Why does Swiss cheese have holes?

Swiss cheese has holes because the bubbles of carbon dioxide that form inside the cheese as it hardens become holes when the cheese is sliced. One of the types of bacteria that is used in the culturing of Swiss cheese produces carbon dioxide as part of its metabolism.

The bacterium Propionibacterium freudenreichii, which is one of several species of bacteria used in the production of Swiss or Emmentaler cheese, produces carbon dioxide from the fermentation of lactic acid while the cheese is curing. This bacterium is found in larger quantities in Swiss and Emmentaler cheeses than in other sorts of cheeses, which explains why bubbles are more evident in these cheeses than in most other cheeses.