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MICROBIOLOGY LEGEND
CYCLE 46 ORGANISM 1
BACTEROIDES FRAGILIS
Bacteroides fragilis is an obligately anaerobic, Gram-negative, rod-shaped bacterium. It is part of the normal microbiota of the human colon and is generally commensal, but can cause infection if displaced into the bloodstream or surrounding tissue following surgery, disease, or trauma. They have been reclassified into distinct species on the basis of DNA homology studies. B. fragilis (formerly known as B. fragilis ssp. fragilis, one of the subspecies of B. fragilis) is often recovered from blood, pleural fluid, peritoneal fluid, wounds and brain abscesses.
Figure 1: Bacteroides fragilis gram stain
Bacteroides Bacteroides species are anaerobic bacteria that are predominant components of the bacterial florae of mucous membranes and are therefore a common cause of endogenous infections. Bacteroides infections can develop in all body sites, including the CNS, the head, the neck, the chest, the abdomen, the pelvis, the skin, and the soft tissues. Inadequate therapy against these anaerobic bacteria may lead to clinical failure.
Because of their fastidiousness, they are difficult to isolate and are often overlooked. Their isolation requires appropriate methods of collection, transportation, and cultivation of specimens. Treatment is complicated by 3 factors: slow growth, increasing resistance to antimicrobial agents, and the polymicrobial synergistic nature of the infection.
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B. fragilis group The B. fragilis group, a member of the Bacteroidaceae family, includes B. fragilis, B. distasonis, B. ovatus, B. thetaiotaomicron, and B. vulgatus. B. fragilis group is the most commonly isolated Bacteroidaceae in anaerobic infections, especially those that originate from the gastrointestinal flora. These bacteria are resistant to penicillins, mostly through the production of beta-lactamase. They are part of the normal GI florae and predominate in intra-abdominal infections and infections that originate from those florae (e.g. perirectal abscesses, decubitus ulcers). Enterotoxigenic B fragilis (ETBF) is also a potential cause of diarrhea.
Although B. fragilis group is the most common species found in clinical specimens, it is the least common Bacteroides present in fecal flora, comprising only 0.5% of the bacteria present in stool. The pathogenicity of this group of organisms probably results from its ability to produce capsular material, which is protective against phagocytosis.
Clinical significance B. fragilis is involved in 90% of anaerobic peritoneal infections. It acts primarily at the surface of the mucosa. It predominates in bacteremia associated with intra-abdominal infections, peritonitis and abscesses following rupture of viscus, and subcutaneous abscesses or burns near the anus.
Laboratory Diagnosis Bacteroides fragilis may be isolated as a single agent, such as in blood cultures, or more typically from mixed infections. The organism is aerotolerant, but requires an anaerobic environment to propagate. Simple identification from blood cultures includes Gram stain and growth on blood agar and Bacteroides-bile-esculin (BBE) agar for isolation and presumptive identification of Bacteroides fragilis group (as well as Bilophila wadsworthia). B. fragilis will appear as dark colonies with brown-black halos on BBE agar due to the hydrolysis of esculin. B. fragilis can be further presumptively identified by resistance to kanamycin, vancomycin and colistin, using a disk test, and will grow in 20% bile, produce catalase (most strains), and is variably indole positive. Many laboratories will confirm the identification using a rapid identification kit or individual fermentation reactions.
Figure 2: Bacteroides fragilis on Bacteroides-bile-esculin (BBE) agar
Treatment In general, B. fragilis is susceptible to metronidazole, carbapenems, tigecycline, beta-lactam/beta-lactamase inhibitor combinations (e.g., Unasyn, Zosyn), and certain antimicrobials of the cephamycin class, including
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cefoxitin. The bacteria have inherent high-level resistance to penicillin. Production of beta lactamase appears to be the main mechanism of antibiotic resistance in B. fragilis. Clindamycin is no longer recommended as the first- line agent for B. fragilis due to emerging high-level resistance (>30% in some reports).
References 1. http://en.wikipedia.org/wiki/Bacteroides_fragilis 2. Bacteroides Infection Author: Itzhak Brook, MD, MSc; Chief Editor: Burke A Cunha, MD 3. www.antimicrobe.org/b85.asp
Questions 1. Discuss the morphological characteristics of Bacteroides fragilis. 2. Discuss the clinical significance of B. fragilis. 3. Discuss the Laboratory Diagnosis of B. fragilis.
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