Bacteria

Rod-shaped Bacillus subtilis
Phylogenetic tree of Bacteria, Archaea and Eucarya. The vertical line at bottom represents the last universal common ancestor.
Bacteria display many cell morphologies and arrangements
The range of sizes shown by prokaryotes (Bacteria), relative to those of other organisms and biomolecules.
Structure and contents of a typical Gram-positive bacterial cell (seen by the fact that only one cell membrane is present).
An electron micrograph of Halothiobacillus neapolitanus cells with carboxysomes inside, with arrows highlighting visible carboxysomes. Scale bars indicate 100 nm.
Helicobacter pylori electron micrograph, showing multiple flagella on the cell surface
Bacillus anthracis (stained purple) growing in cerebrospinal fluid
Many bacteria reproduce through binary fission, which is compared to mitosis and meiosis in this image.
A culture of ''Salmonella
A colony of Escherichia coli
Helium ion microscopy image showing T4 phage infecting E. coli. Some of the attached phage have contracted tails indicating that they have injected their DNA into the host. The bacterial cells are ~ 0.5 µm wide.
Transmission electron micrograph of Desulfovibrio vulgaris showing a single flagellum at one end of the cell. Scale bar is 0.5 micrometers long.
The different arrangements of bacterial flagella: A-Monotrichous; B-Lophotrichous; C-Amphitrichous; D-Peritrichous
Streptococcus mutans visualised with a Gram stain.
Phylogenetic tree showing the diversity of bacteria, compared to other organisms. Here bacteria are represented by three main supergroups: the CPR ultramicrobacterias, Terrabacteria and Gracilicutes according to recent genomic analyzes (2019).
Overview of bacterial infections and main species involved.
Colour-enhanced scanning electron micrograph showing Salmonella typhimurium (red) invading cultured human cells
In bacterial vaginosis, beneficial bacteria in the vagina (top) are displaced by pathogens (bottom). Gram stain.
Antonie van Leeuwenhoek, the first microbiologist and the first person to observe bacteria using a microscope.

Bacteria (singular bacterium, common noun bacteria) are ubiquitous, mostly free-living organisms often consisting of one biological cell.

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A cluster of Escherichia coli bacteria magnified 10,000 times

Microorganism

Organism of microscopic size, which may exist in its single-celled form or as a colony of cells.

Organism of microscopic size, which may exist in its single-celled form or as a colony of cells.

A cluster of Escherichia coli bacteria magnified 10,000 times
Antonie van Leeuwenhoek was the first to study microscopic organisms.
Lazzaro Spallanzani showed that boiling a broth stopped it from decaying.
Vardhmana Mahavira postulated the existence of microscopic creatures in the sixth century BC.
Louis Pasteur showed that Spallanzani's findings held even if air could enter through a filter that kept particles out.
Robert Koch showed that microorganisms caused disease.
Staphylococcus aureus bacteria magnified about 10,000x
Euglena mutabilis, a photosynthetic flagellate
A tetrad of Deinococcus radiodurans, a radioresistant extremophile bacterium
The photosynthetic cyanobacterium Hyella caespitosa (round shapes) with fungal hyphae (translucent threads) in the lichen Pyrenocollema halodytes
Wastewater treatment plants rely largely on microorganisms to oxidise organic matter.
A laboratory fermentation vessel
The eukaryotic parasite Plasmodium falciparum (spiky blue shapes), a causative agent of malaria, in human blood

Two of the three domains Archaea and Bacteria, only contain microorganisms.

An agar plate streaked with microorganisms

Bacteriology

An agar plate streaked with microorganisms
Antonie van Leeuwenhoek, the first microbiologist and the first person to observe bacteria using a microscope.
Louis Pasteur in his laboratory, painting by A. Edelfeldt in 1885
Statue of Koch in Berlin

Bacteriology is the branch and specialty of biology that studies the morphology, ecology, genetics and biochemistry of bacteria as well as many other aspects related to them.

Neisseria gonorrhoeae (small red dots) in pus from a man with a urethral discharge (Gram stain)

Pathogenic bacteria

Neisseria gonorrhoeae (small red dots) in pus from a man with a urethral discharge (Gram stain)
Commensals vs pathogenic bacteria in COPD
An abscess caused by opportunistic S. aureus bacteria.
Protein structure of botulinum toxin.

Pathogenic bacteria are bacteria that can cause disease.

A rotten apple after it fell from a tree

Decomposition

Process by which dead organic substances are broken down into simpler organic or inorganic matter such as carbon dioxide, water, simple sugars and mineral salts.

Process by which dead organic substances are broken down into simpler organic or inorganic matter such as carbon dioxide, water, simple sugars and mineral salts.

A rotten apple after it fell from a tree
Decomposing fallen nurse log in a forest
Ants eating a dead snake
Pig carcass in the different stages of decomposition: Fresh > Bloat > Active decay > Advanced decay > Dry remains
A decaying peach over a period of six days. Each frame is approximately 12 hours apart, as the fruit shrivels and becomes covered with mold.
A punnet of rotten peaches

Decomposition begins at the moment of death, caused by two factors: 1.) autolysis, the breaking down of tissues by the body's own internal chemicals and enzymes, and 2.) putrefaction, the breakdown of tissues by bacteria.

A bottle of Yakult, a probiotic drink containing Lactobacillus paracasei

Probiotic

Probiotics are live microorganisms promoted with claims that they provide health benefits when consumed, generally by improving or restoring the gut flora.

Probiotics are live microorganisms promoted with claims that they provide health benefits when consumed, generally by improving or restoring the gut flora.

A bottle of Yakult, a probiotic drink containing Lactobacillus paracasei
Élie Metchnikoff first suggested the possibility of colonizing the gut with beneficial bacteria in the early 20th century.

Probiotics are considered generally safe to consume, but may cause bacteria-host interactions and unwanted side effects in rare cases.

The bright colors of Grand Prismatic Spring, Yellowstone National Park, are produced by Thermophiles, a type of extremophile.

Extremophile

Organism that is able to live (or in some cases thrive) in extreme environments, i.e. environments that make survival challenging such as due to extreme temperature, radiation, salinity, or pH level.

Organism that is able to live (or in some cases thrive) in extreme environments, i.e. environments that make survival challenging such as due to extreme temperature, radiation, salinity, or pH level.

The bright colors of Grand Prismatic Spring, Yellowstone National Park, are produced by Thermophiles, a type of extremophile.
Microscopic image from the hypersaline Lake Tyrrell (salinity> 20% w/v), in which the eukaryotic chlorophyte, Dunaliella salina, can be tentatively identified. Dunaliella salina is grown commercially for the carotenoid, β-carotene, which is widely used as a natural food colorant as well as a precursor to vitamin A. Alongside is the haloarchaeon, Haloquadratum walsbyi, which has flat square-shaped cells with gas vesicles that allow flotation to the surface, most likely to acquire oxygen.

Some bacteria were found living in the cold and dark in a lake buried a half-mile deep under the ice in Antarctica, and in the Marianas Trench, the deepest place in Earth's oceans.

Electron micrograph of Treponema pallidum

Syphilis

Electron micrograph of Treponema pallidum
Primary chancre of syphilis at the site of infection on the penis
Typical presentation of secondary syphilis with a rash on the palms of the hands
Reddish papules and nodules over much of the body due to secondary syphilis
Model of a head of a person with tertiary (gummatous) syphilis, Musée de l'Homme, Paris
Histopathology of Treponema pallidum spirochetes using a modified Steiner silver stain
This poster acknowledges the social stigma of syphilis, while urging those who possibly have the disease to be tested (circa 1936).
Micrograph of secondary syphilis skin lesions. (A/B) H&E stain of SS lesions. (C/D) IHC staining reveals abundant spirochetes embedded within a mixed cellular inflammatory infiltrate (shown in the red box) in the papillary dermis. The blue arrow points to a tissue histiocyte and the read arrows to two dermal lymphocytes.
Portrait of Mr. J. Kay, affected with what is now believed to have been congenital syphilis c. 1820
Jarisch–Herxheimer reaction in a person with syphilis and human immunodeficiency virus
Syphilis deaths per million persons in 2012
Portrait of Gerard de Lairesse by Rembrandt van Rijn, circa 1665–67, oil on canvas. De Lairesse, himself a painter and art theorist, had congenital syphilis that deformed his face and eventually blinded him.
An early medical illustration of people with syphilis, Vienna, 1498
A Work Projects Administration poster about syphilis c. 1940
Preparation and Use of Guayaco for Treating Syphilis, after Stradanus, 1590

Syphilis is a sexually transmitted infection caused by the bacterium Treponema pallidum subspecies pallidum.

Schematic representation of the nitrogen cycle. Abiotic nitrogen fixation has been omitted.

Nitrogen fixation

Chemical process by which molecular nitrogen, with a strong triple covalent bond, in the air is converted into ammonia or related nitrogenous compounds, typically in soil or aquatic systems but also in industry.

Chemical process by which molecular nitrogen, with a strong triple covalent bond, in the air is converted into ammonia or related nitrogenous compounds, typically in soil or aquatic systems but also in industry.

Schematic representation of the nitrogen cycle. Abiotic nitrogen fixation has been omitted.
Nodules are visible on this broad bean root
A sectioned alder tree root nodule
Equipment for a study of nitrogen fixation by alpha rays (Fixed Nitrogen Research Laboratory, 1926)
Lightning heats the air around it breaking the bonds of starting the formation of nitrous acid.

Nitrogen fixation is carried out naturally in soil by microorganisms termed diazotrophs that include bacteria, such as Azotobacter, and archaea.

A person with severe dehydration due to cholera, causing sunken eyes and wrinkled hands and skin.

Cholera

A person with severe dehydration due to cholera, causing sunken eyes and wrinkled hands and skin.
Typical cholera diarrhea that looks like "rice water"
Scanning electron microscope image of Vibrio cholerae
Vibrio cholerae, the bacterium that causes cholera
The role of biofilm in the intestinal colonization of Vibrio cholerae
Preventive inoculation against cholera in 1966
Euvichol-plus oral vaccine for cholera
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Cholera patient being treated by oral rehydration therapy in 1992
Map of the 2008–2009 cholera outbreak in sub-Saharan Africa showing the statistics as of 12 February 2009
Robert Koch (third from the right) on a cholera research expedition in Egypt in 1884, one year after he identified V. cholerae
How to avoid the cholera leaflet; Aberystwyth; August 1849
Prof. Sambhu Nath De, who discovered the cholera toxin and successfully demonstrated the transmission of cholera pathogen by bacterial enteric toxin
Dumping of sewage or fecal sludge from a UN camp into a lake in the surroundings of Port-au-Prince is thought to have contributed to the spread of cholera after the Haiti earthquake in 2010, killing thousands.
Example of a urine-diverting dry toilet in a cholera-affected area in Haiti. This type of toilet stops transmission of disease via the fecal-oral route due to water pollution.
Cholera hospital in Dhaka, showing typical "cholera beds".
Drawing of Death bringing cholera, in Le Petit Journal (1912).
Emperor Pedro II of Brazil visiting people with cholera in 1855.
Hand bill from the New York City Board of Health, 1832—the outdated public health advice demonstrates the lack of understanding of the disease and its actual causative factors.

Cholera is an infection of the small intestine by some strains of the bacterium Vibrio cholerae.

Rash on the chest and abdomen caused by leprosy

Leprosy

Rash on the chest and abdomen caused by leprosy
M. leprae, one of the causative agents of leprosy: As an acid-fast bacterium, M. leprae appears red when a Ziehl–Neelsen stain is used.
Testing for loss of sensation with monofilament
MDT antileprosy drugs: standard regimens from 2010
New cases of leprosy in 2016.
G. H. A. Hansen, discoverer of M. leprae
Distribution of leprosy around the world in 1891
Two lepers denied entrance to town, 14th century
Medieval leper bell
A 24-year-old man with leprosy (1886)
Father Damien on his deathbed in 1889
Paucibacillary leprosy (PB): Pale skin patch with loss of sensation
Skin lesions on the thigh of a person with leprosy
Hands deformed by leprosy
Leprosy in Tahiti, c. 1895
A 26-year-old woman with leprous lesions
A 13-year-old boy with severe leprosy

Leprosy, also known as Hansen's disease (HD), is a long-term infection by the bacteria Mycobacterium leprae or Mycobacterium lepromatosis.