Bacteroides fragilis represents a group of Gram negative, non-sporulated, obligate anaerobic bacillary bacteria. It includes B. fragilis, B. distasonis, B. ovatus, B. vulgatus, B. thetaiotaomicron, B. caccae, B. eggerthii, B. merdae, B. stercoris Y B. uniformis.
All these bacilli have similarity in virulence factors and resistance to antibiotics. In addition, they represent 1% of the human colonic microbiota, where they normally live in a harmless relationship between host and host..
However, the strains of the group Bacteroides fragilis are the most commonly encountered pathogens in clinically important infections caused by anaerobic bacteria or mixed infections.
Which means that what is important is not the amount in which these bacteria are found in the colon, but their virulence factors, which are what make them thrive as important infectious agents..
On the other hand, these microorganisms are relatively tolerant to oxygen, and their participation in polymicrobial infections is important. That is, they help other anaerobes to remain viable, helping to reduce the potential for redox..
Most infections are opportunistic and endogenous. This means that they appear due to the eruption of the intestinal mucosa due to tumors, surgeries, diverticulosis or other causes, and when leaving their niche they produce septicemia and abdominal abscesses..
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Bacteroides fragilis It is one of the species of this genus that is resistant to bile and also has high resistance to antimicrobial agents..
This resistance occurs mainly against beta-lactam antibiotics (penicillin and cephalosporins) due to the production of beta-lactamases, among which cephalosporinases predominate..
However, certain beta-lactam antibiotics are resistant to attack by these enzymes and are therefore sometimes useful against B. fragilis. These antibiotics are ticarcillin, piperacillin, cefoxitin, and imipenem..
But the resistance problem is not static, so bacteria are increasingly acquiring more mechanisms to evade the action of these drugs. Therefore, percentages of resistance to the aforementioned drugs and also to tetracyclines and clindamycin have already been registered..
So far metronidazole and chloramphenicol are effective against almost all strains of B. fragilis.
Kingdom: Bacterium
Phylum: Bacterioids
Class: Bacteroidia
Order: Bacteroidal
Family: Bacteroidaceae
Gender: Bacteroides
Species: fragilis
Microscopically they are relatively short, pale Gram negative rods with rounded ends, giving them a coccobacillary appearance..
Bacilli are 0.5 to 0.8 µm in diameter by 1.5 to 9 µm in length.
They also have a certain polymorphism (both in size and shape) when they come from liquid cultures and also present irregularity in staining and some vacuoles..
These bacilli do not form spores and do not have flagella, that is, they are immobile..
Colonies are white to gray, semi-opaque, smooth, and non-hemolytic. They present whorls or annular structures within the colony. They measure 1 - 3 mm in diameter.
Bacteroides fragilis it is a quite virulent microorganism.
It produces the enzymes neuraminidase, hyaluronidase, gelatinase, fibrinolysin, superoxide dismutase, catalase, DNase, and heparinase. Most of these enzymes collaborate for the invasion of tissues.
Superoxide dismutase and catalase help you to scavenge toxic free radicals such as the superoxide ion Otwo- and hydrogen peroxide HtwoORtwo respectively.
This represents a virulence factor, since it gives it a greater survival and proliferation advantage in tissues compared to other obligate anaerobes that do not have these enzymes..
Likewise, it has a polysaccharide capsule that can be easily demonstrated with ruthenium red staining, India ink or electron microscopy. The capsule is an essential element to evade phagocytosis by the cells of the immune system.
It also has an endototoxin in its cell wall like all Gram negative bacteria. However, it does not contain lipid A, 2-ketodeoxyyoctanate, heptose, or beta-hydroxymyristic acid..
Therefore, it has a weak biological activity compared to the endotoxins of other Gram negative bacteria. It also produces an enterotoxin (B toxin).
Finally, resistance to antibiotics is a characteristic that increases virulence, because it makes treatment difficult..
All the virulence factors mentioned above play a fundamental role in pathogenesis..
Causes inflammatory diarrhea, although asymptomatic colonization is common.
Human studies suggest an association between infection with Bacteroides fragilis enterotoxigenic with inflammatory bowel disease and colon cancer.
It is frequently present in polymicrobial infections.
The group of B. fragilis It can be identified due to certain biochemical tests:
It is resistant to 2U penicillin discs and 1µg disc kanamycin. It is sensitive to rifampicin 15 µg disk.
It grows in media with 20% bile, it ferments sucrose, it does not produce pigment, it hydrolyzes esculin, the reduction of nitrates is negative and it is negative indole..
Likewise, the acids it produces B. fragilis made from yeast glucose peptone broth are acetic acid, propionic acid, succinic acid and phenylacetic acid.
It is catalase positive, which is an unusual feature in anaerobic bacteria. This is a mechanism that in polymicrobial infections favors the proliferation of other anaerobic bacteria, since this microorganism collaborates in the elimination of toxic substances from oxygen.
The best sample will always be pus or fluid taken directly from the lesion. Sampling and transport should be carried out in an oxygen-free atmosphere and transported as soon as possible to the laboratory..
Special tubes can be used to transport anaerobes or it can be transported in the syringe without leaving air inside and protecting it from the environment.
They grow on blood agar under anaerobic conditions at 37 ° C.
It should be noted that most anaerobic infections are polymicrobial and for this reason, aerobic microorganisms, such as Enterobacteriaceae, may be present together. For this reason, it is necessary to use antibiotics in the culture media for the isolation of anaerobes..
The most widely used antibiotic for this purpose is the aminoglycoside, since all anaerobes are resistant to it..
Gram staining of direct clinical material showing Gram positive and Gram negative bacteria or both is highly suggestive of anaerobic infection. Therefore, the Gram stain is usually useful in the management of these infections..
Bacteroides fragilis looks like a Gram negative rod.
The approach is almost always performed empirically, due to how difficult and slow the cultures are, coupled with the fact that in anaerobes the antibiogram technique is less standardized for these microorganisms.
Therefore, antibiotics with expected susceptibility of anaerobes that commonly cause infection are selected according to the site of infection..
In the case of abdominal infections, antibiotics resistant to beta-lactamases are required.
In order to B. fragilis, metronidazole, imipenem, aztreonam, or ceftriaxone is usually used.
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