Bacteria are usually the most abundant micro-organisms on earth. They are single-celled prokaryotes whose cells lack a nucleus. Most of them have cell walls that contain peptidoglycan as its structural molecule. They are mostly present in the soil, water, and air and are also major inhabitants of our skin, mouth, and intestines. Some bacteria live in extreme habitats such as hot springs, deserts, snow and deep oceans where very few other life forms can survive.