MacConkey agar

MacConkey agar uses for culture of gram negative bacteria and therefore enterobacteriaceae belonging bacteria grow well on this medium.

It is selective, differential and indicator medium because of following properties-

Selective due to bile salts that inhibits gram positive bacteria and selects gram negative bacilli.

Indicator medium is due to having neutral red incorporated in it.

Differential medium is due to separate whether lactose fermenter or non lactose fermenter bacteria.

Above picture is showing lactose and non lactose fermenter colony of bacteria.

Composition of MacConkey agar

Ingredients Gms / Litre
Peptones (meat and casein):  3.000
Pancreatic digest of gelatin: 17.000
Lactose monohydrate:  10.000
Bile salts:  1.500
Sodium chloride: 5.000
Crystal violet:  0.001
Neutral red:  0.030
Agar : 13.500
pH after sterilization( at 25°C) 7.1±0.2

Preparation of MacConkey agar

1. Suspend 49.53 grams of dehydrated medium in 1000 ml purified/distilled water.
2. Heat to boiling to dissolve the medium completely.
3. Sterilize by autoclaving at 15 lbs pressure (121°C) for 15 minutes i.e. validated cycle.
4. Cool to 45-50°C.
5. Mix well before pouring into sterile Petri plates.
6. Leave for drying .
7. Store in refrigerator at 2-8°C.
8. Warm up the agar  and use but the surface of the medium should be dry when inoculated.

It is of various types on the purpose of uses like

MacConkey agar without bile salt- It uses both gram negative and gram positive bacteria

Macconkey agar with bile salt- Selective for gram negative bacteria but Enterococcus species may grow.

MacConkey agar with bole sat and crystal violet: Strict selective medium for gram negative bacteria that also inhibits Entercoccus  species due to having crystal violet in its composition.

References

1. Bailey & Scott’s Diagnostic Microbiology. Editors: Bettey A. Forbes, Daniel F. Sahm & Alice S. Weissfeld, 12th ed 2007, Publisher Elsevier.
2. Clinical Microbiology Procedure Hand book Vol. I & II, Chief in editor H.D. Isenberg, Albert Einstein College of Medicine, New York, Publisher ASM (American Society for Microbiology), Washington DC.
3. Colour Atlas and Text book of Diagnostic Microbiology. Editors: Koneman E.W., Allen D.D., Dowell V.R. Jr and Sommers H.M.
4. Jawetz, Melnick and Adelberg’s Medical Microbiology. Editors: Geo. F. Brook, Janet S. Butel & Stephen A. Morse, 21st ed 1998, Publisher Appleton & Lance, Co Stamford Connecticut.
5. Mackie and Mc Cartney Practical Medical Microbiology. Editors: J.G. Colle, A.G. Fraser, B.P. Marmion, A. Simmous, 4th ed, Publisher Churchill Living Stone, New York, Melborne, Sans Franscisco 1996.
6.  Text book of Diagnostic Microbiology. Editors: Connie R. Mahon, Donald G. Lehman & George Manuselis, 3rd edition2007, Publisher Elsevier.