How do bacteria use fermentation to obtain energy?

Bacteria use fermentation as an anaerobic process to obtain energy when external electron acceptors are unavailable. This metabolic pathway allows bacteria to break down organic molecules, such as sugars or amino acids, into smaller organic end products.1

During this process, the breakdown of the energy source releases electrons, which are then transferred to the resulting smaller organic molecules. These catabolic reactions include specific steps that directly form ATP, the cell's primary energy currency. Because organic molecules are only partially oxidized during fermentation, the energy yield is relatively low—for instance, the breakdown of glucose into lactic acid typically produces only two molecules of ATP per glucose molecule.1

To sustain growth, fermentative bacteria must consume large quantities of nutrients, resulting in the production of significant amounts of organic end products. The specific end products—such as acetic acid, ethanol, or carbon dioxide—vary by species and are often used by scientists to identify different types of bacteria.12