These tiny, single-celled organisms—bacteria—are among the most ancient, numerous, and influential forms of life on Earth. They are so small that millions can live in a single drop of water, yet their impact spans the globe, from shaping ecosystems to influencing human health and disease. What are bacteria and where can they be found? Bacteria are microscopic single-celled organisms that live in almost every environment on Earth, from deep-sea vents to human digestive tracts. They are prokaryotes, lacking a membrane-bound nucleus. Some are harmful, but others support life. They play a crucial role in human health and are used in medicine and industry. Learn about the types, lifecycles, uses, and hazards of bacteria here. 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 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 single-celled organisms with a unique internal structure. Humans and other multicellular organisms are eukaryotes, which means our cells have distinct nuclei bound with a membrane. 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. Bacteria are microbes with a much simpler cell structure than many other organisms, but they are by no means simple. The more scientists look, the more they understand about how complex bacteria are. 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. What is bacteria and how does it look under a microscope? Learn types, characteristics, & reproduction using examples & labeled picture.