Quality Control on MacConkey Agar

Description

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.

Formulation

1. Sodium taurocholate/ Bile salt                  5g
2. Peptone                                                      20g
3. Sodium chloride                                         5g
4. Lactose                                                       10g
5. Agar                                                            15g
6. Neutral red                                                  35ml
7. Distilled water                                             1000ml

Preparation

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.

Quality Control

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

References

www.hardydiagnostics.com/catalog2/hugo/MacConkeyAgar.htm

www.searo.who.int/en/Section10/Section17/Section53/Section482_1783.htm

www.pmlmicro.com/assets/TDS/475.pdf