Microorganisms are living entities requiring a microscope for observation, encompassing a staggering range of life forms. This microscopic world includes members from all three domains of life—Bacteria, Archaea, and Eukarya—plus the acellular viruses. Differences in size reflect variations in their structure, complexity, and function. Understanding this scale is key to grasping how these unseen organisms impact the world and how scientists study them.
Units of Measurement and Visual Anchors
To comprehend the scale of microorganisms, one must become familiar with the units of measurement used in the microscopic world. The most common units are the micrometer (\(\mu\)m) and the nanometer (nm), both fractions of a meter. A micrometer represents one-millionth of a meter and is the standard unit for measuring most cells, including bacteria and larger microbes.
The nanometer is an even smaller unit, representing one-billionth of a meter, meaning there are 1,000 nanometers within a single micrometer. This relationship defines the distinct size classes of the smallest biological entities. For context, the average human hair is roughly 50 to 100 micrometers in diameter, while a human red blood cell is approximately 7 micrometers across, serving as a visual anchor for the micrometer scale.
The Smallest Scales: Viruses and Bacteria
The smallest biological structures capable of causing infection are the viruses, which exist almost entirely within the nanometer scale. Most known viruses range from 20 nm to 300 nm in diameter. Their small size is due to their simple, acellular structure, consisting only of genetic material encased in a protein shell, and sometimes an outer envelope.
Viruses lack the cellular machinery for metabolism, so their size is constrained only by the volume needed to house their genome and protective capsid. For example, the influenza virus is typically around 100 nm, and the poliovirus is only about 30 nm. The largest viruses, such as Mimiviruses, can reach sizes up to 400 nm, or 0.4 \(\mu\)m.
Bacteria, in contrast, are prokaryotic cells, and their size is orders of magnitude larger than most viruses. The typical bacterium falls within the range of 0.5 \(\mu\)m to 5.0 \(\mu\)m in length. Escherichia coli (E. coli), a commonly studied rod-shaped bacterium, measures about 1 to 2 \(\mu\)m long and 0.5 \(\mu\)m wide.
While most bacteria adhere to this micrometer scale, there are exceptions at both extremes. The smallest bacteria, such as those in the genus Mycoplasma, can be as small as 0.2 to 0.3 \(\mu\)m, overlapping with the size of the largest viruses. Conversely, species like Thiomargarita magnifica have been found to reach lengths of up to 2 centimeters, making them visible to the unaided eye.
The Upper Limits: Fungi and Protists
Fungi and protists are eukaryotic microorganisms, which are larger than bacteria due to their complex internal structures, including a membrane-bound nucleus and organelles. Fungi exist in single-celled forms, such as yeasts, or in filamentous forms, known as molds. Unicellular yeasts, like Saccharomyces cerevisiae, are generally spherical and range from 3 to 8 \(\mu\)m in diameter, though some species can reach 40 \(\mu\)m.
Molds grow as a network of branching filaments called hyphae, which form the fungal colony. The width of individual hyphal filaments typically measures between 2 and 10 \(\mu\)m, though some can reach 50 \(\mu\)m. While individual hyphae are microscopic, the collective colony (mycelium) or the fruiting body of a mushroom can be macroscopic.
Protists often represent the largest single-celled organisms. Most protists, such as those that cause malaria, measure between 1 \(\mu\)m and 50 \(\mu\)m. However, the size range of protists extends up to several millimeters in length.
Large ciliated protists like Paramecium or amoebas such as Amoeba proteus can be hundreds of micrometers long, making them easily visible under low-power magnification. This size diversity reflects that protists are a collection of diverse, predominantly unicellular eukaryotes. Their large volume allows for sophisticated internal organization and mobility mechanisms like cilia or pseudopods.
How Size Dictates Study and Behavior
The size of a microorganism fundamentally determines the methods scientists must use to observe and handle it. The light microscope, which uses visible light, is effective for viewing bacteria and larger microbes like yeasts and protists. However, since the wavelength of visible light is 400 to 700 nanometers, the light microscope cannot resolve objects smaller than about 200 nanometers.
This limitation means that viruses, which are in the 20 to 300 nanometer range, are submicroscopic and require the use of an electron microscope. Electron microscopes use a beam of electrons instead of light, allowing for the extreme magnification needed to visualize the fine structure of a viral capsid or the internal components of a tiny bacterium.
Size also dictates the efficacy of physical barriers, such as filtration systems, used to sterilize liquids and air. Standard sterilization filters in laboratories and industry are designed with a pore size of 0.2 \(\mu\)m (200 nm) to effectively remove most bacteria. However, this pore size is insufficient to reliably remove all viruses, many of which can pass right through.
For example, to remove small waterborne viruses like poliovirus (around 30 nm), specialized ultrafiltration membranes with much smaller pore sizes are necessary. Furthermore, the size of a cell is tied to its internal complexity, where the small volume of a prokaryote restricts its internal organization, while the large volume of a protist allows for the inclusion of multiple organelles and intricate cellular systems.