Bacteria in the human body
Encyclopedia
The human microbiome is the assemblage of microorganism
s that reside on the surface and in deep layers of skin, in the saliva and oral mucosa, in the conjunctiva
, and in the gastrointestinal tracts. They include bacteria
, fungi, and archaea
. Some of these organisms perform tasks that are useful for the human host. However, the majority have no known beneficial or harmful effect. Those that are expected to be present, and that under normal circumstances do not cause disease, but instead participate in maintaining health, are deemed members of the normal flora.
Though widely known as "microflora", this is, in technical terms, a misnomer, since the word root "flora" pertains to plants, and biota refers to the total collection of organisms in a particular ecosystem. Recently, the more appropriate term "microbiota" is applied, though its use has not eclipsed the entrenched use and recognition of "flora" with regard to bacteria and other microorganisms. Both terms are being used in different literature. Studies in 2009 questioned whether the decline in biota
(including microfauna
) as a result of human intervention might impede human health
and yeasts
) inhabit the skin and mucosa. Their role forms part of normal, healthy human physiology, however if microbe numbers grow beyond their typical ranges (often due to a compromised immune system) or if microbes populate atypical areas of the body (such as through poor hygiene or injury), disease can result.
It is estimated that 500 to 1000 species of bacteria live in the human gut and a roughly similar number on the skin. Bacterial cells are much smaller than human cells
, and there are at least ten times as many bacteria as human cells in the body (approximately 1014
versus 1013). Though members of the flora are found on all surfaces exposed to the environment (on the skin and eyes, in the mouth, nose, small intestine
), the vast majority of bacteria live in the large intestine
.
Many of the bacteria in the digestive tract, collectively referred to as the gut flora
, are able to break down certain nutrients such as carbohydrate
s that humans otherwise could not digest
. The majority of these commensal bacteria are anaerobes, meaning they survive in an environment with no oxygen
. Normal flora bacteria can act as opportunistic pathogens at times of lowered immunity
.
Escherichia coli
(a.k.a. E. coli) is a bacterium that lives in the colon; it is an extensively studied model organism
and probably the best-understood bacterium of all. Certain mutated strains of these gut bacteria do cause disease; an example is E. coli O157:H7.
A number of types of bacteria, such as Actinomyces
viscosus and A. naeslundii, live in the mouth, where they are part of a sticky substance called plaque
. If this is not removed by brushing, it hardens into calculus
(also called tartar). The same bacteria also secrete acids that dissolve tooth enamel
, causing tooth decay.
The vaginal microflora
consist mostly of various lactobacillus
species. It was long thought that the most common of these species was Lactobacillus acidophilus
, but it has later been shown that the most common one is L. iners followed by L. crispatus. Other lactobacilli found in the vagina are L. delbruekii and L. gasseri. Disturbance of the vaginal flora can lead to bacterial vaginosis
.
are present in the human gut, but, in contrast to the enormous variety of bacteria
in this organ, the numbers of archaeal species are much more limited. The dominant group are the methanogen
s, particularly Methanobrevibacter smithii
and Methanosphaera stadtmanae
. However, colonization by methanogens is variable, and only about 50% of humans have easily-detectable populations of these organisms.
As of 2007, no clear examples of archaeal pathogen
s are known, although a relationship has been proposed between the presence of some methanogens and human periodontal disease
.
s, are present in the human gut. The best-studied of these are Candida
species. This is because of their ability to become pathogenic in immunocompromised hosts
. Yeasts are also present on the skin, particularly Malassezia species, where they consume oils secreted from the sebaceous gland
s.
(51.8%), Firmicutes
(24.4%), Proteobacteria
(16.5%), and Bacteroidetes
(6.3%).
A small number of bacteria are normally present in the conjunctiva
. Staphylococcus epidermidis
and certain coryneforms such as Propionibacterium acnes
are dominant. Staphylococcus aureus
, streptococci, Haemophilus
sp. and Neisseria
sp. sometimes occur. The lachrymal glands continuously secrete, keeping the conjunctiva moist, while intermittent blinking lubricates the conjunctiva and washes away foreign material. Tears contain bactericides such as lysozyme
, so that microorganisms have difficulty in surviving the lysozyme and settling on the epithelial surfaces.
Some pathogens able to infect the conjunctiva, such as Neisseria gonorrhoeae
and Chlamydia trachomatis
are thought to have special processes allowing them to attach to the conjunctival epithelium. Newborn infants are particularly prone to bacterial attachment. Chlamydia and Neisseria may be present in an infected mother and show up on the cervical and vaginal epithelium - in such cases the newborn's eyes may be treated with silver nitrate
or antibiotics.
The gut flora is the human flora of microorganisms that normally live in the digestive tract and can perform a number of useful functions for their hosts. The average human body, consisting of about 1013 (10,000,000,000,000 or about ten trillion) cells, has about ten times that number of microorganisms in the gut. The metabolic activity performed by these bacteria is equal to that of a virtual organ, leading to gut bacteria being termed a "forgotten" organ.
Bacteria
make up most of the flora in the colon
and 60% of the dry mass of feces
. This fact makes feces an ideal source to test for gut flora for any tests and experiments by extracting the nucleic acid from fecal specimens, and bacterial 16S rRNA gene sequences are generated with bacterial primers. This form of testing is also often preferable to more invasive techniques, such as biopsies. Somewhere between 300 and 1000 different species
live in the gut, with most estimates at about 500. However, it is probable that 99% of the bacteria come from about 30 or 40 species. Fungi and protozoa
also make up a part of the gut flora, but little is known about their activities.
Research suggests that the relationship between gut flora
and humans is not merely commensal
(a non-harmful coexistence), but rather is a mutualistic, symbiotic relationship. Though people can survive with no gut flora, the microorganisms perform a host of useful functions, such as fermenting
unused energy substrates, training the immune system
, preventing growth of harmful species, regulating the development of the gut, producing vitamins for the host (such as biotin
and vitamin K
), and producing hormones to direct the host to store fats. However, in certain conditions, some species are thought to be capable of causing disease
by causing infection
or increasing cancer
risk for the host.
Some bacteria found in the large intestine of humans:
Microorganism
A microorganism or microbe is a microscopic organism that comprises either a single cell , cell clusters, or no cell at all...
s that reside on the surface and in deep layers of skin, in the saliva and oral mucosa, in the conjunctiva
Conjunctiva
The conjunctiva covers the sclera and lines the inside of the eyelids. It is composed of rare stratified columnar epithelium.-Function:...
, and in the gastrointestinal tracts. They include bacteria
Bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...
, fungi, and archaea
Archaea
The Archaea are a group of single-celled microorganisms. A single individual or species from this domain is called an archaeon...
. Some of these organisms perform tasks that are useful for the human host. However, the majority have no known beneficial or harmful effect. Those that are expected to be present, and that under normal circumstances do not cause disease, but instead participate in maintaining health, are deemed members of the normal flora.
Though widely known as "microflora", this is, in technical terms, a misnomer, since the word root "flora" pertains to plants, and biota refers to the total collection of organisms in a particular ecosystem. Recently, the more appropriate term "microbiota" is applied, though its use has not eclipsed the entrenched use and recognition of "flora" with regard to bacteria and other microorganisms. Both terms are being used in different literature. Studies in 2009 questioned whether the decline in biota
Biota (ecology)
Biota are the total collection of organisms of a geographic region or a time period, from local geographic scales and instantaneous temporal scales all the way up to whole-planet and whole-timescale spatiotemporal scales. The biota of the Earth lives in the biosphere.-See...
(including microfauna
Microfauna
Microfauna refers to microscopic organisms that exhibit animal-like qualities. Microfauna are represented in the animal kingdom and the protist kingdom...
) as a result of human intervention might impede human health
Bacteria
Populations of microbes (such as bacteriaBacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...
and yeasts
Yeast
Yeasts are eukaryotic micro-organisms classified in the kingdom Fungi, with 1,500 species currently described estimated to be only 1% of all fungal species. Most reproduce asexually by mitosis, and many do so by an asymmetric division process called budding...
) inhabit the skin and mucosa. Their role forms part of normal, healthy human physiology, however if microbe numbers grow beyond their typical ranges (often due to a compromised immune system) or if microbes populate atypical areas of the body (such as through poor hygiene or injury), disease can result.
It is estimated that 500 to 1000 species of bacteria live in the human gut and a roughly similar number on the skin. Bacterial cells are much smaller than human cells
Cell (biology)
The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....
, and there are at least ten times as many bacteria as human cells in the body (approximately 1014
Orders of magnitude (numbers)
This list contains selected positive numbers in increasing order, including counts of things, dimensionless quantity and probabilities. Each number is given a name in the short scale, which is used in English speaking countries, as well as a name in the long scale, which is used in some of the...
versus 1013). Though members of the flora are found on all surfaces exposed to the environment (on the skin and eyes, in the mouth, nose, small intestine
Small intestine
The small intestine is the part of the gastrointestinal tract following the stomach and followed by the large intestine, and is where much of the digestion and absorption of food takes place. In invertebrates such as worms, the terms "gastrointestinal tract" and "large intestine" are often used to...
), the vast majority of bacteria live in the large intestine
Large intestine
The large intestine is the third-to-last part of the digestive system — — in vertebrate animals. Its function is to absorb water from the remaining indigestible food matter, and then to pass useless waste material from the body...
.
Many of the bacteria in the digestive tract, collectively referred to as the gut flora
Gut flora
Gut flora consists of microorganisms that live in the digestive tracts of animals and is the largest reservoir of human flora. In this context, gut is synonymous with intestinal, and flora with microbiota and microflora....
, are able to break down certain nutrients such as carbohydrate
Carbohydrate
A carbohydrate is an organic compound with the empirical formula ; that is, consists only of carbon, hydrogen, and oxygen, with a hydrogen:oxygen atom ratio of 2:1 . However, there are exceptions to this. One common example would be deoxyribose, a component of DNA, which has the empirical...
s that humans otherwise could not digest
Digestion
Digestion is the mechanical and chemical breakdown of food into smaller components that are more easily absorbed into a blood stream, for instance. Digestion is a form of catabolism: a breakdown of large food molecules to smaller ones....
. The majority of these commensal bacteria are anaerobes, meaning they survive in an environment with no oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
. Normal flora bacteria can act as opportunistic pathogens at times of lowered immunity
Immune system
An immune system is a system of biological structures and processes within an organism that protects against disease by identifying and killing pathogens and tumor cells. It detects a wide variety of agents, from viruses to parasitic worms, and needs to distinguish them from the organism's own...
.
Escherichia coli
Escherichia coli
Escherichia coli is a Gram-negative, rod-shaped bacterium that is commonly found in the lower intestine of warm-blooded organisms . Most E. coli strains are harmless, but some serotypes can cause serious food poisoning in humans, and are occasionally responsible for product recalls...
(a.k.a. E. coli) is a bacterium that lives in the colon; it is an extensively studied model organism
Model organism
A model organism is a non-human species that is extensively studied to understand particular biological phenomena, with the expectation that discoveries made in the organism model will provide insight into the workings of other organisms. Model organisms are in vivo models and are widely used to...
and probably the best-understood bacterium of all. Certain mutated strains of these gut bacteria do cause disease; an example is E. coli O157:H7.
A number of types of bacteria, such as Actinomyces
Actinomyces
Actinomyces from Greek "actino" that means mucus and fungus, is a genus of the actinobacteria class of bacteria. They are all Gram-positive and are characterized by contiguous spread, suppurative and granulomatous inflammation, and formation of multiple abscesses and sinus tracts that may...
viscosus and A. naeslundii, live in the mouth, where they are part of a sticky substance called plaque
Dental plaque
Dental plaque is a biofilm, usually a pale yellow, that develops naturally on the teeth. Like any biofilm, dental plaque is formed by colonizing bacteria trying to attach themselves to a smooth surface...
. If this is not removed by brushing, it hardens into calculus
Calculus (dental)
In dentistry, calculus or tartar is a form of hardened dental plaque. It is caused by the continual accumulation of minerals from saliva on plaque on the teeth...
(also called tartar). The same bacteria also secrete acids that dissolve tooth enamel
Tooth enamel
Tooth enamel, along with dentin, cementum, and dental pulp is one of the four major tissues that make up the tooth in vertebrates. It is the hardest and most highly mineralized substance in the human body. Tooth enamel is also found in the dermal denticles of sharks...
, causing tooth decay.
The vaginal microflora
Vaginal flora
The bacteria that colonize the vagina, collectively referred to as the vaginal microbiome, were discovered by the German gynecologist Albert Döderlein in 1892. The amount and type of bacteria present have significant implications for a woman's overall health...
consist mostly of various lactobacillus
Lactobacillus
Lactobacillus is a genus of Gram-positive facultative anaerobic or microaerophilic rod-shaped bacteria. They are a major part of the lactic acid bacteria group, named as such because most of its members convert lactose and other sugars to lactic acid. They are common and usually benign...
species. It was long thought that the most common of these species was Lactobacillus acidophilus
Lactobacillus acidophilus
Lactobacillus acidophilus is a species in the genus Lactobacillus. L. acidophilus is a homofermentative species, fermenting sugars into lactic acid, which grows readily at rather low pH values and has an optimum growth temperature of 37 °C . L...
, but it has later been shown that the most common one is L. iners followed by L. crispatus. Other lactobacilli found in the vagina are L. delbruekii and L. gasseri. Disturbance of the vaginal flora can lead to bacterial vaginosis
Bacterial vaginosis
Bacterial vaginosis or less commonly vaginal bacteriosis is a disease of the vagina caused by bacteria. The U.S. Centers for Disease Control and Prevention states it is not clear what role sexual activity plays in the development. However, it is known that BV is associated with having a new sex...
.
Archaea
ArchaeaArchaea
The Archaea are a group of single-celled microorganisms. A single individual or species from this domain is called an archaeon...
are present in the human gut, but, in contrast to the enormous variety of bacteria
Bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...
in this organ, the numbers of archaeal species are much more limited. The dominant group are the methanogen
Methanogen
Methanogens are microorganisms that produce methane as a metabolic byproduct in anoxic conditions. They are classified as archaea, a group quite distinct from bacteria...
s, particularly Methanobrevibacter smithii
Methanobrevibacter
In taxonomy, Methanobrevibacter is a genus of the Methanobacteriaceae.-External links:...
and Methanosphaera stadtmanae
Methanosphaera
In taxonomy, Methanosphaera is a genus of the Methanobacteriaceae.-External links:...
. However, colonization by methanogens is variable, and only about 50% of humans have easily-detectable populations of these organisms.
As of 2007, no clear examples of archaeal pathogen
Pathogen
A pathogen gignomai "I give birth to") or infectious agent — colloquially, a germ — is a microbe or microorganism such as a virus, bacterium, prion, or fungus that causes disease in its animal or plant host...
s are known, although a relationship has been proposed between the presence of some methanogens and human periodontal disease
Periodontal disease
Periodontitis is a set of inflammatory diseases affecting the periodontium, i.e., the tissues that surround and support the teeth. Periodontitis involves progressive loss of the alveolar bone around the teeth, and if left untreated, can lead to the loosening and subsequent loss of teeth...
.
Fungal flora
Fungi, in particular yeastYeast
Yeasts are eukaryotic micro-organisms classified in the kingdom Fungi, with 1,500 species currently described estimated to be only 1% of all fungal species. Most reproduce asexually by mitosis, and many do so by an asymmetric division process called budding...
s, are present in the human gut. The best-studied of these are Candida
Candida (genus)
Candida is a genus of yeasts. Many species are harmless commensals or endosymbionts of animal hosts including humans, but other species, or harmless species in the wrong location, can cause disease. Candida albicans can cause infections in humans and other animals, especially in immunocompromised...
species. This is because of their ability to become pathogenic in immunocompromised hosts
Immunodeficiency
Immunodeficiency is a state in which the immune system's ability to fight infectious disease is compromised or entirely absent. Immunodeficiency may also decrease cancer immunosurveillance. Most cases of immunodeficiency are acquired but some people are born with defects in their immune system,...
. Yeasts are also present on the skin, particularly Malassezia species, where they consume oils secreted from the sebaceous gland
Sebaceous gland
The sebaceous glands are microscopic glands in the skin that secrete an oily/waxy matter, called sebum, to lubricate and waterproof the skin and hair of mammals...
s.
Skin flora
A study of 20 skin sites on each of 10 healthy humans found 205 identified genera in nineteen bacterial phyla, with most sequences assigned to four phyla: ActinobacteriaActinobacteria
Actinobacteria are a group of Gram-positive bacteria with high guanine and cytosine content. They can be terrestrial or aquatic. Actinobacteria is one of the dominant phyla of the bacteria....
(51.8%), Firmicutes
Firmicutes
The Firmicutes are a phylum of bacteria, most of which have Gram-positive cell wall structure. A few, however, such as Megasphaera, Pectinatus, Selenomonas and Zymophilus, have a porous pseudo-outer-membrane that causes them to stain Gram-negative...
(24.4%), Proteobacteria
Proteobacteria
The Proteobacteria are a major group of bacteria. They include a wide variety of pathogens, such as Escherichia, Salmonella, Vibrio, Helicobacter, and many other notable genera....
(16.5%), and Bacteroidetes
Bacteroidetes
The phylum Bacteroidetes is composed of three large classes of bacteria that are widely distributed in the environment, including in soil, in sediments, sea water and in the guts and on the skin of animals....
(6.3%).
Conjunctival flora
These are sparse in occurrence, but Gram-positive cocci and Gram-negative rods are present.A small number of bacteria are normally present in the conjunctiva
Conjunctiva
The conjunctiva covers the sclera and lines the inside of the eyelids. It is composed of rare stratified columnar epithelium.-Function:...
. Staphylococcus epidermidis
Staphylococcus epidermidis
Staphylococcus epidermidis is one of thirty-three known species belonging to the genus Staphylococcus. It is part of human skin flora, and consequently part of human flora. It can also be found in the mucous membranes and in animals. Due to contamination, it is probably the most common species...
and certain coryneforms such as Propionibacterium acnes
Propionibacterium acnes
Propionibacterium acnes is a relatively slow growing, typically aerotolerant anaerobic gram positive bacterium that is linked to the skin condition acne; it can also cause chronic blepharitis and endophthalmitis, the latter particularly following intraocular surgery...
are dominant. Staphylococcus aureus
Staphylococcus aureus
Staphylococcus aureus is a facultative anaerobic Gram-positive coccal bacterium. It is frequently found as part of the normal skin flora on the skin and nasal passages. It is estimated that 20% of the human population are long-term carriers of S. aureus. S. aureus is the most common species of...
, streptococci, Haemophilus
Haemophilus
Not to be confused with Haemophilia.Haemophilus is a genus of Gram-negative, pleomorphic, coccobacilli bacteria belonging to the Pasteurellaceae family. While Haemophilus bacteria are typically small coccobacilli, they are categorized as pleomorphic bacteria because of the wide range of shapes they...
sp. and Neisseria
Neisseria
The Neisseria is a large genus of commensal bacteria that colonize the mucosal surfaces of many animals. Of the 11 species that colonize humans, only two are pathogens. N. meningitidis and N. gonorrhoeae often cause asymptomatic infections, a commensal-like behavior...
sp. sometimes occur. The lachrymal glands continuously secrete, keeping the conjunctiva moist, while intermittent blinking lubricates the conjunctiva and washes away foreign material. Tears contain bactericides such as lysozyme
Lysozyme
Lysozyme, also known as muramidase or N-acetylmuramide glycanhydrolase, are glycoside hydrolases, enzymes that damage bacterial cell walls by catalyzing hydrolysis of 1,4-beta-linkages between N-acetylmuramic acid and N-acetyl-D-glucosamine residues in a peptidoglycan and between...
, so that microorganisms have difficulty in surviving the lysozyme and settling on the epithelial surfaces.
Some pathogens able to infect the conjunctiva, such as Neisseria gonorrhoeae
Neisseria gonorrhoeae
Neisseria gonorrhoeae, also known as gonococci , or gonococcus , is a species of Gram-negative coffee bean-shaped diplococci bacteria responsible for the sexually transmitted infection gonorrhea.N...
and Chlamydia trachomatis
Chlamydia trachomatis
Chlamydia trachomatis, an obligate intracellular human pathogen, is one of three bacterial species in the genus Chlamydia. C. trachomatis is a Gram-negative bacteria, therefore its cell wall components retain the counter-stain safranin and appear pink under a light microscope.The inclusion bodies...
are thought to have special processes allowing them to attach to the conjunctival epithelium. Newborn infants are particularly prone to bacterial attachment. Chlamydia and Neisseria may be present in an infected mother and show up on the cervical and vaginal epithelium - in such cases the newborn's eyes may be treated with silver nitrate
Silver nitrate
Silver nitrate is an inorganic compound with chemical formula . This compound is a versatile precursor to many other silver compounds, such as those used in photography. It is far less sensitive to light than the halides...
or antibiotics.
The gut flora is the human flora of microorganisms that normally live in the digestive tract and can perform a number of useful functions for their hosts. The average human body, consisting of about 1013 (10,000,000,000,000 or about ten trillion) cells, has about ten times that number of microorganisms in the gut. The metabolic activity performed by these bacteria is equal to that of a virtual organ, leading to gut bacteria being termed a "forgotten" organ.
Bacteria
Bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...
make up most of the flora in the colon
Colon (anatomy)
The colon is the last part of the digestive system in most vertebrates; it extracts water and salt from solid wastes before they are eliminated from the body, and is the site in which flora-aided fermentation of unabsorbed material occurs. Unlike the small intestine, the colon does not play a...
and 60% of the dry mass of feces
Feces
Feces, faeces, or fæces is a waste product from an animal's digestive tract expelled through the anus or cloaca during defecation.-Etymology:...
. This fact makes feces an ideal source to test for gut flora for any tests and experiments by extracting the nucleic acid from fecal specimens, and bacterial 16S rRNA gene sequences are generated with bacterial primers. This form of testing is also often preferable to more invasive techniques, such as biopsies. Somewhere between 300 and 1000 different species
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...
live in the gut, with most estimates at about 500. However, it is probable that 99% of the bacteria come from about 30 or 40 species. Fungi and protozoa
Protozoa
Protozoa are a diverse group of single-cells eukaryotic organisms, many of which are motile. Throughout history, protozoa have been defined as single-cell protists with animal-like behavior, e.g., movement...
also make up a part of the gut flora, but little is known about their activities.
Research suggests that the relationship between gut flora
Flora (microbiology)
In microbiology, flora refers to the collective bacteria and other microorganisms in an ecosystem . While the term microflora is widely used, it is technically a misnomer since the word root flora pertains to the kingdom plantae...
and humans is not merely commensal
Commensalism
In ecology, commensalism is a class of relationship between two organisms where one organism benefits but the other is neutral...
(a non-harmful coexistence), but rather is a mutualistic, symbiotic relationship. Though people can survive with no gut flora, the microorganisms perform a host of useful functions, such as fermenting
Fermentation (biochemistry)
Fermentation is the process of extracting energy from the oxidation of organic compounds, such as carbohydrates, using an endogenous electron acceptor, which is usually an organic compound. In contrast, respiration is where electrons are donated to an exogenous electron acceptor, such as oxygen,...
unused energy substrates, training the immune system
Immune system
An immune system is a system of biological structures and processes within an organism that protects against disease by identifying and killing pathogens and tumor cells. It detects a wide variety of agents, from viruses to parasitic worms, and needs to distinguish them from the organism's own...
, preventing growth of harmful species, regulating the development of the gut, producing vitamins for the host (such as biotin
Biotin
Biotin, also known as Vitamin H or Coenzyme R, is a water-soluble B-complex vitamin discovered by Bateman in 1916. It is composed of a ureido ring fused with a tetrahydrothiophene ring. A valeric acid substituent is attached to one of the carbon atoms of the tetrahydrothiophene ring...
and vitamin K
Vitamin K
Vitamin K is a group of structurally similar, fat soluble vitamins that are needed for the posttranslational modification of certain proteins required for blood coagulation and in metabolic pathways in bone and other tissue. They are 2-methyl-1,4-naphthoquinone derivatives...
), and producing hormones to direct the host to store fats. However, in certain conditions, some species are thought to be capable of causing disease
Disease
A disease is an abnormal condition affecting the body of an organism. It is often construed to be a medical condition associated with specific symptoms and signs. It may be caused by external factors, such as infectious disease, or it may be caused by internal dysfunctions, such as autoimmune...
by causing infection
Infection
An infection is the colonization of a host organism by parasite species. Infecting parasites seek to use the host's resources to reproduce, often resulting in disease...
or increasing cancer
Cancer
Cancer , known medically as a malignant neoplasm, is a large group of different diseases, all involving unregulated cell growth. In cancer, cells divide and grow uncontrollably, forming malignant tumors, and invade nearby parts of the body. The cancer may also spread to more distant parts of the...
risk for the host.
Some bacteria found in the large intestine of humans:
| Bacterium | Range of Incidence |
|---|---|
| Bacteroides fragilis | 100 |
| Bacteroides melaninogenicus | 100 |
| Bacteroides oralis | 100 |
| Lactobacillus | 20-60 |
| Clostridium perfringens | 25-35 |
| Clostridium septicum | 5-25 |
| Clostridium tetani | 1-35 |
| Bifidobacterium bifidum | 30-70 |
| Staphylococcus aureus | 30-50 |
| Enterococcus faecalis | 100 |
| Escherichia coli | 100 |
| Salmonella enteritidis | 3-7 |
| Klebsiella sp. | 40-80 |
| Enterobacter sp. | 40-80 |
| Proteus mirabilis | 5-55 |
| Pseudomonas aeruginosa | 3-11 |
| Peptostreptococcus sp. | ?common |
| Peptococcus sp. | ?common |