Anaeróbios

Bacteroides thetaiotaomicron

Pleomorphic anaerobic Gram-negative bacilli

Identification & Growth

Identification

B. fragilis group: grows in 20% bile, resistant to kanamycin, vancomycin and colistin
Species identification by MALDI-TOF

Growth Conditions

Anaerobic blood agar with hemin and vitamin K, 35–37 °C, 48 h

Clinical Significance

Polymicrobial intra-abdominal infection, pelvic abscess and bacteremia

Intrinsic Resistance

Intrinsic resistance to aminoglycosides, aztreonam, quinolones and polymyxins
Broad cephamycinase: lower susceptibility than B. fragilis to piperacillin-tazobactam and clindamycin

Bench Alerts

The most resistant species of the group — always test when isolated from a sterile site

Clinical Notes

Clinical presentations

Polymicrobial intra-abdominal infections (abscess, secondary peritonitis) following bowel perforation, surgery or abdominal trauma.
Bacteremia of intra-abdominal or pelvic origin, often in the context of colorectal malignancy or complicated diverticular disease.
Polymicrobial gynecologic and soft-tissue infections, including tubo-ovarian abscess and pelvic surgical site infection.

Specimens and collection

Abscess material or peritoneal fluid collected by aspiration, avoiding skin/mucosal flora contamination, transported in appropriate anaerobic media.
Blood culture with a dedicated anaerobic bottle when intra-abdominal or pelvic origin is suspected.
Avoid superficial wound swabs; prioritize deep tissue or aspirate, as superficial samples have low yield for anaerobes.

Epidemiology and at-risk populations

Predominant member of the anaerobic colonic microbiota; the leading Gram-negative anaerobe isolated in intra-abdominal infections.
Invasive disease arises from translocation following disruption of the intestinal barrier (surgery, perforation, malignancy, diverticulitis).
The Bacteroides fragilis group (including B. thetaiotaomicron) is the leading anaerobic genus associated with abdominal surgical infections and anaerobic bacteremia.

Resistance and therapeutic implications

Resistance to penicillin is nearly universal due to beta-lactamase production; requires combination with a beta-lactamase inhibitor or use of a carbapenem/metronidazole.
Metronidazole and carbapenems retain high activity and remain treatment mainstays; resistance to clindamycin is increasing and relevant to empirical choice.
EUCAST publishes breakpoints for the Bacteroides fragilis group; susceptibility testing is recommended in severe or refractory infections given the variable resistance pattern.

Bench and reporting notes

Strictly anaerobic, non-spore-forming Gram-negative rod that grows on selective anaerobic media (bile-esculin agar), being bile-resistant.
Frequently isolated in polymicrobial infection; report alongside concurrent aerobes/facultative organisms to guide combination therapy.
Anaerobe susceptibility testing is not part of routine practice in all laboratories; consider referral to a reference laboratory in severe or refractory cases.

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 thetaiotaomicron in the lab?

Bacteroides thetaiotaomicron is identified through b. fragilis group: grows in 20% bile, resistant to kanamycin, vancomycin and colistin, species identification by maldi-tof.

What are the intrinsic resistances of Bacteroides thetaiotaomicron?

This organism is naturally resistant to intrinsic resistance to aminoglycosides, aztreonam, quinolones and polymyxins, broad cephamycinase: lower susceptibility than b. fragilis to piperacillin-tazobactam and clindamycin. These drugs should not be reported as susceptible.

Where is Bacteroides thetaiotaomicron commonly found?

It is typically associated with polymicrobial intra-abdominal infection, pelvic abscess and bacteremia.

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