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MacConkey agar with various bacterial colonies

MacConkey agar having the following types of the colony-

Mucoid lactose fermenter (MLF)

Lactose fermenter (LF)-Large

small lactose fermenter

and non-lactose fermenter (NLF)

as shown above picture.

MacConkey agar

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

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

Selective due to bile salts that inhibit 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.

The above picture is showing the 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 the 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

McConkey 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 Enterococcus species due to having crystal violet in its composition.

Further Readings

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 Handbook 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 Textbook 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.  Textbook of Diagnostic Microbiology. Editors: Connie R. Mahon, Donald G. Lehman & George Manuselis, 3rd edition2007, Publisher Elsevier.