Microorganisms digest the lactose in sugar in milk and transform it into lactic acid. They contain enzymes that have the ability to break the sugars down into glucose and galactose. These microorganisms, also known as starter culture, are added to milk that has been boiled to kill unwanted bacteria as it cools, and are available commercially at health stores or pharmacies, usually in tablet form.
Streptococcus lactis is used in the production of butter, soft cheeses like Brie and Camembert, and yoghurt. It is commonly known as lactococcus, to avoid confusion with the organism that causes infections in humans.
Lactobacillus bulgaricus is usually found in thick Bulgarian yoghurt -- hence its name. In addition to its role in yoghurt, it has antibiotic properties and can help remove harmful toxins from the digestive system -- and is also sold in capsules as a dietary supplement.
Lactobacillus acidophilus is the most widely used microorganism in making yoghurt. It also has many health benefits and can be prescribed in supplement form to patients taking antibiotics, as it's known to destroy both the harmful and "friendly" bacteria that exists in the digestive system. It can also help treat diarrhoea, constipation and irritable bowel syndrome.
Streptococcus thermophilus is one of the most commonly used starter cultures in making yoghurt and Mozzarella cheese. It's also used in the production of low-fat dairy products and allows yoghurt to retain all the flavour of regular yoghurt. That's because it breaks down the proteins into the smaller elements needed for the texture and flavour of full-fat cheese and yoghurt.