Bacteroides Infection

Updated: Oct 27, 2022
  • Author: Itzhak Brook, MD, MSc; Chief Editor: Michael Stuart Bronze, MD  more...
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This article describes infections caused by the Bacteroides fragilis [1]  group and other anaerobic gram-negative bacilli (AGNB) that were previously included in the Bacteroides genus but are now included in the Prevotella and Porphyromonas genera. In addition, many new genera and several new species have been created to accommodate pathogens such as Bilophila wadsworthia, Sutterella wadsworthensis,Centipeda periodontii, and Anaerobiospirillum thomasii. Infections due to AGNB are common, yet the specific identification of AGNB in these infections is difficult.

Bacteroides species are anaerobic bacteria that are predominant components of the bacterial florae of mucous membranes [2] and are therefore a common cause of endogenous infections. [1]  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 often are overlooked. Their isolation requires appropriate methods of collection, transportation, and cultivation of specimens. [3] Treatment is complicated by 3 factors: slow growth, increasing resistance to antimicrobial agents, [4] and the polymicrobial synergistic nature of the infection. [5]

Bacteroides spp. possess virulence factors or special characteristics to compete successfully for microbial niches, including pili and fimbriae and adhesins (which enhance adherence), hemagglutination, enzymes (collagenase, phospholipase A, hemolysin, peroxidase, protease, fibrolysin, heparinase, neuraminase, superoxide dismutase), toxins, and lipopolysaccharide endotoxin, and capsular polysaccharide (which enhance invasion and evasion from host phagocytosis). [1, 6]

Bacteroides spp possess important immunomodulatory effects and the body’s energy balance. B fragilis’ polysaccharide A enhances homeostatic immune function both in the gut and systemically and balances T-cell subset population size and function. [1, 6]

The B fragilis group, a member of the Bacteroidaceae family, includes B fragilis (causes the most clinical infections), Bacteroides ovatus, Bacteroides thetaiotaomicron,Parabacteroides distasonis (previously Bacteroides distasonis), [7] and Bacteroides vulgatus. These bacteria are resistant to penicillins, mostly through the production of beta-lactamase. They are part of the normal GI florae [2] and predominate in intra-abdominal infections and infections that originate from those florae (eg, perirectal abscesses, decubitus ulcers). Enterotoxigenic B fragilis (ETBF) is also a potential cause of diarrhea. [8]

Pigmented Prevotella, such as Prevotella melaninogenica and Prevotella intermedia (which were previously called the Bacteroides melaninogenicus group), Porphyromonas (eg, Porphyromonas asaccharolytica), and nonpigmented Prevotella (eg, Prevotella oralis, Prevotella oris) are part of the normal oral and vaginal florae and are the predominant AGNB isolated from respiratory tract infections and their complications, including aspiration pneumonia, lung abscess, chronic otitis media, chronic sinusitis, abscesses around the oral cavity, human bites, paronychia, brain abscesses, and osteomyelitis. Prevotella bivia and Prevotella disiens (previously called Bacteroides) are important in obstetric and gynecologic infections.



Although AGNB perform beneficial functions as part of the GI flora, they are also consummate opportunistic pathogens that can cause serious infections, typically in synergistic infections in combination with other anaerobic as well as aerobic bacteria. Most infections due to AGNB originate from the endogenous mucosal membrane florae. Knowledge of the common mode of distribution allows for a logical choice of antimicrobial therapy for infections in these sites.

AGNB infections are generally polymicrobial. The number of isolates can reach 5-10 organisms. The type of copathogens depends on the infection site and the circumstances of the infection. Antimicrobial therapy should be directed at all major aerobic and anaerobic pathogens. AGNB promote infection through synergy with their aerobic and anaerobic counterparts and with each other.

An indirect pathogenic role of AGNB is their ability to produce the enzyme beta-lactamase, which allows them to protect themselves and other penicillin-susceptible organisms from the activity of penicillins.




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The exact frequency of AGNB infection is difficult to calculate because of inappropriate methods of collection, transportation, and cultivation of specimens. AGNB are more commonly found in chronic infections. The rate of recovery of anaerobic bacteria in blood cultures is 5-15% and is higher among immunocompromised patients and those who have predisposing conditions.


The frequency of these infections appears to be higher in developing countries, where therapy is often inadequate or delayed. However, because of difficulties in isolation of these and other anaerobic bacteria, their role is underestimated.


Mortality has decreased over the past 4 decades because of early recognition, medical and surgical intervention, and the initiation of proper prophylactic and therapeutic antimicrobial therapies.


AGNB infections can occur in patients of all ages; however, the frequency of upper respiratory tract and head and neck infections is higher in children than in adults.