Discovery and the birth of microbiology
In 1676 Antonie van Leeuwenhoek, using microscopes he made himself, saw tiny wriggling "animalcules" in rainwater and dental plaque and described them in letters to the Royal Society. The idea that bacteria cause disease took two more centuries to win acceptance. In the 1860s Louis Pasteur's swan-necked flask experiments refuted spontaneous generation and led to pasteurisation, while Robert Koch identified the anthrax bacillus (1876), the tuberculosis bacterium (1882) and the cholera vibrio (1883), formulating "Koch's postulates" linking a specific microbe to a specific disease. Hans Christian Gram's 1884 staining method still divides bacteria into Gram-positive and Gram-negative.
Structure and types
Bacteria are prokaryotes: their DNA floats in the cytoplasm without a nuclear membrane, surrounded by a cell membrane, cell wall and ribosomes, and some swim with flagella. In good conditions they can divide every 20 minutes, and they swap genes with neighbours via plasmids. In 1977 Carl Woese compared ribosomal RNA and showed that bacteria and archaea are separate domains of life, redrawing the tree of life. By shape they are commonly grouped as follows.
| Group | Shape | Examples |
|---|---|---|
| Cocci | Spherical | Staphylococcus, Streptococcus pneumoniae |
| Bacilli | Rod-shaped | E. coli, Mycobacterium tuberculosis |
| Spiral forms | Spiral or curved | Helicobacter pylori, Treponema pallidum |
Helpful and harmful bacteria
The human body hosts around 38 trillion bacteria, roughly as many as its own cells, and the gut microbiome aids digestion, immunity and vitamin production. Lactic acid bacteria ferment yoghurt and kimchi, and root-nodule bacteria fix nitrogen for plants. Pathogens such as Mycobacterium tuberculosis, Salmonella and Staphylococcus aureus invade tissue or release toxins; white blood cells fight them and antibiotics treat them. Overuse of antibiotics breeds antibiotic resistance, which the World Health Organization ranks among the top global health threats. Antibiotics do nothing against viral illnesses such as colds.
This article is general health information and is not a substitute for diagnosis or treatment. If symptoms persist, see a doctor.
FAQ
- What is the difference between bacteria and viruses?
- Bacteria are living cells that feed and divide on their own. Viruses are much smaller, have no cellular structure and can reproduce only inside a host cell, which is why antibiotics work on bacteria but not on viruses.
- Does hand sanitiser kill all bacteria?
- Sanitisers with at least 60% alcohol quickly reduce most bacteria, but soap and water work better on visibly dirty hands and against norovirus or Clostridioides difficile spores.
Sources reviewed: 2026-10-26
References
- Leeuwenhoek, A. van (1677). Observations concerning little animals observed in rain-, well-, sea- and snow-water. Philosophical Transactions, 12, 821–831.
- Pasteur, L. (1862). Mémoire sur les corpuscules organisés qui existent dans l'atmosphère. Annales des Sciences Naturelles (Zoologie), 4e sér., 16, 5–98.
- Koch, R. (1882). Die Ätiologie der Tuberculose. Berliner Klinische Wochenschrift, 19, 221–230.
- Gram, C. (1884). Über die isolierte Färbung der Schizomyceten in Schnitt- und Trockenpräparaten. Fortschritte der Medizin, 2, 185–189.
- Woese, C. R., & Fox, G. E. (1977). Phylogenetic structure of the prokaryotic domain: The primary kingdoms. PNAS, 74(11), 5088–5090.
- Sender, R., Fuchs, S., & Milo, R. (2016). Revised estimates for the number of human and bacteria cells in the body. PLOS Biology, 14(8), e1002533.
- Madigan, M. T., et al. (2019). Brock Biology of Microorganisms (15th ed.). Pearson.