What type of medium is often used to isolate Enterobacteriaceae from clinical samples?

• A. Nutrient agar
• B. MacConkey agar
• C. Blood agar
• D. Potato dextrose agar

Answer: (B)

The medium commonly used to isolate Enterobacteriaceae from clinical samples is MacConkey agar. This selective and differential medium is specifically formulated to inhibit the growth of Gram-positive bacteria while allowing Gram-negative bacteria, specifically members of the Enterobacteriaceae family, to grow.

MacConkey agar contains bile salts and crystal violet, which suppress the growth of non-enteric bacteria. It also contains lactose and a pH indicator, allowing for differentiation based on lactose fermentation. Enterobacteriaceae such as Escherichia coli ferment lactose, producing acid that lowers the pH and changes the color of the medium, typically to pink or red colonies. Non-lactose fermenters, however, do not produce acid and remain colorless on the medium. This characteristic makes MacConkey agar particularly useful in clinical microbiology for isolating and identifying Enterobacteriaceae, facilitating the quick differentiation of pathogenic strains in samples.

In contrast, nutrient agar is a general-purpose medium that supports a wide range of bacterial growth, but it lacks the selective properties needed for isolating Enterobacteriaceae. Blood agar is primarily used for growing fastidious organisms and can support the growth of many bacterial types, but it does not provide the same selective advantage for Enter