Search Search Toggle the table of contents Pathogenic bacteria 16 languagesAdd topic Learn what bacteria are, where they live, how they affect human health, and why some bacteria are beneficial while others can cause illness. Bacteria are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria were among the first life forms to appear on Earth, and are present in most of its habitats. What are bacteria? Bacteria are single-celled, or simple, organisms that are invisible to the naked eye. Many bacteria are found both inside and outside of organisms, including humans. Bacteria (singular: bacterium) are very small organisms. They are prokaryotic microorganisms. Bacterial cells do not have a nucleus, and most have no organelles with membranes around them. Most have a cell wall. They do have DNA, and their biochemistry is basically the same as other living things. bacteria, any of a group of microscopic single-celled organisms that live in enormous numbers in almost every environment on Earth, from deep-sea vents to deep below Earth’s surface to the digestive tracts of humans. What are bacteria? Bacteria are microscopic living organisms that have only one cell. The word for just one is “bacterium.” Millions (if not billions) of different types of bacteria can be found all over the world, including in your body. They’re on your skin and in your airways and mouth. Bacteria are small single-celled organisms. Bacteria are found almost everywhere on Earth and are vital to the planet's ecosystems. Some species can live under extreme conditions of temperature and pressure. The human body is full of bacteria, and in fact is estimated to contain more bacterial cells than human cells. What is bacteria and how does it look under a microscope? Learn types, characteristics, & reproduction using examples & labeled picture. Bacteria are single-celled microorganisms with prokaryotic cells, which are single cells that do not have organelles or a true nucleus and are less complex than eukaryotic cells.