MacConkey Agar is recommended for selective isolation and differentiation of coliforms from pharmaceutical products using the microbial limit testing in compliance with the harmonized methodology of USP/EP/BP/JP.

Summary and Explanation

MacConkey Agar is based on the bile salt-neutral red-lactose agar of MacConkey.(1)The original MacConkey medium was used to differentiate strains of Salmonella typhosa from members of the coliform group. Formula modifications improved the growth of Shigella and Salmonella strains. These modifications included the addition of 0.5% sodi um chloride, decreased agar content, and altered bile salts and neutral red concentrations. The formula improvements gave improved differential reactions between these enteric pathogens and the coliform group. MacConkey Agar is the earliest selective and differential medium for cultivation of enteric microorganisms from a variety of clinical specimens.(1, 2) Subsequently MacConkey Agar and Broth have been recommended for use in microbiological examinati on of foodstuffs(3) and for direct plating / inoculation of water samples for coliform counts.(4) This medium is also accepted by the Standard Methods for the Examination of Milk and Dairy Products.(5) British pharmacopoeia(6) has recommended this medium for the subculture and identification of Escherichia coli. It is also cited as Agar Medium H. It is also recommended by and in accordance with the harmonized method of USP/BP/EP/JP.(7, 6, 8, 9).

Principles of the Procedure

MacConkey Agar contains pancreatic digest of gelatin and peptones (meat and casein) which provides the essential nutrients, vitamins and nitrogenous factors required for growth of microorganisms. Lactose monohydrate is the fermentable source of carbohydrate. Bile salts and crystal violet attributes to the selective action of this medium which is inhibitory to most species of gram-positive bacteria. Gram-negative bacteria usually grow well on the medi um and are differentiated by their ability to ferment lactose. Neutral red is the indicator dye. Lactose fermentation produces acid, which causes absorption of neutral red and a subsequent colour change of the dye giving pink to red colour colonies, when the pH of medium falls. Sodium chloride in the medium helps to maintain osmotic balance of the cells.