Canadian Society for Medical Laboratory Science (CSMLS) Practice Exam

Which type of agar is specifically used for selective growth of gram-negative bacteria?

1. Chocolate agar

2. MacConkey agar

3. Blood agar

4. Mueller Hinton agar

The correct answer is: MacConkey agar

MacConkey agar is specifically designed for the selective growth of gram-negative bacteria. This agar contains bile salts and crystal violet, which inhibit the growth of gram-positive bacteria, allowing for a more concentrated isolation of gram-negative species. Additionally, MacConkey agar includes lactose as a fermentable carbohydrate and a pH indicator that allows for the differentiation of lactose-fermenting bacteria from non-fermenters. When lactose is fermented, acid is produced, leading to a color change in the medium, which helps in identification. Chocolate agar supports the growth of a wide range of organisms, including both gram-positive and gram-negative bacteria, but it does not selectively favor gram-negative species. Blood agar is a rich medium that supports the growth of many organisms but is not selective; it allows for both gram-positive and gram-negative bacteria to grow. Mueller Hinton agar is primarily used for antibiotic susceptibility testing and is not selective for any specific group of bacteria.