Microbiology is the study of microorganisms – biological entities too small to be seen with the unaided eye. Most major advances in microbiology have occurred within the past 150 years, and several important subdisciplines of microbiology have developed during this time, including microbial ecology, molecular biology, immunology, industrial microbiology and biotechnology. Microorganisms of various types exist in all three domains of life (the , and ), and they are by far the most abundant life forms on Earth. Microscopic biological agents include bacteria, archaea, protists (protozoa and algae), fungi, parasitic worms (helminths) and viruses. Although a small percentage of microorganisms are harmful to certain plants and animals and may cause serious disease in humans, the vast majority of microorganisms provide beneficial services, such as assisting in water purification and the production of certain foods, and many are essential for the proper functioning of Earth's ecosystems.

Key Concepts

  • The science of microbiology has advanced rapidly since the late 1800s and has spawned numerous important environmental, industrial and medical subdisciplines.
  • Microbes are found in all domains of life and include bacteria, archaea, fungi, protozoa, algae, certain helminths and viruses.
  • Microorganisms are essential to the proper functioning of all of Earth's ecosystems; all nutrient (elemental) cycles are driven primarily by microbial activities.
  • While the vast majority of microorganisms are beneficial and provide valuable services, some microbes cause serious disease and have developed resistance to many of our most useful antimicrobial drugs.

Keywords: bacteria; archaea; prokaryotes; microorganisms; viruses; fungi; protists; pathogens

Figure 1. Universal phylogenetic tree showing relationships between major lineages of the three domains of life ( , and ). Tree topology and branch lengths were determined by comparative small subunit (SSU) rRNA gene sequence analysis.
Figure 2. Major morphological forms of bacterial cells. (a) coccus (plural, cocci); (b) rod (bacillus; plural, bacilli); (c) spirillum (plural, spirilla) and (d) spirochete.
Figure 3. Major morphological forms of viruses. Most viruses are considerably smaller than cells (a representation of an Escherichia coli cell is shown for size comparison). Viruses of eukaryotes typically have either icosahedral (e.g. rhinoviruses) or helical (e.g. ebola virus) symmetry, whereas viruses that infect bacteria (bacteriophages) often exhibit a complex ‘head‐and‐tail’ combination of these forms.
Figure 4. Key elemental cycles driven predominantly by the activity of microorganisms. (a) Carbon is cycled between inorganic (CO2) and organic (CH2O) forms by the actions of autotrophs (1) and heterotrophs (2). In addition, methane (CH4) is produced (3) or consumed (4) by methanogens or methanotrophs, respectively. (b) Nitrogen compounds are cycled by nitrogen‐fixing (1), nitrosifying (2), nitrifying (3) and denitrifying (4) bacteria. (c) In the sulphur cycle, sulphide is produced by sulphate‐ and sulphur‐reducing bacteria (1 and 2, respectively). Sulphur chemolithotrophic bacteria oxidise sulphide to sulphate through an elemental sulphur intermediate (3).


Arnon SS , Schechter R , Ingelsby TV , et al. (2001) Botulinum toxin as a biological weapon: medical and public health management. The Journal of the American Medical Association 285 (8): 1059–1070.

Crompton DWT (1999) How much human helminthiasis is there in the world? Journal of Parasitology 85: 397–403.

Faurez F , Dory D , Grasland B and Jestin A (2009) Replication of porcine circoviruses. Virology Journal 6: 60.

Gregersen LH , Bryant DA and Frigaard N‐U (2011) Mechanisms and evolution of oxidative sulfur metabolism in green sulfur bacteria. Frontiers in Microbiology 2: 116.

López‐García P and Moreira D (2008) Tracking microbial biodiversity through molecular and genomic ecology. Research in Microbiology 159: 67–73.

Madigan MT , Martinko JM , Bender KS , Buckley DH and Stahl DA (2015) Brock Biology of Microorganisms, 14th edn. San Francisco, CA: Benjamin Cummings/Pearson.

Niagro FD , Forsthoefel AN , Lawther RP , et al. (1998) Beak and feather disease virus and porcine circovirus genomes: intermediates between the geminiviruses and plant circoviruses. Archives of Virology 143: 1723–1744.

The Healthy Human Microbiome Project Consortium (2012) Structure, function and diversity of the healthy human microbiome. Nature 486: 207–214.

Widdel F and Bak F (1992) Gram‐negative mesophilic sulfate‐reducing bacteria. In: Balows A , Trüper HG , Dworkin M , Harder W and Schleifer KH (eds) The Prokaryotes, 2nd edn, pp. 3353–3378. New York, NY: Springer.

Winogradsky S (1949) Microbiologie du Sol. Problemès et Méthodes. Cinquante Ans de Recherches. Paris, France: Masson.

Woese CR , Kandler O and Wheelis ML (1990) Towards a natural system of organisms: proposal for the domains Archaea, Bacteria, and Eucarya. Proceedings of the National Academy of Sciences USA 87: 4576–4579.

Further Reading

Brock TD (1961) Milestones in Microbiology. Upper Saddle River, NJ: Prentice Hall.

Dixon B (2009) Animalcules: The Activities, Impacts, and Investigators of Microbes. Washington, DC: ASM Press.

Henig RM (1993) A Dancing Matrix: Voyages Along the Viral Frontier. New York, NY: Alfred A. Knopf, Inc.

Kirchman DL (2012) Processes in Microbial Ecology. New York, NY: Oxford University Press, Inc.

Kohanski MA , Dwyer DJ and Collins JJ (2010) How antibiotics kill bacteria: from targets to networks. Nature Reviews Microbiology 8: 423–435.

Konstantinidis KT and Tiedje JM (2007) Prokaryotic taxonomy and phylogeny in the genomic era: advancements and challenges ahead. Current Opinions in Microbiology 10: 504–509.

Strelkauskas AJ , Strelkauskas JE and Moszyk‐Strelkauskas D (2010) Microbiology: A Clinical Approach. New York, NY: Garland Science, Taylor and Francis Group, LLC.

Tortora GJ , Funke BR and Case CL (2016) Microbiology: An Introduction, 12th edn. San Francisco, CA: Benjamin Cummings/Pearson.

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Sattley, W Matthew, and Madigan, Michael T(Aug 2015) Microbiology. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0000459.pub2]