MacConkey Agar Nº 2 is a mildly selective and differential medium that is specifically useful for the recognition of enterococci in the presence of coliforms and non-lactose fermenters from water, sewage, food, and clinical samples.

Bacteriological peptone provides nitrogen, vitamins, minerals, and amino acids essential for growth. Lactose is the fermentable carbohydrate energy source, causing a pH drop and a colour change in the pH indicator (Neutral red), and bile precipitation. Bile salts Nº 2 and crystal violet are the selective agents, inhibiting Gram-positive, bile-tolerant microorganisms such as staphylococci and non-faecal streptococci. Sodium chloride supplies essential electrolytes for transport and osmotic balance. Bacteriological agar is the solidifying agent.

The presence of intestinal enterococci is an indicator for faecal contamination, especially when the contamination occurred a long before and the less resistant coliform bacteria, including Escherichia coli, may already be dead when the analysis is carried out.