Microorganisms are organisms too small to be seen clearly without the aid of a microscope. They represent the most ancient and diverse forms of life on Earth and are ubiquitous, existing in nearly every environment, from deep-sea vents to the human digestive tract. Microscopic life is categorized into major groups based on fundamental differences in cellular structure and organization.
The primary distinction rests on whether their cells have a membrane-bound nucleus. This divides cellular microbes into two super-groups: prokaryotes, which lack a nucleus, and eukaryotes, which possess one. A third, separate category includes infectious agents that are not composed of cells at all.
Cellular Life Without a Nucleus: Prokaryotes (Bacteria and Archaea)
Prokaryotes are unicellular organisms lacking internal membrane-bound organelles, including the nucleus. Their genetic material, typically a single, circular chromosome of DNA, is concentrated in the cell. Prokaryotic cells are generally small, with most species ranging in diameter from 0.5 to 5.0 micrometers (µm). They are divided into two distinct domains of life: Bacteria and Archaea.
Bacteria
The domain Bacteria includes organisms found in almost every habitat on the planet. Their cell walls are defined by the presence of peptidoglycan, a unique polymer made of sugars and amino acids. Bacterial cells are commonly classified by shape, including spheres (cocci), rods (bacilli), and spirals (spirilla).
Bacteria play varied roles in global ecology and human health. Many species are decomposers, breaking down dead organic matter to recycle nutrients back into the environment. Others are integral to biogeochemical cycles, such as nitrogen-fixing bacteria that convert atmospheric nitrogen gas into forms usable by plants. While some bacteria cause disease, many others live symbiotically within the human body, aiding digestion and training the immune system.
Archaea
Archaea are recognized as a separate domain of life due to profound biochemical differences from bacteria. Unlike bacteria, archaeal cell walls do not contain peptidoglycan, instead being composed of pseudopeptidoglycan, polysaccharides, or proteins. Furthermore, their cell membranes have a unique molecular structure, utilizing ether bonds to link branched phytanyl units to glycerol, a contrast to the ester bonds found in bacterial and eukaryotic membranes.
Many archaea are known as extremophiles. This includes methanogens, which produce methane gas and are found in anaerobic conditions like wetlands and animal guts. Other archaeal species are halophiles, flourishing in highly saline waters such as the Great Salt Lake. Thermophiles inhabit high-temperature environments like hot springs and deep-sea hydrothermal vents.
Microscopic Life With a Nucleus: Eukaryotes (Protists and Fungi)
Eukaryotic microbes are distinguished by their complex cellular architecture, possessing a true membrane-bound nucleus that houses the genetic material. They also contain specialized internal structures called organelles, such as mitochondria. Eukaryotic cells are typically much larger than prokaryotes, with diameters ranging from 10 to 100 µm. This group includes two primary categories of microscopic organisms: protists and fungi.
Protists
Protists are diverse eukaryotic organisms that do not fit into the kingdoms of plants, animals, or fungi, often referred to as the “catch-all” kingdom. They are predominantly unicellular, and their functional roles are varied, often classified based on how they obtain nutrients.
Animal-like protists, known as protozoa, are heterotrophs that consume other organisms. Many are motile, using structures like flagella, cilia, or pseudopods for movement. Plant-like protists, such as algae and phytoplankton, are photosynthetic autotrophs that use light energy to produce their own food. These organisms form the base of many aquatic food webs and are responsible for a significant portion of the Earth’s oxygen production.
Microscopic Fungi
Microscopic fungi include single-celled yeasts and multicellular molds. They are heterotrophs that obtain nutrients by absorbing dissolved organic material from their surroundings. Fungi have rigid cell walls composed of chitin, a structural polysaccharide. Yeasts are well-known for their metabolic activities in fermentation processes, such as baking and brewing.
Molds are characterized by their filamentous growth structure, forming microscopic threads called hyphae, which collectively create a network known as a mycelium. Fungi are essential decomposers in terrestrial ecosystems, breaking down complex organic materials. Some fungi also form symbiotic relationships with plants, enhancing nutrient absorption.
Non-Living Microscopic Entities (Viruses)
Viruses represent a separate category from cellular microbes because they are acellular. A virus particle, known as a virion, consists of a simple structure: genetic material (either DNA or RNA but never both) enclosed within a protective protein shell called a capsid. Some viruses also possess an outer lipid membrane called an envelope, derived from the host cell.
Viruses are obligate intracellular parasites, lacking the metabolic machinery required to generate energy or synthesize proteins independently. They must invade a living host cell—which can be bacterial, plant, animal, or fungal—and hijack its cellular mechanisms to replicate themselves. Outside of a host cell, the virus exists as an inert particle. This absolute dependence on a host for reproduction is the primary reason viruses are classified on the borderline between living organisms and non-living chemical entities.