exercise 25 KTTK: MacConkey Agar (Micro. lab)

MacConkey agar is

selective and differential

selective

due to inhibition of Gram-positive bacteria by
• Bile salts
• Crystal violet

differential

• Organisms able to ferment lactose produce acidic end products
• Neutral red dye is pH indicator indicating presence or absence of acidic end products
• Clear above pH of 6.8
• Pink to red below pH of 6.8

what MacConkey agar contains

pancreatic digest of gelatin, pancreatic digest of casein, peptic digest of animal tissue, lactose, bile salts, sodium chloride, neutral red dye, crystal violet, agar, distilled/deionized water

coliforms are

gram negative and ferment lactose

to detect coliforms, MAC agar is formulated to select for Gram-negative bacteria and then differentiates this broad grouping of microbes based on their ability to ferment lactose.

bile salts and crystal violet

in the media inhibit the growth of gram positive bacteria while allowing most gram negative bacteria to grow

selective agent

lactose

fermentable carbohydrate

It serves as the primary sugar source. The ability of gram-negative bacteria to ferment lactose is the basis of differentiation on the agar.

neutral red dye

pH indicator

This dye turns pink-red in the presence of acid. When lactose is fermented, acid is produced, causing the indicator to change color.

Peptones (e.g., peptone, proteose peptone)

nutritional base

These provide essential nutrients, including nitrogenous compounds and vitamins, required for microbial growth.

agar

solidifying agent

This polysaccharide, derived from red algae, solidifies the medium to provide a surface for bacterial growth.

sodium chloride

maintains osmotic balance

This component ensures the proper osmotic environment, preventing water from rushing into or out of the bacterial cells.

Know what bile salts and crystal violet do

act as selective agents. In the media inhibit the growth of Gram-positive bacteria while allowing most Gram-negative bacteria to grow.

what neutral red dye is and why it is added to the agar

s pH indicator indicating presence or absence of acidic end products
• Clear above pH of 6.8
• Pink to red below pH of 6.8

Know why (application) MacConkey agar is used

• Used to isolate and differentiate members of the Enterobacteriaceae based on lactose-fermenting ability
• Certain variations can also be used

Know how Escherichia coli should have grown on the MacConkey agar

pink colonies, gram negative and lactose fermenters (+)

Know what results you would expect and how to interpret them

enterobacter aerogenes

ferments lactose (+). (pink colonies), gram negative

staphylococcus epidermidis

gram positive, colorless or not present and non lactose fermenter (-)

providencia stuartii

clear, colorless and non lactose fermenter (-). Gram positive

MacConkey results and interpretations