Bacteroides fragilis group / Parabacteroides spp

Related species include Ezakiella spp, Parabacteroides spp and Phocaeicola spp

Gram Stain

• Gram negative bacilli (anaerobic)

Clinical Significance

These organisms are part of human gastrointestinal flora. 

B. fragilis group are the most commonly encountered anaerobes and the most virulent.  They are associated with bacteremia, brain abscesses, skin/soft tissue, pleuropulmonary, bone/joint, intraabdominal, and pelvic infections. 

Infections tend to be polymicrobial and involve abscess formation. 

Monomicrobial infections have been described including meningitis, endocarditis, septic arthritis, and osteomyelitis.

Usual Susceptibility Pattern

These organisms are susceptible to metronidazole, piperacillin-tazobactam, and carbapenems. 

Resistance to amoxicillin-clavulanate is increasing. Resistance to piperacillin-tazobactam is common in B. thetaiotomicron.

Bacteroides fragilis group may possess an inducible chromosomal carbapenemase (metalloenzyme) that is not usually expressed.  

Antibiotic pressure may result in induction and hyperproduction of this carbapenemase.  

Prolonged exposure to beta-lactamase inhibitor drugs may also result in resistance.

They are resistant to penicillins.  Resistance to clindamycin and moxifloxacin is significant and increasing.