MacConkey agar is used for isolation and differentiation of gram negative enteric bacilli from specimen containing swarming strains of Proteus spp. This is a useful medium to differentiate Lactose fermenters from non-lactose fermenting organisms. The selective agents of MacConkey Agar inhibit the growth of gram positive organisms.
Principle of Medium
Peptone is source of nitrogen and other nutrients. Lactose is fermentable carbohydrate. when lactose is fermented, a local pH drop around the colony cause a colour change in pH indicator(neutral red) and bile precipitation. Bile salts, oxgall and crystal violet are selective agents that inhibits growth of gram positive organisms. Agar is the solidified agent.
- Sodium taurocholate/ Bile salt 5g
- Peptone 20g
- Sodium chloride 5g
- Lactose 10g
- Agar 15g
- Neutral red 35ml
- Distilled water 1000ml
Prepare as instructed by manufacture. Sterilize the solution by autoclaving at 1210C for 5 minutes. when the medium has cooled to (50 – 55)0C , mix well and pour into sterile petridishes. Date the medium and give them to a batch number.
Storage and Self life
The MacConkey agar plates should be stored at (2-8)0C preferably in plastic bags to prevent loss of moisture. The MacConkey agar is light and temperature sensitive. Therefore it should be protected from light, excessive heat, moisture and freezing.
The self life of media is indicated on agar bottle. if there is no change in the appearance of medium suggesting contamination or alteration of pH,; medium can be used upto 4 weeks.
Media should not be used, if there are signs of contamination, deterioration (cracking, shrinking or discolouration) or if the expiration date is passed.
The quality control is tested with at least one known organism to demonstrate growth or positive reaction and at least one organism to demonstrate inhibition or negative reaction
|Control Organism||Expected Results|
|E. coli (ATCC 25922)||Pink colonies|
|Staphylococcus aureus (ATCC 25923)||Growth|
|Streptococcus faecalis (ATCC 29212)||Growth with magenta pink colonies|
|Proteus mirabilis (ATCC 12453)||No swarming|
|Beta haemolytic Streptococcus||No growth|