Cheese is a fermented product, which means that cheesemaking requires the growth of millions of bacteria — we call them starters (to start the acid development) — to acidify the product. Initially, these bacteria originate from the raw milk itself (cow, milker, equipment, etc.).
Starter cultures used in cheesemaking are all lactic acid bacteria, which means they readily grow in milk and ferment lactose (and produce lactic acid). When raw milk was used, cheese quality was variable, as acidification and flavor development depended on the types and amounts of bacteria present in that particular raw milk.