A microorganism is an organism or biological entity that is sufficiently small that it generally requires magnification or specialised methods to be observed and studied effectively. The term is primarily descriptive rather than taxonomic. Microorganisms include bacteria, archaea, many fungi, many protists and microscopic algae, while viruses and other acellular entities are often included in microbiology even though they are not universally considered living organisms. Microorganisms therefore do not belong to one evolutionary group.
Some are prokaryotic and lack a membrane bound nucleus, while others are eukaryotic and possess nuclei and membrane bound organelles. Their sizes vary considerably, from viruses and very small bacteria to microscopic multicellular organisms that can be seen under suitable conditions. Microorganisms occur almost everywhere on Earth, including soil, oceans, freshwater, air, rocks, sediments, food, industrial environments and the bodies of plants and animals. Their enormous diversity makes microorganisms fundamental to biology and environmental science.
Bacteria and archaea are prokaryotic microorganisms. Their cells generally lack a membrane bound nucleus and contain genetic material in a nucleoid region. Eukaryotic microorganisms include yeasts, filamentous fungi, algae and many protists. These organisms have nuclei and other membrane bound organelles and can possess much larger and more structurally complex cells than typical bacteria. Viruses are microscopic but acellular, consisting of genetic material enclosed in protein and sometimes lipid structures.
Viroids and prions are additional biological entities studied within microbiology and infectious disease research. This diversity means that microorganism should never be treated as a synonym for bacterium. Bacteria are one category of microorganisms, but microorganisms include many other forms of life and biological entities. The boundaries of microbiology are consequently defined more by scale and practical study than by a single taxonomic classification.
Microorganisms perform enormous numbers of biochemical reactions and are responsible for processes essential to ecosystem functioning. Photosynthetic microorganisms contribute substantially to primary production and oxygen generation, while bacteria and archaea drive major transformations in carbon, nitrogen, sulfur and phosphorus cycles. Decomposer microorganisms break down organic matter and return nutrients to the environment. Nitrogen fixing microorganisms convert atmospheric nitrogen into biologically useful compounds, while nitrifying and denitrifying microorganisms transform nitrogen between different chemical forms.
Methanogenic archaea produce methane under anaerobic conditions, while other microorganisms consume methane. These processes occur in soils, sediments, oceans, wetlands and animal digestive systems. Because microorganisms have extremely large population sizes and rapid generation times in many environments, they can exert major ecological effects despite being individually microscopic.
Microorganisms are important components of food webs. Bacteria and microscopic algae can act as primary producers, while protozoa and other microorganisms consume bacteria and other small organisms. These microbial interactions can transfer energy and nutrients to larger organisms. In aquatic systems, microorganisms form a major part of planktonic communities and contribute to the microbial loop, through which dissolved organic matter is processed and incorporated into microbial biomass before being transferred to larger consumers.
In soils, microbial communities interact with plant roots, mineral particles, organic matter and each other. Microbial decomposers can determine how quickly nutrients become available to plants. Because microorganisms have rapid biochemical activity, changes in their communities can alter ecosystem processes even before obvious changes appear in larger organisms.
Microorganisms are also fundamental to human health and biotechnology. The human body contains extensive communities of bacteria, archaea, fungi and other microorganisms that interact with host tissues and influence digestion, metabolism, immune development and resistance to colonisation by some pathogens. Industrial microbiology uses microorganisms to manufacture foods, enzymes, antibiotics, organic acids, biofuels and other products. Fermentation by yeasts and bacteria has been used for thousands of years in bread, yoghurt, cheese and fermented beverages.
Modern biotechnology uses genetically engineered microorganisms to produce medicines, vaccines, hormones and industrial chemicals. Microbial systems can also be used in wastewater treatment and environmental remediation. Their small size does not limit their importance because populations containing billions or trillions of cells can perform large scale biochemical transformations.
Microorganisms are essential to scientific research because many fundamental biological processes were first understood through microbial systems. Bacteria have provided important models for studying DNA replication, gene regulation, mutation, metabolism and cell division. Microorganisms also enabled major advances in genetics and molecular biology because they can grow rapidly and often have relatively simple genomes. The discovery of antibiotics and the development of fermentation biotechnology further demonstrated their practical value.
Modern microbiology combines microscopy, culture methods, molecular sequencing, metagenomics, proteomics and computational analysis. Many microorganisms cannot be readily cultured using standard laboratory techniques, so modern researchers increasingly study microbial communities directly through their DNA and other molecular signals. This has revealed an enormous hidden diversity that was missed by traditional culture based approaches.
Not all microorganisms are harmless, and a minority are responsible for important infectious diseases. Bacteria can cause diseases such as tuberculosis and bacterial meningitis, fungi can cause infections, protozoan organisms can cause malaria, and viruses cause numerous infectious diseases. However, pathogenicity is highly context dependent. A microorganism may be harmless in one environment but cause disease when it enters a different body site or when host defences are impaired. Many microorganisms are beneficial, neutral or essential. Even potentially harmful organisms can have ecological roles independent of their effects on humans. The presence of a microorganism therefore does not automatically imply danger. Risk depends on the organism, dose, route of exposure, host susceptibility, environmental conditions and biological mechanisms involved.
A common misconception is that microorganisms are synonymous with germs. Germ is an informal term usually referring to microorganisms or biological agents capable of causing disease, whereas microorganism includes an enormous diversity of organisms that are harmless or beneficial. Another misconception is that all microorganisms are single celled. Many fungi and algae can form multicellular microscopic structures, and some microorganisms exist as multicellular organisms that remain microscopic.
Microorganisms are best understood as a diverse collection of tiny biological systems rather than a single category of life. Their ubiquity, metabolic diversity and ability to interact rapidly with their surroundings make them essential to nutrient cycling, ecological stability, evolution, medicine, agriculture, industry and the functioning of the biosphere.
