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Home›Biology›Microbes

What microbes are

You can just see something about 100 micrometres wide with the naked eye; a typical bacterium is about one hundredth of that.

Too small to see

A microbe, or microorganism, is a living thing too small to see without magnification. OpenStax Microbiology gives the scale: an object has to be about 100 micrometres (µm) across before the eye can see it. A typical animal cell is about 10 µm, a typical bacterium about 1 µm, and a typical virus about 100 nanometres (nm), ten times smaller again. A micrometre is a millionth of a metre and a nanometre a billionth.

Sizes on a log scale, each step ten times larger. 1 a typical virus (about 100 nm), 2 a typical bacterium (about 1 µm), 3 a typical animal cell (about 10 µm), 4 the smallest thing the eye can see (about 100 µm).

Microbes are found in all three domains of life. Bacteria and Archaea are prokaryotes: their cells have no nucleus. Microbes in the domain Eukarya, such as protists and microscopic fungi like yeasts, have a nucleus. Viruses fit into none of the three domains, because they are not made of cells at all. Parasitic worms called helminths are studied by microbiologists too, although most are big enough to see, because their eggs and larvae are microscopic.

The first person to see them

BackgroundLittle animals in a drop of water. Antonie van Leeuwenhoek was a cloth merchant in Delft who ground his own tiny lenses. His first observations reached the Royal Society in London in 1673, introduced by the physician Regnier de Graaf. In 1675 he saw living creatures in rain water that had stood a few days in a new pot, and on 9 October 1676 he wrote a long letter in Dutch to Henry Oldenburg, the Society’s secretary, describing “little animals” in rain, well, sea and snow water and in water where pepper had soaked. He claimed upwards of a million in one drop of pepper water. The letter was read aloud over several meetings. Oldenburg translated less than half of it and published it in Philosophical Transactions on 25 March 1677. The Fellows could not see the creatures at first, but later that year Robert Hooke managed it.

From his drawings, scientists now know he was looking at bacteria and protists. Leeuwenhoek kept writing to the Royal Society for fifty years, 190 letters in all, until his death on 26 August 1723 at the age of 90. Nearly two centuries later came what OpenStax calls the Golden Age of Microbiology, from 1857 to 1914, when Louis Pasteur, Robert Koch and others linked particular microbes to fermentation and disease.

  • by 7000 BC: A fermented drink of rice, honey and fruit in Neolithic China.
  • 1673: Leeuwenhoek’s first observations reach the Royal Society.
  • 1676: His letter of 9 October on “little animals”.
  • 1677: Published in Philosophical Transactions; Hooke confirms.
  • 1723: Leeuwenhoek dies at 90 after 190 letters.
  • 1857 to 1914: The Golden Age of Microbiology.

Mostly harmless, often useful

People used microbes long before anyone saw them. Bread, beer, wine, yogurt, cheese and pickles all depend on fermentation by bacteria, yeasts or molds, and pottery from a Neolithic village in China shows a fermented drink of rice, honey and fruit by 7000 BC. In bread, the yeast Saccharomyces cerevisiae breaks down sugars in the flour and gives off carbon dioxide, which makes the dough rise. OpenStax stresses that most bacteria are harmless or helpful; only some cause disease.

Microbes also live on and in us in large numbers. A 2016 estimate by Ron Sender, Shai Fuchs and Ron Milo put the bacteria in a 70 kg reference man at about 3.8 × 10¹³, compared with about 3.0 × 10¹³ human cells. That is a ratio of about 1.3 to 1, not the 10 to 1 often repeated in older books, and the bacteria weigh only about 0.2 kg in total.

BacteriaProkaryotes, usually about 1 µm; most have peptidoglycan walls.
ArchaeaProkaryotes with a separate history; no peptidoglycan; many in extreme places.
ProtistsEukaryotes that are not plants, animals or fungi: algae and protozoa.
FungiEukaryotes with chitin walls: yeasts and molds.
VirusNot a cell; must infect a host cell to reproduce.
Micrometre (µm)One millionth of a metre.
Key points
  • Visible limit about 100 µm; animal cell about 10 µm; bacterium about 1 µm; virus about 100 nm.
  • Bacteria and archaea are prokaryotes; protists and fungi are eukaryotes; viruses are not cells.
  • Leeuwenhoek’s letter of 9 October 1676 to the Royal Society described microbes.
  • Most bacteria are harmless or helpful.
  1. About how wide must something be (in µm) for the naked eye to see it?
  2. A typical bacterium is about 1 µm and a typical virus about 100 nm. How many times larger is the bacterium? (1 µm = 1,000 nm)
  3. Which group of microbes is not made of cells?
    • Viruses
    • Archaea
    • Yeasts
    • Protozoa
  4. Bacteria and archaea are both…
    • prokaryotes, with no nucleus
    • eukaryotes, with a nucleus
    • viruses
    • fungi
  5. To whom did Leeuwenhoek send his letter of 9 October 1676?
    • Henry Oldenburg of the Royal Society
    • Louis Pasteur
    • Robert Koch
    • Charles Darwin
  6. In which liquid did Leeuwenhoek count upwards of a million creatures in one drop?
    • Water in which pepper had soaked
    • Milk
    • Blood
    • Wine
  7. Leeuwenhoek wrote to the Royal Society from 1673 until his death in 1723. How many years is that?
  8. What makes bread dough rise?
    • Carbon dioxide from yeast breaking down sugars
    • Oxygen from bacteria
    • Steam from the flour
    • Nitrogen from the air
  9. Sender and colleagues (2016) estimated 3.8 × 10¹³ bacteria and 3.0 × 10¹³ human cells. What is the ratio of bacteria to human cells? (one decimal place)
  10. Which statement about bacteria is true?
    • Most are harmless or helpful
    • All cause disease
    • They all live only in water
    • They all have a nucleus
Sources for this course (15)
  • OpenStax Microbiology 1.1 What Our Ancestors Knew (fermented drink from rice, honey and fruit in Neolithic China by 7000 BC, Saccharomyces cerevisiae in bread, Leeuwenhoek 1675 "animalcules" in rain water, Golden Age of Microbiology 1857 to 1914; openstax.org/books/microbiology/pages/1-1-what-our-ancestors-knew)
  • 1.3 Types of Microorganisms (about 100 µm to see with the eye, animal cell about 10 µm, bacteria about 1 µm, viruses about 100 nm, three domains, coccus/bacillus/spirillum, archaea without peptidoglycan and in extreme places such as Morning Glory pool, protists, algae, diatoms 2 to 200 µm, protozoa with cilia, flagella or pseudopods, fungi with chitin walls, more than 1000 yeast species, molds and penicillin, helminths, prions about one-tenth of a virus; .../1-3-types-of-microorganisms)
  • 3.1 Spontaneous Generation (Aristotle, van Helmont's mice recipe of 3 weeks, Redi 1668 with six jars, Needham 1745, Spallanzani, Pasteur 1858 gun-cotton filter, swan-neck flasks, Alhumbert Prize 1862, 1864 lecture; .../3-1-spontaneous-generation)
  • 3.2 Foundations of Modern Cell Theory (Semmelweis 1847, 10 to 20 percent vs 1 percent, chlorinated lime; Snow; Pasteur 1856 fermentation; Lister 1867 carbolic acid; Koch's postulates; .../3-2-foundations-of-modern-cell-theory)
  • 6.1 Viruses (acellular, obligate intracellular parasites, DNA or RNA never both, capsid, envelope, spikes, helical/polyhedral/complex, 20 nm to 900 nm, Ivanovski 1892 with the Chamberland filter of 1884, pore 0.1 µm, Beijerinck 1898 and virus = poison, Stanley 1935, host range, bacteriophages, FDA 2006 phage spray on lunch meats; .../6-1-viruses)
  • 9.1 How Microbes Grow (binary fission, FtsZ Z ring, generation time, E. coli 20 minutes, human generation 25 years, M. tuberculosis 15 to 20 hours, M. leprae 14 days, 2^48 = 281,474,976,710,656 after 24 h at 30 minutes, Nn = N0 2^n, lag/log/stationary/death, budding, fragmentation, Epulopiscium; .../9-1-how-microbes-grow)
  • Royal Society: Philosophical Transactions 12:821-831, published 25 March 1677, Leeuwenhoek's Dutch letter of 9 October 1676 "concerning little animals" (doi.org/10.1098/rstl.1677.0003)
  • Royal Society archive EL/L1/22 (letter to Oldenburg dated at Delft, 18 pages, partial translation by Oldenburg)
  • Royal Society blog "Celebrating Leeuwenhoek", 2023 (first letter via de Graaf, Phil. Trans. 19 May 1673; 190 letters in fifty years; died 26 August 1723 aged 90)
  • N. Lane 2015, Phil. Trans. R. Soc. B 370:20140344 (upwards of 1,000,000 creatures in a drop of pepper-water; letter read aloud over several sessions)
  • Lens on Leeuwenhoek, letter L-040 (Oldenburg printed less than half; Hooke succeeded later in 1677)
  • CDC MMWR 53(34):783, 2004, 150th Anniversary of John Snow and the Pump Handle (31 August 1854, handle removed 8 September 1854, Vibrio cholerae isolated 1883; cdc.gov/mmwr/preview/mmwrhtml/mm5334a1.htm)
  • CDC History of Smallpox and Smallpox timeline (Jenner 1796; 33rd World Health Assembly declared the world free of smallpox on 8 May 1980; cdc.gov/smallpox/about/history.html)
  • American Chemical Society, Discovery and Development of Penicillin landmark (Fleming 1928 at St Mary's Hospital London, paper June 1929, Florey and Chain at Oxford from 1939; acs.org/education/whatischemistry/landmarks/flemingpenicillin.html)
  • Fleming 1929, Br J Exp Pathol 10:226. Sender, Fuchs and Milo 2016, PLoS Biol 14:e1002533 (3.8 × 10^13 bacteria and 3.0 × 10^13 human cells in a 70 kg reference man, ratio about 1.3, bacteria about 0.2 kg).

Checked: 2026-10-17