Microbial (or microorganism) is a tiny form of life that can only be seen with a microscope. These organisms are found everywhere on Earth and play an important role in many ecological processes. There are four main groups of microbes: bacteria, archaea, fungi, and protozoa.
Bacteria are the most common and widespread group of microbes. They are single-celled organisms that come in a variety of shapes, including spheres, rods, and spirals. Bacteria can live in almost any environment, including soil, water, air, and even inside other animals. Some bacteria cause diseases in humans, plants, and animals; but many more are harmless or even beneficial. For example, bacteria help us digest food and produce vitamins.
Archaea are similar to bacteria in size and shape but differ from them in several ways. Archaea live in extremely hot or cold environments that would kill most other life forms—such as deep-sea hydrothermal vents or polar ice caps. Like bacteria, some archaea cause disease; but many more are helpful to humans or other organisms by breaking down environmental pollutants or producing useful chemicals such as methane gas.
Fungi are another group of single-celled microbes that includes yeasts and molds. Fungi typically live on dead organic matter—such as leaves or wood—and release enzymes that break down the complex molecules into simpler ones that the fungi can use for food. Some fungi also form symbiotic relationships with plants; for example , certain types of mushrooms help trees absorb nutrients from the soil . Fungi can cause diseases in humans , plants , and animals ; but many species are harmless or even beneficial , such as those used to make bread rise or blue cheese taste moldy .
Protozoa are single-celled eukaryotes—organisms whose cells have a nucleus enclosed within membrane -bound organelles . Most protozoa live in watery environments ; but some , such as amoebas , can also be found crawling around on land . Protozoa range in size from very small ( less than 0 . 1 millimeter ) to quite large ( over 1 meter ) . Many protozoa feed on smaller organisms ; but others get their energy from plant material using photosynthesis like algae do . Some protozoa cause disease ; but many more are helpful to humans by serving as prey for fish or controlling populations of harmful insects .