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Cholera bacteria (Vibrio cholerae) (secondary electron microscopy).  Typical dimensions are 2-3 micrometers long, 0.5 micrometers thick.

Cholera bacteria ( Vibrio cholerae ) ( secondary electron microscopy ). Typical dimensions are 2-3 micrometers long, 0.5 micrometers thick.

Classification : Creature
Domain : bacteria
Scientific name
Haeckel , 1894


Helicobacter pylori , causing stomach ulcers (secondary electron microscopy)

The bacteria (Bacteria) ( singular the bacterium , obsolete also the bacterium ; from ancient Greek βακτήριον baktērion , rods, coll. Also bacillus ) form one of the three basic domains , in addition to the eukaryotes and archaea , into which all living beings are divided.

Bacteria, like archaea, are prokaryotes , which means that their DNA is not contained in a cell nucleus separated from the cytoplasm by a double membrane , as in eukaryotes, but with them, as with all prokaryotes, the DNA lies freely in the cytoplasm, compressed into a narrow space, the nucleoid ( nuclear equivalent ).

The science and doctrine of bacteria is bacteriology .


Bacteria were first observed in water and in human saliva by Antoni van Leeuwenhoek using a self-made microscope and described by him in reports to the Royal Society of London in 1676 .

Until the end of the last century, the term "bacteria" was used in microbiology for all microscopic, mostly unicellular organisms that do not have a real nucleus and therefore belong to the prokaryotes. However, this also applies to the archaea, which have been assigned to a separate domain since around 1990. In order to distinguish it from the archaea, the term “real bacteria” (“eubacteria”) or “real bacteria” was used in the transition period up to the definition of the three living being domains and the scientific names Eubacteria and Archaebacteria were used. Eubacteria was an unfortunate name because there is also a genus of bacteria called Eubacterium . Today the two domains of the prokaryotes are called Bacteria and Archaea , the third domain is that of the eukaryotes .

More than three hundred years after the first description of bacteria and despite countless already described and cataloged species, it can be assumed, based on current knowledge, that the vast majority (approx. 95 to 99%) of all bacterial species existing on our planet are neither known nor described (Status: 2006). So there are always new discoveries. As the largest previously known bacterium was discovered in 1999: The so-called sulfur pearl of Namibia, Thiomargarita namibiensis , with a maximum diameter of 0.7 mm, a visible with the naked eye sulfur bacteria . The bacterium with the fewest genes is Carsonella ruddii . It has only 159,662 base pairs and 182 genes. This bacterium lacks essential genes that a bacterium needs to live independently. It lives endosymbiotic in specialized cells of leaf fleas . The bacterium with the smallest genome that lives parasitically is Mycoplasma genitalium with 582,970 base pairs. The bacterium with the smallest genome that lives independently, i.e. neither symbiotic nor parasitic, is Pelagibacter ubique and has around 1.3 million base pairs.


Shape and size

Forms and aggregates of bacteria (selection)

Bacteria come in various external forms (examples in brackets): spherical, so-called cocci ( Micrococcus ), cylindrical, so-called rods ( Bacillus , Escherichia ) with more or less rounded ends, helical ( spirilla , spirochete ), with stems ( Caulobacter ), with appendages ( Hyphomicrobium ), forming multicellular trichomes ( Caryophanon , Oscillatoria ), forming long, branched threads, so-called hyphae , which branch and form a thread mass called mycelium ( streptomycetes ), as well as structures with several irregularly arranged cells ( pleurocapsa ). Bacteria often occur in aggregates: spherical chains ( Streptococcus ), flat arrangement of spherical cells ( Merismopedia ), regular three-dimensional arrangement of spheres ( Sarcina ), rod chains ( Streptobacillus ), rod chains enclosed in tubes ( Leptothrix ).

The size of bacteria is very different: their diameter is between about 0.1 and 700 µm, for most known species it is about 0.6 to 1.0 µm. Their length is in a larger range: for single cells between about 0.6 µm (in cocci ) and 700 µm, hyphae can be even longer, most bacteria are 1 to 5 µm long. The volume of most bacteria is on the order of 1 µm³. Apart from a few exceptions, individual bacterial cells cannot be seen with the naked eye, as the resolution of the human eye is around 50 µm. Mycoplasmas are particularly small , the diameter of the smallest is around 0.3 µm. Many cyanobacteria are particularly large ; their diameter is usually between 2 and 8 µm. The largest known bacterium to date is Thiomargarita namibiensis : roughly spherical with a diameter of 300–700 µm, so visible to the naked eye. The volume of the largest bacterium (diameter d about 700 µm, volume of a sphere = 0.523 · d 3 ) is about 10 billion times larger than the volume of the smallest (diameter about 0.3 µm).


Scheme of a bacterial cell

Most bacteria have a cell wall , all have a cell membrane that encloses the cytoplasm and the ribosomes . The DNA is free in the cytoplasm as a strand-shaped, self-contained molecule - a so-called bacterial chromosome . Some bacteria also have two bacterial chromosomes, for example Ralstonia eutropha strain H16. Often there is further DNA in the cytoplasm in the form of smaller, also strand-like, self-contained molecules, the plasmids , which are multiplied independently of the bacterial chromosome and passed on during reproduction or can be transferred from one individual to another. The genome of the intestinal bacterium Escherichia coli consists of almost 4.7 million base pairs, the sequence of which is fully known. The DNA molecule is around 1.4 millimeters long with a diameter of only 2 nanometers and contains around 4400 genes . Despite its length of more than a thousand times the cell diameter, it is condensed into an area of ​​about half the cell diameter (presumably highly ordered) (nucleoid). Many more bacterial genomes are now fully known. A specialty of the bacteria is also the RNA polymerase . You only have one, and it consists of only 5 subunits (α (2x), β, β 'and ω). In contrast, the RNA polymerase of archaea consists of 11–12 subunits, and eukaryotes have several RNA polymerases, which consist of up to 12 subunits.

Explanations of the bacteria scheme:

  • A longitudinal section of a bacterium is shown schematically.
  • Not all structural elements shown are always present in all bacteria.
  • The following are always present in all bacteria: cytoplasmic membrane , cytoplasm , nucleoid and ribosomes .
  • Thylakoids (used for phototrophy ) are present in very different forms in all phototrophic bacteria, with the exception of chlorobia .
  • Chlorosomes (used for phototrophy) are present in chlorobia.
  • If a cell wall is present (by far most bacteria), it is thin in gram- negative bacteria and thick in gram-positive bacteria.
  • Gram-negative bacteria have another biomembrane outside the cell wall, the so-called outer membrane , which is not shown in the diagram in the figure.
  • As far as flagella are present, their number (1 to many) and their arrangement differ depending on the type of bacteria. Their length also varies. They are always helical.
  • As far as there are pili , their number (1 to many), length and arrangement vary.
  • If there is a mucous membrane , glycocalyx outside the cell wall, it can be of different thicknesses and consist of different mucous substances depending on the type of bacteria and external conditions.
  • As far as plasmids are present, their number is different.
  • If gas vesicles are present, their size and number vary depending on the type of bacteria and external circumstances.

Way of life and reproduction

Way of life

The way of life and metabolism of the bacteria are very different. There are bacteria that need oxygen ( aerobic bacteria or aerobes), bacteria for which oxygen is poison (obligate anaerobic bacteria or obligate anaerobes), and bacteria that are tolerant of oxygen (facultative anaerobes). Some bacteria are capable of photosynthesis , i.e. phototrophic , for example the cyanobacteria (formerly also called blue-green algae) , while most of them are chemotrophic . Most of the chemotrophs are heterotrophic , but some are chemoautotrophic , namely lithoautotrophic.

Some bacteria (e.g. Bacillus ) develop permanent stages ( spores ) in which the entire metabolism comes to a standstill. In this state, the bacteria can withstand unfavorable - even extreme - environmental conditions and last for several thousand years. Other types of bacteria have developed a different strategy and adapted their metabolism directly to extreme environmental conditions. They are known as extremophiles .

Most bacteria live together in nature in the form of biofilms .


Bacteria multiply asexually through cell division . This can be done by equal or inequitable transverse division (especially in the case of cylindrical bacteria, for example in Pseudomonas , Bacillus ), by budding (for example in Planctomyces ), by multiple spore formation ( e.g. in Crenothrix ) or in another way. When the endospores are formed , there is usually no increase because predominantly only one endospore per cell is formed; only a few bacteria, for example Anaerobacter polyendosporus and Metabacterium , do several endospores per cell. All offspring of asexual reproduction have identical genomes and therefore form a clone .

The reproduction in a bacterial population is described under bacterial growth .

Gene transfer

During conjugation , bacteria can exchange DNA with one another with the help of so-called sexpili ( protein tubes ) ( horizontal and vertical gene transfer ). Using the Sexpili, the cells can approach each other and then transfer DNA (the bacterial “chromosome” in whole or in part and plasmids) from one cell to the other via a plasma bridge. Since the pili are not directly involved in DNA transmission, this can also take place without pili, if two bacterial cells lie close together. This gene transfer is mainly practiced by Gram-negative bacteria. In the case of Gram-positive bacteria, the mechanism of transduction predominates. Here, bacteriophages are used as vectors . Transformation , the uptake of naked DNA, is rarely widespread.


Bacteria usually move freely in the liquid medium, swimming through flagella , also known as flagella, which, unlike the flagella of eukaryotes (e.g. protists), are not built according to the "9 + 2 pattern", but rather from a long, helical, 15 to 20 nm thick protein thread . In addition, the flagella of the bacteria do not act as a driving force by changing their shape like the flagella of the eukaryotes, but they are turned like a propeller. The rotary motion is generated on a complicated basal structure by a stream of protons , similar to a turbine that is driven by a stream of liquid or gas. A proton concentration gradient is required for this. Spirochaetes move by rotating around themselves and, thanks to their helical bodies, to a certain extent screw themselves through the surrounding medium. Some bacteria do not move freely but by crawling, for example myxobacteria and some cyanobacteria .

Various environmental factors can influence the direction in which the bacteria move. These reactions are known as phototaxis , chemotaxis (chemotaxis versus oxygen: aerotaxis), mechanotaxis and magnetotaxis .

Endosymbiotic hypothesis

On the basis of biochemical studies, it is now assumed that some organelles found in the cells of many eukaryotes were originally independent bacteria ( endosymbiotic theory ); this affects the chloroplasts and the mitochondria . These organelles are characterized by a double membrane and contain their own circular DNA, which can contain 5 to 62 genes depending on the type. Evidence for this are the results of rRNA sequencing and the organelle proteins, which show a stronger homology to the bacterial proteins than to the eukaryotes. The codons of mitochondrion and chloroplast are also more similar to the codon usage of bacteria.


Bacteria on and in people

A human is made up of about 10 trillion (10 13 ) cells that have emerged from the fertilized egg cell. In addition, there are about ten times as many bacteria on and in it.

A total of around 10 10 bacteria live in a person's mouth .

With average hygiene, there are around a hundred times as many bacteria on human skin, namely around a trillion in total, but distributed very differently: on the arms there are only a few thousand, in greasier regions such as the forehead a few million and in damp regions like the Armpits several billion per square centimeter. There they feed on around ten billion skin flakes that are released daily and on minerals and lipids that are secreted from the skin pores.

99% of all microorganisms living in and on the human body, namely more than 10 14 of at least 400 different species, including mainly bacteria, live in the digestive tract, especially in the large intestine. There they form the intestinal flora .

Even in the lungs of healthy people, 128 types of bacteria have recently been discovered as a result of a new research method as part of the microbiome project (around 2007). Until then, microbiologists had never been able to reproduce bacteria from the lungs in the laboratory. Hence it was thought that the lungs were sterile.


The ability of a large number of bacteria to produce substances important for humans such as antibiotics and enzymes is used in many ways in biotechnology . In addition to classic processes in food and chemical production ( white biotechnology ; especially bioethanol , acetic acid , lactic acid , acetone ), their abilities to eliminate problematic waste and to produce drugs (especially antibiotics , insulin ) are also used. Above all, Escherichia coli and various types of Clostridia , Corynebacterium , Lactobacillus , Acetobacter and a large number of other bacteria play a role by making their metabolism specifically usable.

For this purpose, useful parts of the genome of certain bacteria are often implanted into the genome of easy-to-keep, easy-to-cultivate and largely harmless bacteria such as Escherichia coli ( genetic manipulation ).


Bacteria can exchange genes with each other, even across species boundaries, and even incorporate fossil DNA fragments that occur in their environment into their own DNA. In this context, a new term was coined: anachronistic evolution , evolution also across time limits.


Bacteria play a major role in the human body. A multitude of bacteria live in the human intestine, which together form the intestinal flora that promotes digestion . The skin of healthy people is colonized by harmless bacteria that make up the skin flora . A particularly large number of bacteria are found on the teeth. But bacteria can also act as pathogens . Some bacteria cause purulent wound infections ( infections ), sepsis (blood poisoning) or the inflammation of organs (e.g. cystitis or pneumonia ). In order to prevent these diseases , two methods of fighting bacteria have been developed by hygiene , a branch of medicine:

Sterilization is a process used to make medical devices and materials sterile.

Disinfection is a process to greatly reduce the number of bacteria on the skin or objects (e.g. with hand disinfectants ).

Once the bacteria have penetrated the body and triggered an infection, antibiotics are now an effective means of combating bacteria, for example penicillins , which are produced by fungi of the genus Penicillium . Penicillin disrupts the synthesis of the bacterial cell wall, so it only works against growing bacteria. However, certain bacteria have developed effective protection against many antibiotics over time. That is why bacteria are examined in microbiological laboratories and a resistance test is carried out. When treating with antibiotics, it must be noted that not only pathogenic (disease-causing) bacteria, but also mutualistic (useful) bacteria can be disrupted or killed by the drug. This can lead to the fact that initially a small number of bacteria of the species Clostridium difficile living in the intestine , which are naturally resistant to many antibiotics, gain the upper hand in the intestine and cause severe diarrhea.

A resistance to antibiotics can be natural or the result of a mutation to be. To prove this, the biologists Max Delbrück and Salvador Edward Luria developed the fluctuation test .

An older method of doctors in the fight against bacterial infections is the operation with opening and cleaning of the pus focus , according to the ancient Latin surgeon saying “Ubi pus, ibi evacua” - in German: “Where there is pus, empty it.” For larger foci of pus, this method in conjunction with the administration of antibiotics is much more effective than the use of antibiotics alone.


Many soil bacteria, which act as destructors or make nutrient salts available for the plants , are indispensable for important geochemical material cycles .

The so-called cyanobacteria form a large group of bacteria. Since they are prokaryotes, they do not belong to the group of algae. They carry out photosynthesis and are therefore independent of organic food, but need light for energy supply. Together with the green algae (Chlorophyta) and other groups of algae, they form the phytoplankton of the oceans and freshwater and thus form the basis of food for many ecosystems .

Special bacteria occur as symbionts in the intestine or in other organs of many living things and are involved in digestion and other physiological processes. Escherichia coli and enterococci are the best known representatives of this group. But it also includes anaerobic bifidobacteria . While these bacteria act as symbionts, other bacteria cause infectious diseases in humans, animals and plants ( bacteriosis ).


Phylogenetic system

Phylogenetic tree of the bacteria, wherein the 16S from the comparison of the base sequence rRNA results

A phylogenetic classification based on morphological and metabolic physiological characteristics is usually not possible for bacteria; it must be based on the molecular structure of these organisms. The classification is mainly done with the help of phylogenetic markers. Such markers are cellular macromolecules, the composition of which differs more and more as the degree of relationship between different organisms decreases. The 16S subunit of ribosomal RNA is currently one of the most important molecules of this type . The base sequence of this RNA is said to reflect the actual evolutionary relationships among organisms.

The phylogenetic system of bacteria currently accepted by most bacteriologists is described in Taxonomic Outline of the Bacteria and Archaea , which also classifies the archaea. This system is shown below, limited to bacteria in the proper sense (domain Bacteria) down to the level of order. However, the diversity of bacterial life forms is significantly greater than this system represents. Based on the 16S rRNA sequences known to date, more than 50 different bacterial phyla are suspected . The existence of this phyla is predicted on the basis of large groups of certain rRNA sequences that repeatedly appear in environmental samples, but so far no bacterium has been cultivated from this phyla.

"Classic" systems

Before one could set up phylogenetically based systems, one had to rely on characteristics that hardly made it possible to establish natural, phylogenetic relationships. Molecular biological characteristics commonly used today, which are required to determine phylogenetic relationships, could not be determined with the methods available at the time. The following system is an example of obsolete (“classic”) systems. The prokaryotes ("Schizophyta") formed a division of the plants. Even today, the community of bacteria occurring in a biotope is sometimes referred to as "bacterial flora ".

Schizophyta department ("fissile plants", includes all prokaryotes = "anucleobionta")

Class Bacteria (bacteria = "split fungi")
Order Eubacteriales (single-celled unbranched bacteria)
Family Coccaceae (globular bacteria)
Family Bacteriaceae (rod-shaped bacteria without spores)
Bacillaceae family (rod-shaped bacteria with spores)
Family Spirillaceae ("screw bacteria", helical)
Order Chlamydobacteriales (thread bacteria in tubes "sheaths")
Order Mycobacteriales (rod-shaped bacteria with branches, mycelium-forming bacteria "ray fungi")
Order Myxobacteriales ("slime bacteria", single-cell, swarm-forming bacteria)
Order Spirochaetales (flexible, helical bacteria with active shape change)
Class Cyanophyceae ("blue-green algae", "split algae")
Order Chroococcales (unicellular, without spores)
Order Chamaesiphonales (unicellular or thread-like, with spores)
Order Hormogonales (thread-like, with hormogonia, often heterocysts)

Practical division

Cocci - Spirilla - Bacilli

For practical reasons, bacteria are sometimes divided according to their shape and organization based on the earlier “classical” systems. Spherical bacteria are called cocci , elongated, cylindrical bacteria are called bacilli and spiral, helical bacteria are called spirils or spirochetes . These basic forms can appear individually or come together to form typical forms (heap cocci = staphylococci , chain cocci = streptococci , double cocci = diplococci ). Furthermore, rod bacteria in particular often form one or more flagella , with the help of which they can move. The number and arrangement of the flagella are distinguishing features. Some bacteria form mucous membranes, "capsules", and some different types of spores . Also important for the subdivision is the way of life, especially the metabolic type, and the possibility of coloring the bacteria in a certain way. The so-called Gram stain (introduced by the Danish bacteriologist Gram) allows conclusions to be drawn about the composition and structure of the cell wall ; the so-called gram-positive bacteria probably even form a natural family group, a monophyletic taxon .

Serologically distinguishable variations of bacteria are called serotypes .

The oldest bacterium

Since 2000, an estimated 250 million year old bacterium has been considered the oldest living thing on earth. The microorganism with the current name "Bacillus permians" was discovered in a laboratory at West Chester University in Pennsylvania by researchers led by Russell H. Vreeland . In a nutrient solution, the bacterium developed activities. It was recovered from drilling in a cave near Carlsbad (New Mexico) , which was used to explore a possible final repository for nuclear waste. It survived the ages in a larger crystal of salt with some brine in it at 2,000 feet (609 meters) deep.

The research team reported on their find in the British science journal Nature on October 19, 2000. The discovery sparked new reflections on the origin of life in the universe. Such a long lifespan for this organism would allow it to travel huge distances in space and make panspermia more likely. It seems like spores could be a key to this. Bacteria and yeast can reduce their functions in bad times so that they become a stable elastic structure. Such spores have already been resuscitated from 118-year-old meat cans and 166-year-old beer bottles.

The resuscitation route was more complex for the oldest living being known from 1995. Bacteria spores around 25 to 40 million years old were brought to life here. They came from the abdomen of a bee that was enclosed in a Dominican amber .

Other researchers took a detached stance towards the discovery of their colleagues and pointed out that reports of finds of ancient bacteria in rock, coal or ancient Egyptian temples have not yet withstood scientific scrutiny. Russell H. Vreeland considers that such a long service life was only faked by contamination with recent bacteria, however, as almost impossible.



  • Martin Dworkin, Stanley Falkow, Eugene Rosenberg, Karl-Heinz Schleifer, Erko Stackebrandt (Eds.): The Prokaryotes, A Handbook of the Biology of Bacteria . 7 volumes, 3rd edition, Springer-Verlag, New York a. a. O., 2006, ISBN 0-387-30740-0 . Also includes archaea.
  • Joseph W. Lengeler, Gerhart Drews , Hans G. Schlegel (Eds.): Biology of the Prokaryotes . Georg Thieme Verlag, Stuttgart 1999, ISBN 3-13-108411-1 . Also includes archaea.
  • Michael T. Madigan, John M. Martinko, Paul V. Dunlop, David P. Clark: Brock - Biology of microorganisms , 12th Ed. (Pearson International Edition), Pearson, Benjamin Cummings, Pearson Education, Inc., San Francisco et al. a. O. 2009, ISBN 978-0-321-53615-0 . Comprehensive textbook, also covers other microorganisms.
  • Michael T. Madigan, John M. Martinko: Brock - Mikrobiologie , 11th revised edition, Pearson Studium, Munich 2006, ISBN 3-8273-7187-2 . Translation of Brock - Biology of microorganisms 11th ed. Into German, also covers other microorganisms.
  • Betsey Dexter Dyer: A field guide to bacteria . Cornell University Press, Ithaca NY, USA 2003, ISBN 0-8014-8854-0 (cardboard), ISBN 0-8014-3902-7 (linen). Observations in the field, also covers Archaea.
  • Karl Bernhard Lehmann & Rudolf Otto Neumann : Atlas and outline of bacteriology and textbook of special bacteriological diagnostics . Lehmann, Munich 1896. Classic (outdated) textbook with a focus on medical bacteriology.


Web links

Wiktionary: Bacteria  - explanations of meanings, word origins, synonyms, translations
Commons : Bacteria  - collection of pictures, videos and audio files

Individual evidence

  1. ^ Carl R. Woese, Otto Kandler, Mark L. Wheelis: Towards a natural system of organisms: Proposal for the domains Archaea, Bacteria, and Eucarya . In: Proceedings of the National Academy of Science, USA . Vol. 87, 1990, pp. 4576-4579.
  2. Bild der Wissenschaft , 1/2007, p. 9.
  3. ^ Dorion Sagan, Lynn Margulis: Garden of Microbial Delights: A Practical Guide to the Subvisible World . Kendall / Hunt Publishing Company, Dubuque, Iowa 1993.
  4. How Microbes Defend and Define Us . In: New York Times , accessed October 25, 2019.
  5. Lucian Haas: Bacteria can incorporate fossil DNA fragments into their genome . Deutschlandfunk , Research News , November 21, 2013.
  6. ^ The Taxonomic Outline of Bacteria and Archaea
  7. George M. Garrity, Timothy G. Lilburn, James R. Cole, Scott H. Harrison, Jean Euzéby, Brian J. Tindall: Taxonomic Outline of the Bacteria and Archaea . Release 7.7, March 6, 2007, Michigan State University Board of Trustees,
  8. ^ Hans Fitting, Walter Schumacher, Richard Harder, Franz Firbas: Textbook of botany for universities . Founded by E. Strasburger, F. Noll, H. Schenk and AFW Schimper. 25th edition. Piscator, Stuttgart 1951, pp. 295-301.
  9. ↑ Primordial bacterium brought to life . Spiegel Online , October 18, 2000; Retrieved December 3, 2010.
  10. ^ Prehistoric bacteria revived from buried salt . In: Science News Online of December 6, 1999, accessed December 3, 2010.
  11. RH Vreeland, WDRosenzweig, DW Powers: Isolation of a 250 million-year-old bacterium from a primary salt crystal . In: Nature , 407, 2000, pp. 897-900.
  12. ^ The Economist (Ed.): Colonizing the galaxy is hard. Why not send bacteria instead? April 12, 2018, ISSN  0013-0613 ( [accessed April 24, 2020]).
  13. Hardcore hibernation . naturenews of October 19, 2000, accessed on December 3, 2010.
  14. RJ Cano, M. Borucki: Revival and identification of bacterial spores in 25 to 40 million year old Dominican amber . In: Science , 268, 1995, pp. 1060-1064.
  15. Alive ... after 250 million years . BBC News of October 18, 2000, accessed December 3, 2010.
  16. ^ Row over ancient bacteria . BBC News of June 7, 2001, accessed December 3, 2010.