Microorganisms, commonly referred to as microbes, represent an enormous category of living things too small to be seen without specialized equipment. These organisms exist in a range typically measured in micrometers, or millionths of a meter. A single bacterium often measures between 0.2 and 5.0 micrometers, making it thousands of times smaller than the diameter of a human hair. Observing these life forms requires the use of a compound light microscope, which can magnify samples up to 1,000 times. The smallest entities necessitate the higher resolution of an electron microscope. This invisible world includes a vast array of cellular life forms, as well as non-living infectious agents, all of which populate virtually every environment on Earth.
The Diversity of Cellular Microorganisms
Cellular microbes are broadly classified into two fundamental structural types: prokaryotes and eukaryotes. Prokaryotic cells, encompassing the domains Bacteria and Archaea, lack a membrane-bound nucleus and other internal compartments. Their genetic material, usually a single, circular chromosome, is located in a region of the cytoplasm called the nucleoid. Bacteria possess a rigid cell wall primarily composed of peptidoglycan, a unique polymer of sugars and amino acids that provides structural support.
Archaea are genetically distinct from bacteria and exhibit differences in their cellular architecture. While they also lack a nucleus, their cell walls do not contain peptidoglycan, often featuring pseudopeptidoglycan or S-layer proteins instead. A key distinction lies in their cell membranes, where Archaea utilize ether-linked lipids with branched chains, providing enhanced stability in the extreme environments they often inhabit. Bacteria feature ester-linked fatty acids in their membrane structure, a composition shared with eukaryotes.
The microscopic eukaryotes are generally larger and structurally more complex, including Protists and microscopic Fungi. These cells contain a true nucleus that houses the linear DNA chromosomes, along with various membrane-enclosed organelles like mitochondria. Protists are an exceptionally diverse group, acting as a functional catch-all for microscopic eukaryotes not classified as fungi, plants, or animals. This category includes animal-like protozoans that consume other microbes and plant-like algae that perform photosynthesis.
Microscopic fungi, such as yeasts, possess a cell wall made of chitin, a tough polysaccharide also found in the exoskeletons of insects. Unlike algae, fungi are non-photosynthetic and absorb nutrients from their environment. Some fungi, like molds, grow as a network of thread-like filaments called hyphae, while yeasts exist as single, oval-shaped cells. These structural differences reflect the vast biological diversity contained within the unseen world of cellular microbes.
The Acellular Entities: Viruses and Viroids
Entities smaller than cellular microbes are acellular infectious agents, not considered true living organisms because they cannot replicate independently. Viruses are obligate intracellular parasites, meaning they must invade a host cell to hijack its machinery for reproduction. A complete virus particle, known as a virion, consists of genetic material (DNA or RNA) encased in a protective protein shell called a capsid. Some viruses also possess an outer lipid envelope, typically derived from the host cell’s membrane.
The viral life cycle involves attachment to a specific cell surface receptor, followed by entry. The virus effectively turns the host cell into a virus-producing factory where the viral genome is replicated and new components are assembled. Newly formed virions are then released, often causing the destruction of the host cell. Viruses range in size from approximately 20 to 300 nanometers, making them significantly smaller than bacteria and visible only with electron microscopy.
Even simpler than viruses are viroids, the smallest known infectious agents. A viroid consists solely of a short strand of circular, single-stranded RNA; it lacks the protective protein capsid found in viruses. These naked RNA molecules primarily infect plant cells and replicate by utilizing the host’s own cellular enzymes. Viroids do not code for any proteins, but instead interfere with the plant’s normal gene expression, leading to various disease states in crops.
The Essential Roles of Invisible Life
The microscopic world plays a role in maintaining the planet’s ecological balance and supporting human health. Microbes are the primary drivers of global nutrient cycling, involving the transformation of elements into forms usable by other life. Specific bacteria and archaea are the only organisms capable of nitrogen fixation, converting inert atmospheric nitrogen gas into ammonia. Ammonia is a compound required for synthesizing proteins and nucleic acids in all living things.
Microscopic fungi and bacteria are the main decomposers in terrestrial and aquatic environments, breaking down complex organic matter from dead organisms and waste. This process of decomposition recycles carbon, sulfur, and phosphorus back into the environment. Without this microbial activity, nutrients would remain locked in organic compounds.
Microbes also form the human microbiome, a vast community of organisms living in and on the body, with the gut harboring the largest and most dynamic population. This intestinal community performs numerous functions supportive of host health. Gut bacteria assist in the digestion of complex carbohydrates that human enzymes cannot break down, and they synthesize essential compounds like Vitamin K and certain B vitamins. They also train the immune system and prevent harmful external pathogens from colonizing the digestive tract.
In contrast to these supportive roles, a small fraction of microbes are pathogens, agents that cause disease by invading and damaging host cells or producing toxins. Pathogens include various bacteria, viruses, fungi, and protozoa. To establish an infection, they must successfully reach a target site, multiply rapidly, and evade the host’s immune response. Bacterial pathogens cause illness by releasing toxins, while viruses cause disease by forcing host cells to prioritize viral replication, leading to cell destruction.