

It helps us visually differentiate lactose fermenters from non-fermenters.

It inhibits the growth of most gram positive bacteria