Lactobacillus helveticus is a lactic-acid producing, rod-shaped bacterium of the genus Lactobacillus. It is most commonly used in the production of American Swiss cheese and Emmental cheese, but is also sometimes used in making other styles of cheese, such as Cheddar, Parmesan, Romano, provolone, and mozzarella. The primary function of L. helveticus culture is to prevent bitterness and produce nutty flavors in the final cheese. In Emmental cheese production, L. helveticus is used in conjunction with a Propionibacter culture, which is responsible for developing the holes (known as "eyes") through production of carbon dioxide gas.