Final answer:
The process wherein bacteria use lactose to produce propionic acid and CO₂ is known as fermentation, specifically propionic acid fermentation, an anaerobic process.
Step-by-step explanation:
When some bacteria use lactose as a source of energy and produce propionic acid and CO₂, this process is known as fermentation. Fermentation is an anaerobic process, meaning it does not require oxygen. The reactant, or starting material shared by aerobic respiration and both types of fermentation—alcoholic fermentation and lactic acid fermentation—is glucose. In the case of propionic acid fermentation, this pathway is particularly important in the production of the distinctive flavor of Swiss cheese and involves the use of prokaryotes, which can switch between aerobic respiration and fermentation depending on oxygen availability.