Bacteria - These small cells can be found everywhere. They live in the water, in the air, and on land. Bacteria are among the smallest forms of life on Earth. The size of each individual bacteria cell in the picture on the left is 1/1,000,000th (one millionth) the size of a meter, or 1 micron! You may have up to 100 million bacteria in your body right now! Some bacteria are harmful and cause diseases; others are helpful. You have bacteria in your digestive tract that kill harmful bacteria and digest your food. Bacteria are necessary for turning milk into cheese, cucumbers into pickles, and cabbage into sauerkraut. Other bacteria help decompose dead plants and animals. In the Great Lakes, bacteria are responsible for recycling nutrients back to the food web. Without them useful nutrients would be buried in the sediments and lost forever.