Solidifying Agents: A couple of solidifying agents commonly used in microbiology labs are agar and gelatin. Each imparts different traits or attributes to the medium. These are discussed below:

1. Agar. This is the most widely used solidifying agent. Agar is a complex polysaccharide extracted from marine red algae. It is typically added to a broth at a concentration around 1.5%. If a firmer medium is desired, higher concentrations can be used.

Useful Properties:

• Most microbes are unable to degrade agar. This keeps the medium a solid while microbes grow on it.

• The melting point of agar is between 95º-100ºC. While the medium is still relatively warm, it can be poured, pipetted, or inoculated with microbes. As it cools to 42º-44ºC, agar begins to solidify the medium. Once solidified, microbes can be incubated at 37ºC (or higher) without the fear of the medium liquefying.

2. Gelatin. This solidifying agent when used at concentrations of 10%-15% creates a firm solid medium. However, its uses are more limited because of its low melting point. Mediums using gelatin liquefy at room temperature.

Useful Property:

• Nutrient gelatin is useful in bacterial identification. Gelatin is a protein that can be broken down by enzymes secreted by some bacteria. Bacteria that secrete these enzymes are identified when the nutrient gelatin they are grown on undergoes liquefaction.