Overview

A microorganism, often called a microbe, is any living or biological entity so small that it cannot be seen with the unaided eye. In biology it is treated as a kind of organism studied by the field of microbiology. Microorganisms range from single cells to simple multicellular forms and include both cellular life and non-cellular infectious agents.

Main groups and basic characteristics

Microorganisms are commonly classified into several major groups: bacteria, archaea, protists, microscopic fungi, and viruses. Most are unicellular organisms consisting of a single cell or one cell, though small multicellular forms exist. Viruses are distinct in that they lack autonomous metabolism and must reproduce inside host cells.

Habitats and extreme environments

Microbes inhabit virtually every environment on Earth where there is liquid water. They are found in soils, the atmosphere, and aquatic systems, including thermal ecosystems such as hot springs and deep-sea vents on the ocean floor. Some live deep inside rocks and within the planet's crust, associated with geological processes beneath the surface of the Earth. Organisms adapted to such conditions are known as extremophiles.

Ecological roles and human uses

Microorganisms are central to ecosystem function. They break down organic matter as decomposers, recycle nutrients and participate in elemental cycles such as the nitrogen cycle. Many form beneficial relationships with plants and animals, fix atmospheric nitrogen, or contribute to soil formation. Humans harness microbes for food fermentation, waste treatment, biotechnology, and pharmaceutical production; conversely, some microbes are pathogens that cause disease.

History of discovery and scientific study

Microbes were among the earliest known life forms in Earth's history and first became visible to scientists after the invention of lenses and microscopes. The development of staining, culturing, and later molecular methods expanded our understanding of microbial diversity and function. Modern approaches such as genomics and environmental sequencing reveal vast numbers of microbes that cannot yet be cultured in the laboratory.

Distinctions, notable facts and practical considerations

  • Cellular versus non-cellular: bacteria, archaea, protists and fungi are cellular; viruses are acellular.
  • Size and complexity vary greatly—from simple single cells to colonies and biofilms with organized structure.
  • Symbiosis: many microbes live in close association with hosts, ranging from mutual benefit to parasitism.
  • Applied importance: microbes are indispensable in agriculture, medicine, industry and environmental remediation.

Microorganisms remain a frontier of discovery: new lineages and metabolic capabilities are regularly identified, reshaping how scientists understand life’s diversity and the processes that sustain ecosystems and human society.