A microorganism, or microbe, is an organism ordinarily too small to be examined in detail without a microscope. Microorganisms include bacteria, archaea, and many microscopic eukaryotes. Most are unicellular, but some form multicellular structures or colonies. The term describes a scale of biological organization rather than a single evolutionary group. Microbiology studies these organisms and commonly includes viruses, although viruses are acellular and their inclusion within the definition of microorganism depends on usage. (uib.no)
Diversity and classification
Microorganisms occur across the major groups of cellular life. Bacteria and archaea are prokaryotes: their genetic material is not enclosed within a membrane-bound nucleus. Despite this shared feature, the two groups differ substantially in evolutionary history, membrane composition, and molecular machinery. Archaea are therefore not simply unusual bacteria. Both groups occupy diverse environments, including soils, waters, and associations with other organisms. (openstax.org)
Microbial eukaryotes have cells containing a nucleus. They include microscopic fungi, such as yeasts and molds, and numerous protists. Yeasts are typically unicellular; molds grow as branching filaments and may form visible colonies. Protozoa is a traditional term for diverse, mostly unicellular protists, many of which consume other organisms or organic material. Microscopic algae include photosynthetic eukaryotes found in aquatic and other environments. These descriptive groups do not all correspond to single evolutionary lineages. (openstax.org)
Viruses differ fundamentally from cellular microbes. They contain genetic material within a protein-based structure, sometimes surrounded by a membrane envelope, and depend on host cells for replication. Microbiology includes their study because of their interactions with organisms and their environmental and medical importance; this disciplinary inclusion does not settle whether viruses should be considered living organisms. (openstax.org)
Structure, metabolism, and reproduction
Microbes vary greatly in size and organization. Many bacterial cells are approximately one micrometer across, whereas microbial eukaryotes are often larger. Shape alone provides limited information about identity: unrelated organisms may look similar under a microscope while differing markedly in physiology and genetic composition. Visible growth, such as a mold colony, may consist of numerous microscopic structures rather than one large cell. (openstax.org)
Microbial metabolism encompasses many ways of obtaining carbon and energy. Some organisms use photosynthesis, including cyanobacteria and microscopic algae. Others obtain energy by transforming organic compounds or oxidizing inorganic substances. Microbes also differ in their relationship to oxygen: some require it, while others grow in oxygen-free conditions. These differences help determine which organisms occupy particular habitats and which chemical transformations they perform. (openstax.org)
Bacteria commonly reproduce by dividing one cell into two daughter cells. Their populations can increase rapidly under favorable conditions, but growth depends on resources and environmental circumstances. A species’ capacity to multiply in one setting does not imply that it will grow equally well elsewhere; microorganisms have distinct nutritional and physiological requirements. (microbiologysociety.org)
Distribution and ecological roles
Microorganisms inhabit soil, freshwater, seawater, and the surfaces and interiors of larger organisms. Some tolerate unusually high temperatures, salinity, acidity, or pressure. Such adaptations are species-specific: microbes found in extreme environments are not representative of all microorganisms, and many thrive under ordinary environmental conditions. (microbiologysociety.org)
Microbial activity is integral to ecosystem functioning. Through decomposition, bacteria and fungi break down organic material and return nutrients to circulation. Microbes participate in the carbon cycle by producing, consuming, and transforming carbon compounds. Photosynthetic microorganisms provide organic matter that supports aquatic food webs, while other microbes consume and recycle that material. (microbiologysociety.org)
Certain bacteria and archaea carry out nitrogen fixation, converting atmospheric nitrogen into ammonia that can enter biological nutrient networks. Other microbial processes transform ammonia into nitrite and nitrate or return nitrogen compounds to atmospheric nitrogen. These activities connect microbial physiology with soil fertility and the movement of nutrients through terrestrial and marine environments. (microbiologysociety.org)
Relationships with humans and practical uses
The human body supports microbial communities on the skin and in locations such as the digestive tract. The term microbiome commonly refers to a microbial community and, in some definitions, its collective genetic material. Many resident microbes are harmless or beneficial: some help break down dietary compounds, while others compete with potential invaders. Microbial presence alone is therefore not evidence of disease. Some microorganisms nevertheless cause infections, and effects can differ between strains and host circumstances. (nigms.nih.gov)
Humans employ microbes in food production, manufacturing, and environmental management. Fermentation by yeasts and bacteria underlies products including bread, alcoholic beverages, and fermented foods. Microbial processes also contribute to sewage treatment and the manufacture of drugs and therapeutic proteins. Molds have supplied medically important substances, including penicillin. These applications exploit particular organisms and metabolic activities rather than a property shared uniformly by all microbes. (microbiologysociety.org)
Discovery and investigation
During the 1670s, Antonie van Leeuwenhoek observed microorganisms using his microscopes. In the nineteenth century, Louis Pasteur investigated microbial fermentation and challenged spontaneous generation. His swan-neck flask experiments showed that heated nutrient liquids could remain uncontaminated when airborne dust carrying microbes was prevented from reaching them, even though air could enter. (microbiologysociety.org)
Modern investigation combines microscopy, cultivation, biochemical analysis, and genetic methods. Cultivation allows direct study of microbial growth, but many organisms are difficult to reproduce under laboratory conditions. Metagenomics examines genetic sequences obtained collectively from organisms in a sample, allowing researchers to investigate microbial communities without first growing every member separately. It is used to study communities from environments such as soil, water, and human skin. (nigms.nih.gov)