BACTEROIDES FRAGILIS: CHARACTERISTICS, MORPHOLOGY, PATHOLOGIES - BIOLOGY - 2025

Bacteroides fragilis represents a group of Gram negative, non-sporulated, obligate anaerobic bacillary bacteria. Includes B. fragilis, B. distasonis, B. ovatus, B. vulgatus, B. thetaiotaomicron, B. caccae, B. eggerthii, B. merdae, B. stercoris, and B. uniformis.

All these bacilli have similarities 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.

Bacteoides fragilis colonies on Bacteroides fragilis blood / Gram agar

However, the strains of the Bacteroides fragilis group are the most commonly encountered pathogens in clinically important infections caused by anaerobic bacteria or mixed infections.

This means that what is important is not the amount in which these bacteria are 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 by tumors, surgeries, diverticulosis or other causes, and when they leave their niche they produce septicemia and abdominal abscesses.

characteristics

Phylum: Bacterioidetes

Class: Bacteroidia

Order: Bacteroidales

Family: Bacteroidaceae

Genus: Bacteroides

Species: fragilis

Morphology

Microscopically, they are relatively short, pale Gram negative rods with rounded ends, giving them a coccobacillary appearance.

The 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.

Virulence factors

Bacteroides fragilis is a fairly 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 the tissues.

Superoxide dismutase and catalase are used to eliminate toxic free radicals such as superoxide ion O ₂ ^- and hydrogen peroxide H ₂ O ₂ 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.

Pathologies

It causes inflammatory diarrhea, although asymptomatic colonization is common.

Human studies suggest an association between enterotoxigenic Bacteroides fragilis infection with inflammatory bowel disease and colon cancer.

It is frequently present in polymicrobial infections.

Biochemical characteristics

The B. fragilis group 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 produced by B. fragilis 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.

Diagnosis

The best sample will always be pus or fluid taken directly from the lesion. Sampling and transportation should be done 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.

Culture

They grow on blood agar under anaerobic conditions at 37 ° C.

It should be noted that the majority of anaerobic infections are polymicrobial and for this reason aerobic microorganisms may be present together, such as Enterobacteriaceae. 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

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 often useful in the management of these infections.

Bacteroides fragilis is seen as a Gram negative rod.

Treatment

The approach is almost always carried out empirically, due to how difficult and slow cultures are, coupled with the fact that in anaerobes the antibiogram technique is less standardized for these microorganisms.

For this reason, antibiotics with expected susceptibility of anaerobes that commonly cause infection are selected according to the site of infection.

Antibiotics resistant to beta-lactamases are required for abdominal infections.

For B. fragilis, metronidazole, imipenem, aztreonam, or ceftriaxone are usually used.

References

1. Ryan KJ, Ray C. Sherris. Medical Microbiology, 6th Edition McGraw-Hill, New York, USA; 2010.
2. Koneman E, Allen S, Janda W, Schreckenberger P, Winn W. (2004). Microbiological Diagnosis. (5th ed.). Argentina, Editorial Panamericana SA
3. Forbes B, Sahm D, Weissfeld A. Bailey & Scott Microbiological Diagnosis. 12 ed. Argentina. Editorial Panamericana SA; 2009.
4. González M, González N. Manual of Medical Microbiology. 2nd edition, Venezuela: Directorate of media and publications of the University of Carabobo; 2011
5. Wikipedia contributors. Bacteroides fragilis. Wikipedia, The Free Encyclopedia. October 31, 2018, 13:51 UTC. Available at: wikipedia.org/
6. Chen LA, Van Meerbeke S, Albesiano E, et al. Fecal detection of enterotoxigenic Bacteroides fragilis. Eur J Clin Microbiol Infect Dis. 2015; 34 (9): 1871-7.