Almost all microbes are far too small to see, yet they differ hugely in size from one another. Most bacteria are about 1 to a few microns long, a few hundredths of a hair’s width. Most viruses are 10 to 100 times smaller, while yeasts and many protists are several times larger than bacteria. The groups overlap at the edges, though: some viruses are bigger than small bacteria, and one bacterium reaches a centimeter.
Typical Sizes of Each Microbe Group
Microbes are measured in microns (micrometers, µm), one millionth of a meter or one thousandth of a millimeter.1ScienceAlert. New Images Reveal The Tiniest Known Life-Forms on Earth Viruses are measured in nanometers (nm), and 1,000 nanometers make one micron. A human hair, about 70 microns wide, gives a handy ruler for both.
- Viruses: most are about 20 to 300 nanometers across, with giant viruses reaching about 700 nanometers.
- Bacteria: most are about 0.5 to 5 microns long. An E. coli cell is about 2 microns long.
- Yeasts: these single-celled fungi measure about 3 to 10 microns.
- Molds: individual threads are only a few microns wide, but they form visible fuzzy networks.2Lumen Learning. Microbiology – Characteristics of Fungi
- Protists: amoebas and algae range from a few microns up to a millimeter or more.
- Micro-animals: rotifers have many cells, yet are under 2 millimeters.3Global Ecology and Biogeography. Patterns of diversity in microscopic animals: are they comparable to those in protists or in larger animals?
Fungi are hard to pin to one size. A yeast is a single round or oval cell, so one number describes it. A mold grows as branching threads that keep lengthening, which is how one mold can cover a slice of bread or a patch of wall. What is microscopic is the width of a thread and the size of its spores, not the visible fuzzy patch. That patch is a huge network of tiny parts, not one giant cell.
Living bacteria and their relatives span volumes from 0.02 to 400 cubic microns,4PubMed. What size should a bacterium be? A question of scale so the largest ordinary bacteria hold about 20,000 times the volume of the smallest. Length and volume give different-looking comparisons because doubling a cell’s length, width and depth together multiplies its volume about eight times. A virus ten times narrower than a bacterium therefore has roughly a thousandth of its volume. Size charts usually give just one dimension, such as length for a rod-shaped bacterium or diameter for a round virus, so two entries may not measure the same thing. A bacterium listed as 2 microns long may be well under 1 micron wide.
Picturing Microbe Sizes With Everyday Comparisons
About 30 to 70 average bacteria could line up across the width of one human hair. Hundreds to thousands of typical viruses could fit across that same width. A red blood cell, about 7 microns, is several times longer than a typical bacterium.
Scaling up helps. If a typical bacterium were enlarged to a grain of rice, a few millimeters long, a typical virus would be a speck of dust beside the grain, and a yeast cell would be about the size of a grape.
Small spaces hold astonishing crowds. A pinhead about 1 millimeter wide could hold hundreds of thousands of bacteria side by side, and a milliliter of seawater can hold about a million.
What You Can See With Your Eyes or a Microscope
The naked eye can make out objects of about 0.1 millimeter, the size of a large amoeba. A standard light microscope cannot sharpen details smaller than about 190 nanometers with visible light.5University of Southampton. Seeing The Virus With Topological Optical Microscopy Bacteria and yeasts show up well, but most viruses, often only tens of nanometers across, do not.6University of Southampton. Seeing The Virus With Topological Optical Microscopy Newer light-based methods have imaged single virus particles at about 60 nanometers.7PubMed Central. Optical chemical imaging of tobacco mosaic virus in solution at 60-nm resolution Most virus pictures you see come from electron microscopes, which use electrons instead of light.
A home or school microscope has its limits. At about 400 times magnification, bacteria appear as tiny dots or short rods, and you may need a stain or careful focusing to be sure of them. Yeast cells from a drop of bread dough mixed in water are large enough to spot easily as small ovals, and some may be budding. A drop of pond water usually shows protists and sometimes rotifers swimming or crawling around. No ordinary microscope will show the virus behind a cold or flu, however carefully you focus.
You can still see mold and bacterial colonies without any microscope. A colony is millions of cells piled together. Seeing a colony is not the same as seeing a single cell.
The Smallest Microbes on Record
The smallest known living cells average 0.009 cubic microns in volume, so about 150 of them could fit inside one E. coli cell.8ScienceAlert. New Images Reveal The Tiniest Known Life-Forms on Earth A common ocean bacterium and a lab-built minimal cell are both about 0.03 cubic microns.9PubMed Central. Minimal Out-of-Equilibrium Metabolism for Synthetic Cells: A Membrane Perspective
Life seems to have a floor. A cell must hold its DNA, its proteins and the machinery that makes proteins, and that sets a minimum size.10PubMed. What size should a bacterium be? A question of scale Cells also carry backup systems to survive environmental shocks, which pushes the minimum higher.11PubMed. What size should a bacterium be? A question of scale Even a lab-made cell with only the essential genes still needs a volume close to a sphere 400 nanometers across.12PubMed Central. Minimal Out-of-Equilibrium Metabolism for Synthetic Cells: A Membrane Perspective Scientists don’t yet agree on the true lower limit for life.
Viruses can be smaller than any cell because a cell must carry the machinery to build its own proteins, and that machinery takes up space. A virus carries none of it and borrows the machinery of the cell it infects. It needs only its genes and a protective shell, which can be far smaller than the contents of a complete cell.
Giant Bacteria and Giant Viruses
One bacterium, Thiomargarita, grows to about a centimeter long, far beyond what was thought possible for bacteria.13PubMed. A centimeter-long bacterium with DNA contained in metabolically active, membrane-bound organelles It carries more than half a million copies of its genome.14PubMed. A centimeter-long bacterium with DNA contained in metabolically active, membrane-bound organelles It also keeps its DNA in membrane-wrapped compartments, something bacteria were thought not to do.15PubMed. A centimeter-long bacterium with DNA contained in metabolically active, membrane-bound organelles Another giant, Epulopiscium from fish guts, keeps tens of thousands of genome copies spread around its edge.16PubMed. DNA replication and genomic architecture of very large bacteria Copies near every part of the cell surface let each region respond to its own surroundings, instead of waiting for signals from a distant center.17PubMed Central. Extreme polyploidy in a large bacterium
Size creates a problem for these giants, because food and oxygen move through a cell mostly by slow drifting, which works poorly over long distances. Many giant bacteria fill at least 80 percent of the cell with a fluid-filled sac. That keeps the active living layer thin and close to the surface, where supplies can reach it quickly.
Among viruses, mimivirus has a shell about 500 nanometers wide, about 700 nanometers including its outer fibers.18PubMed Central. Mimiviruses: Giant viruses with novel and intriguing features That makes it larger than the smallest bacteria, and it can be seen with a light microscope. Pandoraviruses carry even more genetic material than mimivirus.19PubMed Central. Mimiviruses: Giant viruses with novel and intriguing features Giant viruses mostly infect amoebas, and they were missed for years because they looked like bacteria.
Why Microbes Stay Small
As a cell grows, its volume rises faster than its surface. A 1-unit cube has 6 units of surface for 1 of volume, while a 10-unit cube has only 0.6 units of surface for each unit of volume. Nutrients enter through the surface, so in the bigger cube the center is far from supply and goes hungry.
A cell with more surface for its size takes in nutrients and grows more efficiently.20PubMed Central. Cell Geometry Limits Bacterial Metabolic Efficiency Bacteria boost their surface by changing shape or folding their membranes.21PubMed Central. Genomic Mysteries of Giant Bacteria: Insights and Implications Larger bacteria often grow faster, though, since the cell balances building surface against building its inner machinery.22PubMed Central. Resource allocation to cell envelopes and the scaling of bacterial growth rate
Energy use also climbs steeply with size in bacteria. It rises faster than size, unlike in animals, where it rises more slowly.23UNM Digital Repository. Allometric Scaling and Metabolic Ecology of Microorganisms and Major Evolutionary Transitions In ocean bacteria, a cell twice as large burns roughly three times the energy.
Predators push on size too. Tiny grazers easily eat freely floating bacteria under 7 microns. Under grazing, more than 80 percent of one bacterium’s cells grew into long filaments, while starved cells shrank into small rods that were easy prey.
Microbes Change Size With Their Conditions
A microbe’s size is not fixed. When E. coli suddenly loses its food, its inner contents shrink away from the cell wall because the cell loses water from inside.24PubMed Central. Starvation induces shrinkage of the bacterial cytoplasm During starvation, cells also get thinner and divide at a smaller size.25PubMed Central. Changes in cell dimensions during amino acid starvation of Escherichia coli Size charts therefore give typical values for well-fed cells.
That is one reason the same species gets different sizes in different books. Measured size depends on whether cells were growing or starving, and drying and staining for a microscope also shrink cells. Ranges are more honest than single numbers.
Why Microbe Size Matters for Filters and Masks
Medicines are made sterile by passing them through filters with openings of about 0.2 microns.26PubMed Central. Development of Isoporous Microslit Silicon Nitride Membranes for Sterile Filtration Applications To earn that rating, a filter must stop every cell of a small test bacterium about 0.4 to 0.8 microns long.27PubMed Central. Development of Isoporous Microslit Silicon Nitride Membranes for Sterile Filtration Applications Some thin, flexible bacteria can still squeeze through such filters.28PubMed. Influence of size, shape, and flexibility on bacterial passage through micropore membrane filters Most viruses, being much smaller than 0.2 microns, are not reliably removed by bacterial filters.
For a home water filter, check the pore size on the label, and for bacteria look for 0.2 microns or smaller. A filter rated about 1 micron can stop many parasites, which are larger, but not all bacteria. Viruses can pass through filters of either size, so boiling or approved disinfection is needed to deal with them.
Masks are not simple sieves. Viruses travel in breath droplets much larger than the virus itself, and mask fibers also catch particles by trapping them, not just by size. A mask can therefore block virus-carrying droplets despite the tiny size of the virus.