Bacteroides Infection Workup

Updated: Oct 27, 2022
  • Author: Itzhak Brook, MD, MSc; Chief Editor: Michael Stuart Bronze, MD  more...
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Laboratory Studies

Collection of specimens of anaerobic bacteria is important because documentation of an anaerobic infection is through culture of organisms from the infected site. Appropriate documentation of anaerobic infection requires proper collection of appropriate specimens, expeditious transportation, and careful laboratory processing. [1]

Specimens must be obtained free of contamination. Inadequate techniques or media can lead to missing the presence of anaerobic bacteria or the assumption that only aerobic organisms are present in a mixed infection.

Because anaerobes are present on skin and mucous membranes, even minimal contamination with normal florae can be misleading.

Unacceptable or inappropriate specimens can yield normal florae and, therefore, have no or little diagnostic value.

Appropriate materials should be obtained by using techniques that bypass the normal florae.

Direct-needle aspiration is the best method of obtaining a culture; the use of swabs is much less desirable.

Specimens obtained from normally sterile sites, such as blood or spinal, joint, or peritoneal fluids, are collected after thorough skin decontamination. Two approaches are used to culture the maxillary sinus by aspiration following sterilization of the canine fossa or the nasal vestibule, via either the canine fossa or the inferior meatus. Urine is collected by percutaneous suprapubic bladder aspiration.

Other specimens can be collected from abscess contents, from deep aspirates of wounds, and via special techniques, such as transtracheal aspirates or direct lung puncture.

Specimens of the lower respiratory tract are difficult to obtain without contamination with indigenous florae. Double-lumen catheter bronchial brushing and bronchoalveolar lavage, cultured quantitatively, can be useful.

Culdocentesis fluid obtained after decontamination of the vagina is acceptable.

Transportation of specimens should be prompt unless transport devices are available. Transport devices generally contain oxygen-free environments provided by a mixture of carbon dioxide, hydrogen, and nitrogen, plus an aerobic condition indicator. Specimens should be placed into an anaerobic transporter as soon as possible.

Liquid or tissue specimens are always preferred to swabs. Liquid specimens are inoculated into an anaerobic transport vial or a syringe and a needle. All air bubbles are expelled from the syringe. Insertion of the needle tip into a sterile rubber stopper is no longer recommended. Because air gradually diffuses through the plastic syringe wall, specimens should be processed in less than 30 minutes.

Swabs are placed in sterilized tubes that contain carbon dioxide or prereduced anaerobically sterile Carey and Blair semisolid media.

Tissue specimens can be transported in an anaerobic jar or in a sealed plastic bag rendered anaerobic.

Gram stain of a smear of the specimen provides important preliminary information regarding the types of organisms present, helps determine appropriate initial therapy, and serves as a quality control.

Cultures should be immediately placed under anaerobic conditions and should be incubated for 48 hours or longer. An additional 36-48 hours is generally required for species- or genus-level identification by using biochemical tests. Kits that contain these tests are commercially available. [3]

A rapid enzymatic test enables identification after only 4 hours of aerobic incubation.

Gas-liquid chromatography of metabolites is often used.

Nucleic acid probers and polymerase chain reaction methods are also being developed for rapid identification. [30]

Detailed procedures of laboratory methods can be found in microbiology manuals. [3]

Antimicrobial susceptibility testing of AGNB has become less predictable because their resistance to several antimicrobials has increased. [31] Screening of AGNB isolates for beta-lactamase activity may be helpful. [16, 32] However, occasional strains may resist beta-lactam antibiotics through other mechanisms.

Routine susceptibility testing is time-consuming and often unnecessary. Poor quality control of in-vitro susceptibility testing and the difficulty to obtain results in a reasonable time make testing less useful. However, testing the susceptibility of isolates recovered from sterile body sites and/or those that are clinically important (ie, blood cultures, bone, CNS, serious infections) and have variable susceptibilities, especially those isolated in pure culture from properly collected specimens, is important. [4] Antimicrobial testing is recommended, however, to monitor local hospital and geographic susceptibility profiles and to determine the activity of new antimicrobials.

Antibiotics that should be tested include penicillin, a broad-spectrum penicillin, a penicillin plus a beta-lactamase inhibitor, clindamycin, chloramphenicol, a second-generation cephalosporin (eg, cefoxitin), tigecycline, newer quinolones (eg, moxifloxacin), chloramphenicol, metronidazole, and a carbapenem. The recommended methods include agar microbroth and macrobroth dilution. Newer methods include the E-test and the spiral gradient end point system.


Imaging Studies

Radiologic or imaging studies and scans (ie, computed tomography and magnetic resonance imaging) are helpful. Abscesses are a common complication of Bacteroides infections. The presence of gas in the infected site is highly suggestive of anaerobic infection. [1]