Register Now


Lost Password

Lost your password? Please enter your email address. You will receive a link and will create a new password via email.

Add question

Sorry, you do not have a permission to add a question.
You must login to ask question.


Register Now

If You are a member Please log in. If You're not already registered, please create a new account.

Bacteroides fragilis

Bacteroides fragilis

Bacteroides fragilis is a Gram negative pleomorphic rod shaped bacteria. It is an obligate anaerobic, non-spore forming, non-motile bacterium. Bacteroides fragilis is a normal inhabitant of human colon but if it is displaced and spread to other surrounding tissues or blood stream it can cause disease. It is a catalase and indole positive bacteria. Among the clinical infections of class Bacteroidaceae B. fragilis is the most common recovered specie.

History of Bacteroides Fragilis

In 1898 Veillon and Zuber firstly described Bacteroides fragilis and named it as Bacillus fragilis. In 1919 the bacteria was moved into a separate newly made genus Bacteroides by Castellani and Chalmers. B. fragilis had been named as Fusiformis fragilis, Ristella fragilis and finally named as Bacteroides fragilis subspecies fragilis by Holdeman and Moore in 1974. Holdeman and Moore defined following supspecies of B. fragilis:

  • Bacteroides fragilis subspecies fragilis
  • Bacteroides fragilis subspecies distasonis
  • Bacteroides fragilis subspecies eggerthii
  • Bacteroides fragilis subspecies ovatus
  • Bacteroides fragilis subspecies thetaiotanomicron
  • Bacteroides fragilis subspecies uniformis
  • Bacteroides fragilis subspecies vulgatus

Taxonomy of Bacteroides fragilis

Classification of B. fragilis
Kingdom Bacteria
Phylum Bacteroidetes
Class Bacteroidia
Order Bacteroidales
Family Bacteroidaceae
Genus Bacteroides
Specie Bacteroides fragilis
Binomial Name: Bacteroides fragilis

Physiology of Bacteroides fragilis

Bacteroides fragilis is a Gram negative bacterium. It is an anaerobic counterpart of E. coli. On blood agar it grows well and gives black brown hemolytic colonies in 3 to 5 days. It can also grow at 20% bile concentration which is a distinguishing microbiology tool help in distinguishing from other genera. On Bacteroides-bile-esculin (BBE) agar it gives dark colonies with brown black halos due to esculin hydrolyzing. It is a non spore forming bacteria but when is observed under microscope it contains large vacuoles which appear smiliar to spores. B. fragilis is a catalase positive and variably Indole positive too. Although it is a Gram negative bacteria but it does not produce endotoxin due to the altered LPS.

Fig: colonies of Bacteroides fragilis on Bacteroides-bile-esculin (BBE) agar

Diseases Caused by Bacteroides fragilis

Following are the infections caused by Bacteroides fragilis:

Intra-abdominal infections:

Bacteroides fragilis is a natural inhabitant of human gastrointestinal tract but it can also cause intra-abdominal infections if displaced into surrounding tissue. It case intra-abdominal abscess. In 70 % cases of intra-abdominal abscess B. fragilis had been isolated.

Pelvic infections:

Bacteroides fragilis cause polymicrobial pelvic infections. These infections include postpartum endometritis, pelvic abscess, post episiotomy soft tissue infections, tuboovarian abscess and pelvic inflammatory disease.


Bacteremia is defined as the presence of bacteria in blood. Bacteroides fragilis is most common anaerobic isolate after blood culture. Presence of bacteria in abdomin, soft tissues and female genital tract is most common source of bacteraemia.

Skin and soft tissue infections:

Human skin has many bacteria as a natural micro flora but B. fragilis is not a part of human skin micro flora. If there is any tissue and this bacteria get entry into the tissue it causes skin infections such as gangrene and necrosis. Diabetic patients are also vulnerable to skin infections caused by B. fragilis.


B. fragilis is also involved in causing pericarditis, endocarditis, meningitis, appendicitis and inflammation of throat.

Transmission of Bacteroides fragilis

Bacteroides fragilis is a common inhabitant of human mucosa. If the bacteria is displaced and transferred to any other body tissue it can cause disease of that tissue. Displacement could be due to trauma, cut, burns, penetration of foreign objects or through improper surgical practices. Invasion of organism by its own is not yet reported.

Pathology of Bacteroides fragilis

The pathogenicity results due to the production of capsule. Its capsular polysaccharide protects it from phagocytosis also it has role in starting abscess formation. Bacteria poses fimbriae and pili which promotes its adhesion. B. fragilis also produce an enterotoxin known as Bacteroides fragilis toxin (BFT) or fragilysin. The toxin is a zinc mettaloprotease. It has an altered LPS so there is no endotoxin production and is Gram negative bacteria so it does not cause toxic shock syndrome. B. fragilis produces multiple enzymes like hemolysin, protease, peroxidase, collagenase, heparinase, hyaluronidase and neuraminidase. Production of catalase and superoxide diamutase are additional virulence factors.

Signs and Symptoms

Following are the clinical manifestations of Bacteroides fragilis:

  • Bacteremia
  • Appendicitis
  • Bed sore
  • Septic arthritis
  • Meningitis
  • Inflammation of throat
  • Upper respiratory infection
  • Skin infections
  • Endocarditis
  • Soft tissue infection
  • Pericarditis

Further complications

Further complications caused by B. fragilis include endocarditis, pericarditis, vascular graft infections, septic arthritis and osteomyelitis. B. fragilis is a component of mixed microbial infections resulting in brain ascess. In the absence of brain abscess in neonate B. fragilis also causes meningitis which is the inflammation of meninges the membranes protecting the brain and spinal cord.

Risk factors

Bacteroides fragilis infection is a polymicrobial or mixed type infection. The patients having intra-abdominal infections are at high risk to have B. fragilis infection. Diabetic patients are also susceptible to infections caused by B. fragilis.


Bacteroides fragilis can grow on 20% bile salt concentration which is a distinguish character of it. Gram staining and growth on blood agar and Bacteroides-bile-esculin (BBE) agar is used for isolation and presumptive identification . It is a catalase positive and variable indole positive bacterium. Bacteria can ferment glucose, lactose, maltose and sucrose. B. fragilis is unable to ferment rhamnose, arabinose, salicin and trehalose. Rapid identification kits and individual fermentation reactions are used for identification of B. fragilis. For checking intra-abdominal infections computed tomography (CT) and ultrasound is done.

Treatment of Bacteroides fragilis infections

Antibiotic Resistance:

Bacteroides fragilis produce beta-lactamase so it is resistant to penicillin. It also develop resistant against clindamycin which was used for its treatment. Due to clindamycin resistance it is not used now as first line treatment for B. fragilis. The organism is also resistant to aminoglycosides. More than 80 % Bacteroides spp. are resistant to tetracyclines now.

Single drug Treatment:

  • Carbapenems can be used for treating B. fragilis infections as Carbapenems are resistant to the hydrolysis produced by a number of beta-lactamases including beta-lactamases produced by Bacteroides spp.
  • Chloramphenicol is also used for treatment as the reported resistance against it is very rare.
  • Metronidazoles have a high activity against B. fragilis as it can penetrate into abscess and the resistance against Metronidazoles is very rare.

Combination drugs:

As B. fragilis produce beta-lactamase so the combination of beta-lactamase inhibitors and penicillins is use for treatment of B. fragilis infections. Ampicillin/sulbactum, ticarcilli/clavulanate and piperacillin/tazobactum are three most common use combination drugs as these are highly potent and also have higher activity.

Prevention from Bacteroides fragilis infections

Following are the preventive measures for Bacteroides fragilis infections:

  • Incase of gastrointestinal surgery prophylaxis need to be done.
  • Abscess and debarment need to be drained in case of skin infection.
  • Good skin care in bed ridden patients and diabitic patients should be given.
  • Use of personal protective equipments should be done while dealing the organism in laboratory.
  • Prophylactic treatment also helps to prevent B. fragilis infections.

There is currently no available vaccine for B. fragilis.


About askmicroAmbassador