Differential and selective media are both types of culture media that can be used to help identify and/or isolate bacterial strains.

Selective media

Selective media contain agents that allow selected bacteria to grow, but inhibit the growth of others. The inhibitory agents could be antimicrobial drugs, dyes or alcohols.

Differential media

Differential media distinguish different types of bacteria based on the appearance of the culture. Differential media contain one or more factors that cause bacteria with certain metabolic or culture characteristics to look different from other bacteria growing on the same agar plate. This type of medium is also sometimes known as an indicator medium. In response to components of the media, the bacterial colonies might change color or affect the appearance of the medium through the production of extracellular enzymes.

Examples

Blood agar

Blood agar is an enriched medium that is differential for hemolysis, the lysis and digestion of red blood cells. In incomplete, or a-hemolysis, the agar around the colonies changes to a green color as hemoglobin is converted to methemoglobin. In complete, or b-hemolysis, the agar around the colonies changes to colorless. No changes to the agar occur in non-hemolytic, or g-hemolytic samples. Hemolysis can be used to help identify bacteria. In some species, such as E. coli, hemolytic activity can vary and can be associated with virulence.

MacConkey agar

MacConkey agar is selective for Gram-negative organisms due to bile salts and crystal violet, which inhibit the growth of Gram-positive bacteria. This means that only Gram-negative bacteria will be able to grow and form colonies on MacConkey agar. MacConkey agar is differential for lactose metabolism. Lactose fermenting bacteria lower the pH of the medium. In response, the pH indicator neutral red turns the colonies and medium pink. Non-lactose fermenting organisms remain white or colorless and the medium changes color to yellow.

Figure 1. Blood and MacConkey agar with possible outcomes