A bacterium is a tiny, single-celled organism. There are many different kinds of bacteria, and they can be found in every environment on Earth, from the hot springs of Yellowstone National Park to the deep ocean floor. Bacteria are essential to the recycling of nutrients in ecosystems and the decomposition of organic matter. Some bacteria cause disease, but many more are beneficial to humans and other organisms.
Bacteria are prokaryotic cells, meaning that they lack a nucleus or any other membrane-bound organelles. Their genetic material is organized into a single circular chromosome that is located in the cytoplasm. Bacteria reproduce by binary fission, a process in which the parent cell splits into two identical daughter cells. Most bacteria are between 0.5 and 5 micrometers in length, although some species can reach sizes up to half a millimeter.
The vast majority of bacteria are harmless to humans and even beneficial; for example, gut bacteria help us digest our food and produce vitamins that we need for good health. However, some types of bacteria can cause serious diseases such as pneumonia, meningitis, tuberculosis, cholera, typhoid fever, and food poisoning. These diseases are usually caused by bacterial infections that occur when harmful bacteria enter our body through cuts in our skin or by inhaling them through our nose or mouth. Treatment for bacterial infections typically involves antibiotics, drugs that kill or inhibit the growth of bacteria.
The study of bacteria is called bacteriology. Scientists who study bacteriology are called bacteriologists.