Agar plates

Topics: Microbiology, Bacteria, Escherichia coli Pages: 7 (1285 words) Published: January 5, 2014
Mac Conkey‘s Agar plate
MacConkey (also McConkey) agar is a culture medium designed to grow Gram-negative bacteria and stain them for lactose fermentation. It contains bile salts (to inhibit most Grampositive bacteria, except Enterococcus and some species of Staphylococcus), crystal violet dye (which also inhibits certain Gram-positive bacteria), neutral red dye (which stains microbes fermenting lactose), lactose and peptone.

Results after 24 hrs at 35º C




Escherichia coli




Proteus mirabilis








Streptococcus faecalis 29212


or partial

Acting as a visual ph indicator, the agar distinguishes those Gram-negative bacteria that can ferment the sugar lactose (Lac+) from those that cannot (Lac-). This medium is also known as an "indicator medium" and a "low selective medium". Absence of electrolytes serves to inhibit swarming by Proteus species.

By utilizing the lactose available in the medium, Lac+ bacteria such as Escherichia coli, Enterobacter and Klebsiella will produce acid, which lowers the pH of the agar below 6.8 and results in the appearance of red/pink colonies.

Eosin methylene blue (EMB) agar –
Eosin methylene blue (EMB) is a 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 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).[1] Organisms that ferment lactose display "nucleated colonies" -- colonies with dark centers.[2]. •

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.

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). It is one of the few bacteria that reacts this way to EMB.[3]

Examples of EMB with different coliform bacteria. 

Eosin-methylene blue (EMB) agar plate inoculated with Escherichia coli (a gram-negative coliform bacterium) showing good growth of dark blue-black colonies with metallic green sheen indicating vigorous fermentation of lactose and acid production which precipitates the green metallic pigment.

EMB agar plate inoculated with Enterobacter aerogenes (a gram-negative coliform bacterium) showing good growth of brown, dark-centered, mucoid colonies indicating lactose fermentation and acid production.

EMB agar plate inoculated with Klebsiella
pneumoniae (a gram-negative coliform bacterium) showing good growth of brown, dark-centered, mucoid colonies (smaller than Enterobacter) indicating lactose fermentation and acid production.


EMB agar plate inoculated with (A)
Escherichia coli, (B) Pseudomonas aeruginosa, (C) Klebsiella pneumoinae, and (D) Enterobacter aerogenes. All four gram negative bacteria grew exhibiting different morphology. Escherischia coli grew with typical lactose fermenter

morphology with excessive acid production and precipitation of green metallic pigment (colonies with green metallic sheen). Pseudomonas aeruginosa grew exhibiting the nonfermenter morphology (pinkish colonies), both Klebsiella pneumoniae and Enterobacter aerogenes grew with lactose fermentation and acid production morphology (with purple dark centered mucoid colonies).

Hektoen enteric agar
Hektoen enteric agar (HEK or HE or HEA) is a selective and differential agar...
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