Bacteroides is a genus
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...

 of Gram-negative
Gram-negative bacteria are bacteria that do not retain crystal violet dye in the Gram staining protocol. In a Gram stain test, a counterstain is added after the crystal violet, coloring all Gram-negative bacteria with a red or pink color...

, bacillus
Bacillus (shape)
The word bacillus may be used to describe any rod-shaped bacterium, and such bacilli are found in many different taxonomic groups of bacteria. However, the name Bacillus, capitalized and italicized, refers to a specific genus of 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...

. Bacteroides species are non-endospore
An endospore is a dormant, tough, and temporarily non-reproductive structure produced by certain bacteria from the Firmicute phylum. The name "endospore" is suggestive of a spore or seed-like form , but it is not a true spore . It is a stripped-down, dormant form to which the bacterium can reduce...

-forming, anaerobes, and may be either motile or non-motile, depending on the species. The DNA base composition is 40-48% GC. Unusual in bacterial organisms, Bacteroides membranes contain sphingolipid
Sphingolipids are a class of lipids containing a backbone of sphingoid bases, a set of aliphatic amino alcohols that includes sphingosine. They were discovered in brain extracts in the 1870s and were named for the mythological Sphinx because of their enigmatic nature. These compounds play...

s. They also contain meso-diaminopimelic acid in their peptidoglycan layer
Peptidoglycan, also known as murein, is a polymer consisting of sugars and amino acids that forms a mesh-like layer outside the plasma membrane of bacteria , forming the cell wall. The sugar component consists of alternating residues of β- linked N-acetylglucosamine and N-acetylmuramic acid...


Bacteroides are normally mutualistic, making up the most substantial portion of the mammalian gastrointestinal 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....

, where they play a fundamental role in processing of complex molecules to simpler ones in the host intestine. As many as 1010-1011 cells per gram of human feces have been reported. They can use simple sugars
Monosaccharides are the most basic units of biologically important carbohydrates. They are the simplest form of sugar and are usually colorless, water-soluble, crystalline solids. Some monosaccharides have a sweet taste. Examples of monosaccharides include glucose , fructose , galactose, xylose...

 when available; however, the main sources of energy for Bacteroides species in the gut are complex host-derived and plant glycans .

One of the most important clinically is Bacteroides fragilis
Bacteroides fragilis
Bacteroides fragilis is a Gram-negative bacillus bacterium species, and an obligate anaerobe of the gut.B. fragilis group is the most commonly isolated bacteriodaceae in anaerobic infections especially those that originate from the gastrointestinal flora. B. fragilis is the most prevalent organism...


Bacteroides melaninogenicus
Bacteroides melaninogenicus
Bacteroides melaninogenicus is a normal flora found in the upper respiratory tract. it is an important human pathogen in various anaerobic infections often mixed with other aerobic and anaerobic bacteria....

has recently been reclassified and split into Prevotella melaninogenica and Prevotella intermedia
Prevotella intermedia
Prevotella intermedia is a gram-negative obligate anaerobic pathogen involved in periodontal infections, including gingivitis and periodontitis, and often found in Acute necrotizing ulcerative gingivitis . It is commonly isolated from Dentoalveolar abscesses, where obligate anaerobes...



Bacteroides species also benefit their host by excluding potential pathogens from colonizing the gut. Some species (B. fragilis, for example) are opportunistic human pathogens, causing infections of the peritoneal cavity, gastrointestinal surgery, and appendicitis
Appendicitis is a condition characterized by inflammation of the appendix. It is classified as a medical emergency and many cases require removal of the inflamed appendix, either by laparotomy or laparoscopy. Untreated, mortality is high, mainly because of the risk of rupture leading to...

 via abscess formation, inhibiting phagocytosis, and inactivating beta-lactam antibiotics. Although Bacteroides species are anaerobic, they are aerotolerant and thus can survive in the abdominal cavity.

In general, Bacteroides are resistant to a wide variety of antibiotic
An antibacterial is a compound or substance that kills or slows down the growth of bacteria.The term is often used synonymously with the term antibiotic; today, however, with increased knowledge of the causative agents of various infectious diseases, antibiotic has come to denote a broader range of...

s — β-lactams, aminoglycosides, and recently many species have acquired resistance to erythromycin
Erythromycin is a macrolide antibiotic that has an antimicrobial spectrum similar to or slightly wider than that of penicillin, and is often used for people who have an allergy to penicillins. For respiratory tract infections, it has better coverage of atypical organisms, including mycoplasma and...

 and tetracycline. This high level of antibiotic resistance has prompted concerns that Bacteroides species may become a reservoir for resistance in other, more highly-pathogenic bacterial strains.

Microbiological Applications

An alternative fecal indicator organism, Bacteroides, has been suggested because they make up a significant portion of the fecal bacterial population, have a high degree of host specificity that reflects differences in the digestive system of the host animal, and have a small potential to grow in the environment. Over the past decade, real-time polymerase chain reaction (PCR) methods have been utilized to detect the presence of various microbial pathogens through the amplification of specific DNA sequences without culturing bacteria. One study has measured the amount of Bacteroides by using qPCR to quantify the 16S rRNA genetic marker that is host-specific. This technique allows quantification of genetic markers that are specific to the host of the bacteria and allow detection of recent contamination. A recent report found that temperature plays a major role in the amount of time the bacteria will persist in the environment, the life span increases with colder temperatures (0-4C)

External links

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