Bacteria are single-celled microorganisms that are found in almost every environment, including soil, water, and air. Some bacteria are responsible for causing infectious diseases in humans, animals, and plants, but most of them are actually beneficial.

For example, bacteria can be used to produce food like yogurt and cheese, and they are also important for breaking down dead organic matter, recycling nutrients, and fixing nitrogen in the soil. In addition, many bacteria can also be used to clean up contaminated soil and water, and some have even been used to develop novel antibiotics.