Anaeróbios

Bacteroides fragilis

Pleomorphic Gram-negative bacilli

Identification & Growth

Identification

Strict anaerobe, grows in 20% bile, resistant to kanamycin, vancomycin and colistin
Catalase-positive (uncommon among anaerobes)

Growth Conditions

Bacteroides Bile Esculin (BBE) agar: dark colonies with a brown-black halo
Anaerobic jar or chamber, 35 °C, 48–72 h

Clinical Significance

Intra-abdominal abscess, peritonitis, polymicrobial soft-tissue infection

Intrinsic Resistance

Aminoglycosides, aztreonam, older quinolones, colistin, vancomycin

Bench Alerts

Produces beta-lactamase: penicillin and ampicillin alone will fail
Metronidazole, piperacillin-tazobactam and carbapenems are the predictable options
The specimen must be sent in anaerobic transport medium, without air exposure

Clinical Notes

Clinical presentations

Is the most frequently isolated gram-negative anaerobe in intra-abdominal infections, including abscesses and postperforation peritonitis.
Causes polymicrobial gynecologic infections, including tubo-ovarian abscess and postsurgical pelvic infection.
Bacteremia due to B. fragilis is frequently associated with a primary intra-abdominal or pelvic source.

Specimens and collection

Specimens should be collected by aspiration or biopsy into an anaerobic container, avoiding prolonged air exposure.
Superficial swabs are inadequate for anaerobic culture due to risk of contamination by surface aerobic flora.
Blood cultures in anaerobic bottles should be collected when bacteremia of intra-abdominal origin is suspected.

Epidemiology and at-risk populations

Is part of the normal gastrointestinal microbiota, becoming pathogenic after disruption of the intestinal barrier.
Patients undergoing abdominal surgery, penetrating trauma, or intestinal perforation have increased risk of invasive infection.

Resistance and therapeutic implications

Is the anaerobic species with the highest rate of beta-lactam resistance among clinically relevant anaerobes, mainly mediated by beta-lactamase production.
Metronidazole, carbapenems, and beta-lactam/beta-lactamase inhibitor combinations retain good activity against most isolates.
Susceptibility testing is not performed routinely but is recommended in treatment failure or recurrent severe infections.

Bench and reporting notes

Anaerobic culture requires an anaerobic chamber or jar and prolonged incubation, often 48-72 hours.
Mass spectrometry (MALDI-TOF) identification significantly accelerates turnaround time compared to traditional biochemical methods.
Polymicrobial cultures of intra-abdominal origin frequently include B. fragilis alongside Enterobacterales and enterococci.

Sources

Educational decision-support content. It does not replace laboratory validation, current guidelines or review by the responsible professional.

FAQ: Frequently Asked Questions

How to identify Bacteroides fragilis in the lab?

Bacteroides fragilis is identified through strict anaerobe, grows in 20% bile, resistant to kanamycin, vancomycin and colistin, catalase-positive (uncommon among anaerobes).

What are the intrinsic resistances of Bacteroides fragilis?

This organism is naturally resistant to aminoglycosides, aztreonam, older quinolones, colistin, vancomycin. These drugs should not be reported as susceptible.

Where is Bacteroides fragilis commonly found?

It is typically associated with intra-abdominal abscess, peritonitis, polymicrobial soft-tissue infection.

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