''Bacteroides fragilis'' is an

anaerobic Anaerobic means "living, active, occurring, or existing in the absence of free oxygen", as opposed to aerobic which means "living, active, or occurring only in the presence of oxygen." Anaerobic may also refer to: * Anaerobic adhesive, a bonding a ...

Gram-negative Gram-negative bacteria are bacteria that do not retain the crystal violet stain used in the Gram staining method of bacterial differentiation. They are characterized by their cell envelopes, which are composed of a thin peptidoglycan cell wall ...

, pleomorphic to

rod-shaped A bacillus (), also called a bacilliform bacterium or often just a rod (when the context makes the sense clear), is a rod-shaped bacterium or archaeon. Bacilli are found in many different taxonomic groups of bacteria. However, the name ''Bacillu ...

bacterium Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria were among ...

. It is part of the normal microbiota of the human colon and is generally

commensal Commensalism is a long-term biological interaction (symbiosis) in which members of one species gain benefits while those of the other species neither benefit nor are harmed. This is in contrast with mutualism, in which both organisms benefit fro ...

, but can cause infection if displaced into the bloodstream or surrounding tissue following surgery, disease, or trauma.

Habitat

''Bacteroides fragilis'' resides in the human gastrointestinal tract and is essential to healthy gastrointestinal function such as

mucosal immunity Mucosal immunology is the study of immune system responses that occur at mucosal membranes of the intestines, the urogenital tract, and the respiratory system. The mucous membranes are in constant contact with microorganisms, food, and inhaled a ...

and host nutrition. As a mesophile, optimal growth occurs at 37 °C and a pH around 7.

Morphology

Cells of ''B. fragilis'' are rod-shaped to pleomorphic with a cell size range of 0.5-1.5 x 1.0-6.0 μm.''B. fragilis'' is a Gram-negative bacterium and does not possess flagella or cilia making it immotile. However, it does utilize peritrichous fimbriae for adhesion to other molecular structures. ''B. fragilis'' also utilizes a complex series of surface proteins, lipopolysaccharide chains, and outer membrane vesicles to help survive the volatile intestinal micro-environment.

Metabolism & mutualism in the gut microbiome

''B. fragilis'' is an aerotolerant, anaerobic chemoorganotroph capable of fermenting a wide variety of glycans available in the human gut microenvironment including glucose, sucrose, & fructose. ''B. fragilis'' can also catabolize a variety of biopolymers, polysaccharides, and glycoproteins into smaller molecules which can then be used and further broken down by other microbes. Fatty acids produced by the fermentation of carbohydrates can serve as a source of energy for the host. Cytochrome ''bd'' oxidase is essential for oxygen consumption in ''B. fragilis'' and can allow other obligate anaerobes to survive in the now oxygen-reduced microenvironment. Animals lacking gut bacteria require 30% more caloric intake to maintain body mass.

Environment-sensing systems

The complex environmental-sensory system allows ''B. fragilis'' to survive/adapt in the ever-changing human gut microbiome. This system is composed of/designed to effectively handle:

Bacteriocin Bacteriocins are proteinaceous or peptidic toxins produced by bacteria to inhibit the growth of similar or closely related bacterial strain(s). They are similar to yeast and paramecium killing factors, and are structurally, functionally, and ec ...

s: ''B. fragilis'' intestinal isolates secrete high levels of bacteriocin proteins and are resistant to other bacteriocins secreted by other closely related isolates. This mechanism is believed to reduce the level of intra-specific competition.

Bile salt Bile acids are steroid acids found predominantly in the bile of mammals and other vertebrates. Diverse bile acids are synthesized in the liver. Bile acids are conjugated with taurine or glycine residues to give anions called bile salts. Prima ...

resistance: Utilizes enzymes such as

bile salt hydrolase Bile salt hydrolases (BSH) are microbial enzymes that deconjugate primary bile acids. They catalyze the first step of bile acid metabolism and maintain the bile acid pool for further modification by the microbiota. BSH enzymes play a role in a ra ...

to resist the degrading effects of bile salts. Detergent activity of bile salts can permeabilize bacterial membranes which can eventually lead to membrane collapse and/or cell damage.

Oxidative Stress Oxidative stress reflects an imbalance between the systemic manifestation of reactive oxygen species and a biological system's ability to readily Detoxification, detoxify the reactive intermediates or to repair the resulting damage. Disturbances ...

Response: Proteins such as catalase, superoxide dismutase, & alkyl hydroperoxide reductase protect the organism from harmful

oxygen radicals In chemistry, a radical, also known as a free radical, is an atom, molecule, or ion that has at least one unpaired valence electron. With some exceptions, these unpaired electrons make radicals highly chemically reactive. Many radicals spont ...

. This permits growth in the presence of nanomolar concentrations of O2.

Antibiotic resistance

Member of the genus ''Bacteroides'' are characterized with having the highest numbers of antibiotic resistance mechanisms accompanied by the highest resistance rates amongst anaerobic bacteria. Species of the Bacteroidaceae have displayed increasing resistance to antimicrobial agents such as

cefoxitin Cefoxitin is a second-generation cephamycin antibiotic developed by Merck & Co., Inc. from Cephamycin C in the year following its discovery, 1972. It was synthesized in order to create an antibiotic with a broader spectrum. It is often grouped wi ...

clindamycin Clindamycin is an antibiotic medication used for the treatment of a number of bacterial infections, including osteomyelitis (bone) or joint infections, pelvic inflammatory disease, strep throat, pneumonia, acute otitis media (middle ear infec ...

metronidazole Metronidazole, sold under the brand name Flagyl among others, is an antibiotic and antiprotozoal medication. It is used either alone or with other antibiotics to treat pelvic inflammatory disease, endocarditis, and bacterial vaginosis. It is ef ...

carbapenem Carbapenems are a class of very effective antibiotic agents most commonly used for the treatment of severe bacterial infections. This class of antibiotics is usually reserved for known or suspected multidrug-resistant (MDR) bacterial infections. ...

s, & fluoroquinalones. Resistance Reservoirs: ''Bacteroides'' species accumulate a variety of antibiotic/antimicrobial resistance genes as they reside in the gastrointestinal tract. This allows the genetic transfer of these genes to other ''Bacteroides'' species and possibly other more virulent bacteria leading to an overall increase in multi-drug resistance. This is exacerbated by the tendency of resistance genes to be relatively stable even without the presence of the antibiotic.

Epidemiology and pathogenesis

The ''B. fragilis'' group is the most commonly isolated

Bacteroidaceae The ''Bacteroidaceae'' are a family of environmental bacteria Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorg ...

anaerobic infection Anaerobic infections are caused by anaerobic bacteria. Obligately anaerobic bacteria do not grow on solid media in room air (0.04% carbon dioxide and 21% oxygen); facultatively anaerobic bacteria can grow in the presence or absence of air. Microaer ...

s, especially those that originate from the gastrointestinal microbiota. ''B. fragilis'' is the most prevalent organism in the ''B. fragilis'' group, accounting for 41% to 78% of the isolates of the group. These organisms are resistant to penicillin by virtue of production of

beta-lactamase Beta-lactamases, (β-lactamases) are enzymes () produced by bacteria that provide multi-resistance to beta-lactam antibiotics such as penicillins, cephalosporins, cephamycins, monobactams and carbapenems (ertapenem), although carbapenems ...

, and by other unknown factors. This group was formerly classified as subspecies of ''B. fragilis'' (i.e. ''B. f.'' ssp. ''fragilis'', ''B. f.'' ssp. ''distasonis'', ''B. f.'' ssp. ''ovatus'', ''B. f.'' ssp. ''thetaiotaomicron'', and ''B. f.'' ssp. ''vulgatus''). They have been reclassified into distinct species on the basis of DNA homology studies. ''B. fragilis'' (formerly known as ''B. f.'' ssp. ''fragilis'') is often recovered from

blood Blood is a body fluid in the circulatory system of humans and other vertebrates that delivers necessary substances such as nutrients and oxygen to the cells, and transports metabolic waste products away from those same cells. Blood in the c ...

pleural fluid The pleural cavity, pleural space, or interpleural space is the potential space between the pleurae of the pleural sac that surrounds each lung. A small amount of serous pleural fluid is maintained in the pleural cavity to enable lubrication be ...

peritoneal fluid Peritoneal fluid is a serous fluid made by the peritoneum in the abdominal cavity which lubricates the surface of tissue that lines the abdominal wall and pelvic cavity. It covers most of the organs in the abdomen. An increased volume of peritone ...

, wounds, and brain

abscess An abscess is a collection of pus that has built up within the tissue of the body. Signs and symptoms of abscesses include redness, pain, warmth, and swelling. The swelling may feel fluid-filled when pressed. The area of redness often extends b ...

es. Although the ''B. fragilis'' group is the most common species found in clinical specimens, it is the least common ''Bacteroides'' present in fecal microbiota, comprising only 0.5% of the bacteria present in stool. Their pathogenicity partly results from their ability to produce capsular polysaccharide, which is protective against

phagocytosis Phagocytosis () is the process by which a cell uses its plasma membrane to engulf a large particle (≥ 0.5 μm), giving rise to an internal compartment called the phagosome. It is one type of endocytosis. A cell that performs phagocytosis is ...

and stimulates abscess formation. ''Bacteroides fragilis'' is involved in 90% of anaerobic

peritoneal The peritoneum is the serous membrane forming the lining of the abdominal cavity or coelom in amniotes and some invertebrates, such as annelids. It covers most of the intra-abdominal (or coelomic) organs, and is composed of a layer of mesothe ...

infections. It also causes bacteremia associated with intra-abdominal infections, peritonitis and abscesses following rupture of viscus, and subcutaneous

abscesses An abscess is a collection of pus that has built up within the tissue of the body. Signs and symptoms of abscesses include redness, pain, warmth, and swelling. The swelling may feel fluid-filled when pressed. The area of redness often extends b ...

burn A burn is an injury to skin, or other tissues, caused by heat, cold, electricity, chemicals, friction, or ultraviolet radiation (like sunburn). Most burns are due to heat from hot liquids (called scalding), solids, or fire. Burns occur mainl ...

snear the anus. Though it is gram negative, it has an altered LPS and does not cause endotoxic shock. Untreated ''B. fragilis'' infections have a 60% mortality rate.

References

External links

''Bacteroides'' references
in Baron's ''Medical Microbiology'' (online at the

NCBI The National Center for Biotechnology Information (NCBI) is part of the United States National Library of Medicine (NLM), a branch of the National Institutes of Health (NIH). It is approved and funded by the government of the United States. The ...

br>bookshelf
.
Type strain of ''Bacteroides fragilis'' at Bac''Dive'' - the Bacterial Diversity Metadatabase
{{Taxonbar, from=Q221817 Bacteroidia Gram-negative bacteria Medically important anaerobes Gut flora bacteria