What Sugars Give a Positive Fermentation Test?

Sugars do not produce positive results in a fermentation test. Fermentation is the process in which sugars are converted into a different chemical product. In a fermentation test, specific sugars are used to determine the presence of bacteria. A positive result would be the formation of acid, gas or alcohol.

The sugars used in a fermentation test are specifically selected based on the products they become and what bacteria lead to that result. For example, mannitol salt agar kills most bacteria except salt-resistant Staphylococcus bacteria. Only certain strains of Staphylococcus react with the mannitol in the agar, such as Staphylococcus aureus, which produces acidic byproducts. Mannitol salt agar has a red pH indicator that turns yellow in the presence of higher acidity levels to signify this reaction.

Fermentation tests are carried out in different media depending on the result desired and the bacteria tested. The two most common media are broths and agar. Agar is a jelly-like compound produced from seaweed that allows most bacteria to grow on its surface, and it can be infused with various nutrients and chemicals. A broth is made up of solutions of chemicals. Media that restrict certain types of growth are referred to as selective. Media that show a physical change and are used to test for certain products are referred to as differential. These differential media are the basis for many confirmatory tests, including fermentation tests.