Bacteroides species

Authors: David W. Hecht, M.D.


Bacteroides spp are non-spore forming gram-negative bacilli that are part of the human resident flora. Microbiologically, they are distinguished from other genera by growth in 20% bile. At present, the Bacteroides fragilis group consists of ten species: B. fragilis (the most frequent isolate), B. distasonisB. thetaiotaomicronB. vulgatusB. ovatusB. eggerrthiiB. merdaeB. stercorisB. uniformis, and B. caccae. Since 1990, many organisms previously designated as Bacteroides have been reclassified (see chapter on Anaerobes other than Bacteroides). Bacteroides, the predominant genus in the human intestine, are important in numerous metabolic activities and may provide some level of protection from invasive pathogens. All 10 species are usually isolated from the colon, although infections caused by or associated with them can include virtually any organ.

Isolation and identification of Bacteroides spp pose a hurdle to many clinical laboratories; most only identify the genus. Other laboratories may identify B. fragilis species, and lump all others into the B. fragilis group. Further, if more than three anaerobic organisms are isolated in a clinical specimen, many laboratories do not perform further identification. Those laboratories that do identifyBacteroides frequently do not perform susceptibility testing. Lack of identification and susceptibility testing of anaerobes can be attributed, in part, to its expense and the lack of timeliness in providing relevant information to the clinician (48). Thus, choice of therapy for infections that may involve Bacteroides is empiric.


Bacteroides fragilis are endogenous organisms of the GI tract. Spread of strains among patients is not known, although this topic has not been well studied. Thus, infections due to this organism are most likely caused by endogenous strains.


The hallmarks of nearly any infection involving Bacteroides spp include abscess formation, and they are frequent isolates in polymicrobial infections. Typical sites of polymicrobial infections involving Bacteroides include the abdomen and pelvis, perirectal, skin and soft tissue, and solid organs. Although isolation of Bacteroides spp as the sole pathogen can occur, it is unusual. Infections where single organism isolation is most commonly associated with Bacteroides include endocarditis, meningitis, septic arthritis and osteomyelitis (36).

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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 include Gram stain and growth on blood agar and Bacteroides-bile-esculin (BBE) agar for isolation and presumptive identification ofBacteroides 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 (20).


Multiple virulence factors have been implicated in the pathogenesis of this organism. They include the capsular polysaccharide (which inhibits opsonophagocytosis and promotes abscess formation), pili and fimbriae (promotes adherence), and production of a number of different enzymes (hyaluronidase, hemolysin, peroxidase, collagenase, protease, heparinase, and neuraminidase). In addition, superoxide dismutase and catalase also considered virulence factors. These enzymes defend B. fragilis against oxygen radicals and increase aerotolerance (14,30,57,82,114,119).


In general, Bacteroides spp grow well on enriched media in an anaerobic environment. Aero tolerance is greater in this genus than more fastidious anaerobes. Historically, susceptibility testing of Bacteroides has utilized a variety of media and methods. The National Committee for Clinical Laboratory Standards (NCCLS) recommends the agar dilution method as the reference standard (76). The testing medium recommended is Brucella agar supplemented with lysed sheep blood, Vit K1, and hemin, which supports the growth of virtually all anaerobes. This method is generally time consuming and expensive to perform, leading to its infrequent use. The alternative method recommended is the broth micro dilution technique (76). In addition to the NCCLS approved methods, the FDA has also approved use of the E-Test gradient method, which is a more rapid but somewhat expensive method (21,94). Currently, the NCCLS recommends surveillance testing on an annual basis to monitor susceptibility patterns as a guide for empiric antibiotic choice. Susceptibility testing for individual patients, isolates, should be performed when there is known resistance of a particular species, failure of a usual regimen, a pivotal role of the antimicrobial agent in determining outcome, and the need for long term therapy. In particular, susceptibility testing is indicated for isolates from brain abscess, meningitis, osteomyelitis, joint infections, infection of prosthetic devices or vascular grafts, and refractory bacteremia (76).

For the purpose of this chapter, in vitro data for each of the antibiotics were chosen from publications using NCCLS agar dilution or broth microdilution methodology. Intermediate and susceptible break points for each antibiotic with activity against Bacteroides were set by the NCCLS (76). The intermediate range was established because of the difficulty in reading end points and the clustering of MICs at breakpoint concentrations. With maximum dosages of appropriate antibiotics, organisms with susceptible or intermediate endpoints are generally amenable to therapy. Susceptibility among the different species of the B. fragilis group can vary significantly. Table 1 contains susceptibility results for the most frequently isolated species from various recent publications.

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Single Drugs

Penicillins: Penicillin hydrolyzing activity in B. fragilis group members was first recognized in 1968 (87). Greater than 90% of clinical isolates produce beta-lactamases that are predominantly active against cephalosporins, have high activity, are cell associated, and are produced constitutively (3,25,81,116). Thus most Bacteroides isolates are resistant to penicillin G and ampicillin, but may remain susceptible to cephamycins, and extended spectrum penicillins. Table 1 illustrates the comparative activity of ticarcillin, and piperacillin. Ticarcillin is moderately active against B. fragilis group members, while piperacillin demonstrates significantly greater activity. However, recent data has demonstrated steadily decreasing activity of piperacillin over the previous decade, noted by increasing MIC50 and MIC90 values (103). Susceptibility may range from 30-90%, depending upon the species tested (50), with B. thetaiotaomicron and B. distasonis the most resistant.

Cephalosporins: Three cephamycins (cefoxitin, cefotetan, cefmetazole) and two extended spectrum cephalosporins (ceftizoxime, moxalactam) have demonstrated generally good activity against B. fragilis group members, although marked differences among species can be noted for some agents. Cefoxitin is the most potent in this class, with 90% of strains below the susceptible or indeterminate breakpoint, although resistance has been noted to occur in clusters (24). Its activity is similar against all members of the B. fragilis group. The two other cephamycins, cefotetan and cefmetazole, have similar activity compared with cefoxitin against B. fragilis. However, their activity against the non-fragilis members of the B. fragilis group is significantly lower, and species dependent (Table 1) (23). Ceftizoxime has been shown to have activity against most Bacteroides spp. Early studies demonstrated activity similar to that of cefoxitin, but more recent data suggest resistance to this agent is increasing (52,80,104,123). Moxalactam also has excellent in vitro activity, but is little used due to serious side effects. Broad spectrum cephalosporins, including cefaclor, cefuroxime, cefotaxime, cetazidime, cefpodoxime, cefepime have generally poor activity against the B. fragilis group, and are generally not considered as effective against these organisms (106).

Carbapenems: The carbapenems, imipenem and meropenem, are resistant to hydrolysis by a number of beta-lactamases, including those of Bacteroides spp. Thus, both agents demonstrate excellent activity against all species within the B. fragilis group. MIC90 values for both imipenem and meropenem are 2 μg/ml or less (50,88,104). Overall, the susceptibility of B. fragilis group members to the carbapenems is 99.6 % in the United States. Most resistant strains produce a zinc metalloenzyme beta-lactamase (58). A significantly higher level of resistance has been reported from Japan, which is attributed to greater usage of imipenem in that country (5). Resistance to carbapenems also confers cross-resistance to all beta-lactam agents, including the beta-lactam/beta-lactamase inhibitor combinations.

Clindamycin: Historically, B. fragilis group members have been highly susceptible to clindamycin, and its predecessor lincomycin (23,39). Throughout the 1970's, clindamycin demonstrated the greatest activity with MIC90 of 4 μg/ml or less for most clinical isolates (109). However, by the late 1970's, isolates with high-level resistance to clindamycin emerged (96). In 1979, three laboratories reported the identification of a clindamycin resistance gene on transferable plasmids (97). Further, these genes were found to share >95% homology among their coding sequences (90). Subsequently, resistance among Bacteroides spp has increased in both the United States, and worldwide. In one report, resistance at four Chicago area medical centers was > 25% (52). High-level clindamycin resistance is also reported worldwide (6). Among the different species, B. fragilisB. vulgatus, are the most susceptible (MIC50 = 1 and 0.25 μg/ml, respectively), although the MIC90 for both species was >256 (50). A national survey has confirmed a statistically significant increase in resistance among all members of the B. fragilis group from 1990 to 1996 (103), with a predicted 15.1% and 8.18% annual increase for B. fragilis and other members of the B.fragilis group, respectively.

Chloramphenicol: Chloramphenicol is very active against all members of the B. fragilis group. Resistance has been reported rarely, and is due to either inactivation by a nitroreductase or chloramphenicol acetyltransferase, and resistance transfer has been reported (10,51,95). Use of this drug for clinical infections involving anaerobes is generally limited, primarily due to concerns about toxicity.

Aminoglycosides: Bacteroides are uniformly resistant to aminoglycosides. This is likely due to the failure to transport the amino glycosides into bacterial cells and their inability to reach the ribosomal target site.

Metronidazole: Nitroimidazoles, including metronidazole, have a high degree of activity against Bacteroides, with MIC90 values < 1 μg/ml (73). They are bactericidal and penetrate well into abscesses frequently making them the ideal candidates in therapy. Resistance to metronidazole is extremely rare and has been limited primarily to strains isolated in France (92). The resistance determinant has been localized to two different transferable plasmids (91), suggesting that resistance may increase over time.

Fluoroquinolones: The activity of several fluoroquinolones have been evaluated against anaerobes including the B. fragilis group. Ciprofloxacin, ofloxacin, levofloxacin, sparfloxacin, and grepafloxacin have generally fair to poor activity against members of the B. fragilis group with MIC90 values that range from 2 μg/ml -32 μg/ml, although they demonstrate greater activity against nonBacteroides anaerobes (47,53). Trovafloxacin has significantly greater activity against all members of the B. fragilis group (MIC90 0.39μg/ml - 2 μg/ml) but has very limited clinical indications due to toxicity. Moxifloxacin has similar activity to that of trovafloxacin, and is currently in clinical trials. (1,56). Investigational agents, including DU6859a and BM5284756 also appear very active in vitro, and await results of clinical trials (1,56,126,127).

Tetracyclines: Once a mainstay in therapy of anaerobic infections in the 1960's, more than 80% of Bacteroides isolates are now resistant (35). Tetracycline derivatives, the glycylcyclines, demonstrate excellent in vitro activity, but are still in early investigational studies (46,125).

Combination Drugs

Penicillins Plus Beta-lactamase Inhibitors: Most of the beta-lactamases produced by B. fragilis group members are inhibited by sulbactam, clavulanic acid, and tazobactam. As a result, nearly all isolates of the B. fragilis group are susceptible to ampicillin/sulbactam, ticarcillin/clavulanate, and piperacillin/ tazobactam. All three combinations demonstrate very potent activity against members of the B. fragilis group, although strains of B. distasonis that do not produce beta-lactamases have somewhat higher MIC values (Table 1) (61,124). Susceptibility of Bacteroides to these agents has not changed significantly from 1990-1996 (103).

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The principles of treatment for infection involving members of the B. fragilis group are a) making the environment such that anaerobic bacteria find it difficult to proliferate, b) checking the spread of anaerobic bacteria into healthy tissues, c) neutralizing the toxins of anaerobes, and d) supportive care (37). Control of the local environment includes debridement of necrotic tissue, drainage of pus collections, and improvement in oxygenation. For intra-abdominal abscesses, this can be performed via surgery or percutaneously. Failure of a patient to improve within 48 hours after initial drainage is an indication for repeat radiographic scanning and drainage, or laparotomy. When a drain has been placed, criteria for removal of the catheter includes clinical resolution of sepsis, minimal drainage from catheter, and radiographic evidence of abscess resolution.

Antimicrobial therapy is an important adjunct to removal of infected tissues. Most infections involving B. fragilis are polymicrobial infections, involving aerobes and anaerobes. Thus, antibiotic therapy usually requires activity against B. fragilis, along with enteric gram -negative and/or gram-positive organisms, depending upon the site of isolation. In general, maximum dosages of antimicrobial agents recommended the manufacturers are suggested for infections involving B. fragilis group members (76).

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Intra-abdominal and Pelvic Infections

B. fragilis is isolated in 70% of cases of intra-abdominal infection involving abscesses (7). Early on, animal models demonstrated the importance of anaerobes, particularly B. fragilis, in intra-abdominal infection and abscess formation (83). The first clinical trial in humans was reported in 1973, comparing cephalothin/kanamycin to clindamycin/kanamycin (117). The infectious complication rate due to anaerobes noted in the cephalothin/kanamycin group was 21% compared with 2% in the clindamycin/kanamycin group. This study established establishing that treatment of the anaerobe was important, and clindamycin/aminoglycoside became the "gold standard" to which other regimens were compared. Treatment of facultative bacteria is also important (19,31). Interested readers are referred to an excellent review for a summary of comparative clinical trials (49). Of particular note are the studies that do not require an aminoglycoside in combination with clindamycin, and the use of monotherapy (69,74).

Drugs of Choice: Table 2 lists suitable agents determined to be effective in clinical trials, either a monotherapy or in combination. Monotherapy with a carbapenem (imipenem, 500 mg i.v. q6h;meropenem 1g i.v. q8h), beta-lactam/beta-lactamase inhibitor combinations (ampicillin/sulbactam  3g i.v. q6h; ticarcillin/clavulanate 3.1g i.v.q 8h; piperacillin/tazobactam 3.75 g i.v. q6h), ormetronidazole (500 mg i.v. or p.o. q6h) combined with either an aminoglycoside or a third generation cephalosporin could be considered the most active. Clindamycin (900 mg i.v. q8h)/aminoglycoside, piperacillin (3g i.v. q6h)/aminoglycoside, cefoxitin (2g i.v.q6h), cefotetan (2g i.v. q12h), and ceftizoxime (2g i.v. q8h) might now be considered second line therapy due to concerns about resistance among aerobic and anaerobic organisms. The principle behind all agents listed as monotherapy or in combination is to treat B. fragilis, other anaerobes, andEnterobacteraciae. However, the choice of antimicrobial therapy may require modification depending upon the clinical setting. For example, a patient that develops an intra-abdominal infection while in hospital may be colonized with more resistant gram-negative organisms, such as Pseudomonas or resistant Enterobacter spp.

Antimicrobial therapy for pelvic infections involving Bacteroides spp is similar to that of intra-abdominal therapy. Polymicrobial infections that include B. fragilis group organisms involving the female genital tract the pelvis include soft tissue infection following episiotomy, postpartum endometritis, pelvic abscess, tuboovarian abscess, and acute pelvic inflammatory disease. Most agents listed in table 2 have demonstrated similar clinical efficacy against B. fragilis as part of a polymicrobial infection in this setting (67,111,113). Failures associated with these regimens are usually due toEnterococcus. A specific regimen is recommended for pelvic inflammatory disease. Current recommendations from the Centers for Disease Control and Prevention include cefoxitin (2g i.v. q6h) or cefotetan (2g i.v. q12h) plus doxycycline (100 mg p.o. bid), or clindamycin (900mg i.v. q8h) plus gentamicin followed by doxycycline (70). These regimens are recommended to treat C. trachomatisand N. gonorrhoeae in addition to B. fragilis and other anaerobes.

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B. fragilis is the most common anaerobe isolated from blood cultures, although the frequency of positive cultures is decreasing overall (11,28,122). In virtually all cases, isolation of a member of the B. fragilis group in blood indicates underlying infection (122), and is associated with 60% mortality if untreated (18). The source of bacteremia is most commonly intra-abdominal, female genital tract, or soft tissue (38,41,85).

Drug of Choice: Empiric antimicrobial therapy of Bacteroides bacteremia includes parenteral agents that are effective against B. fragilis and aerobic organisms, depending upon the source of bacteremia (Table 2). Specific choices of an agent against Bacteroides should be guided by local patterns of resistance, or obtaining the susceptibility results for the isolate. Bacteroides strains demonstrating higher or resistant MICs were correlated with treatment failure and a trend towards increased mortality (78,102). Identification and drainage of the source of bacteremia remains a mainstay of nonantibiotic therapy.

Skin and Soft Tissue

B. fragilis is not considered part of the normal skin flora. However, these organisms are an important cause of disease when soft tissues are damaged and in diabetes. The most serious infections involving B. fragilis include synergistic necrotizing fasciitis and Fournier's gangrene. Other sites of infection include wound infections following abdominal or pelvic surgery, pilonidal cysts, decubitus wounds, and diabetic foot ulcers (44,107). The role of anaerobes in an uncomplicated diabetic foot infection has been debated, with only 13% of these infections associated with anaerobes (mostly gram-positive) in one study (4,42).

Drug of Choice: Treatment of B. fragilis in serious soft tissue infection requires parenteral therapy resembling that used for abdominal infections (Table 2). Thus, empiric coverage for gram-negative and gram-positive anaerobes, plus enteric gram-negative organisms is required. Empiric coverage for S. aureus and Streptococci may be considered in more serious infections until culture results are available. Ampicillin/sulbactam (3g i.v. q6h)+ aminoglycoside, ticarcillin/clavulanate (3.1g i.v. q8h), piperacillin/tazobactam (3.75g i.v. q8h), imipenem (500mg i.v. q6h) or meropenem (1g i.v. q8h), or a combination of metronidazole (500 mg i.v. or p.o. q6h)/aminoglycoside (if Staphylococci or Streptococci are not suspected) may be suitable for empiric therapy. Patients who have been hospitalized may have resistant gram-negative aerobic bacteria as well, with an antimicrobial choice modified accordingly. Changes in the empirical regimen should be guided by culture results. For less serious soft tissue infections, such as the diabetic foot, parenteral or oral agents can be used. Currently, recommendations for empirical parenteral therapy include beta-lactam/beta-lactamase inhibitor combinations (ampicillin/sulbactam 3g i.v. q6h; ticarcillin/clavulanate 3.1g i.v.q 8h; piperacillin/tazobactam 3.75 g i.v. q6h), clindamycin (900 mg i.v. q8h) plus a third generation cephalosporin or fluoroquinolone, cefoxitin (2g i.v. q6h) or ceftizoxime (2g i.v. q8h) or metronidazole (500 mg i.v. or p.o. q6h) plus a fluoroquinolone (ciprofloxacin 750mg p.o. bid, ofloxacin 200 mg p.o. bid) (42). Therapy may also require coverage for Pseudomonas, depending upon culture results. Examples of effective oral therapy include clindamycin (300mg p.o qid) or metronidazole (250-500 mg p.o. q.i.d.) in combination with a fluoroquinolone, or monotherapy with amoxicillin/clavulanate (500mg p.o t.i.d. or 875mg p.o. b.i.d.), or trovafloxacin (200 mg p.o. q.d.) when Bacteroidesspp are involved (42,62,86).

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Special Situations

Bacteroides spp can be the sole cause of infection in certain rare clinical situations requiring special consideration. Endocarditis, osteomyelitis, septic arthritis, and meningitis are four such examples. In addition, brain abscesses caused by B. fragilis alone, or in a mixed infection require special consideration. In all circumstances, antibiotic susceptibility of B. fragilis should be tested to determine the most appropriate antibiotic.

Endocarditis, Pericarditis, and Vascular Graft Infections

Bacteroides fragilis is one of the most common causes of anaerobic infective endocarditis, although overall quite rare. It is usually associated with large valvular vegetations and peripheral embolization (38). Thrombophlebitis and congestive heart failure are common complications as well (73). Mortality is reported to be as high as 46% (33).

Drug of Choice: Bactericidal antimicrobials are indicated in treatment of anaerobic infective endocarditis. In experimental endocarditis due to B. fragilismetronidazole alone or in combination withclindamycin were superior to clindamycin or cefoxitin alone (45). The potent activity of carbapenems, and possibly beta-lactam/beta-lactamase inhibitors may also prove effective. In one report of one patient, imipenem was administered, followed by a combination of ampicillin/clavulanate and metronidazole with a successful outcome (60). B. fragilis is an extremely rare cause of pericarditis, mycotic aneurysm, and vascular graft infection. All three clinical situations are associated with a high mortality, and no specific data on antimicrobial efficacy is available (65,99,101). This author suggests choosing the most potent antimicrobial that is bactericidal (such as metronidazole) in conjunction with appropriate surgical intervention.


 B. fragilis is a rare cause of meningitis in the absence of brain abscess and is found most commonly in neonates as a result of congenital malformations, necrotizing enterocolitis, bowel perforation, or shunts. Anaerobes are so rare a cause of meningitis that clinical laboratories do not routinely culture cerebrospinal fluid for them. Thus, the only clue may be positive blood cultures (32).

Drug of Choice: Therapy of B. fragilis meningitis requires bactericidal therapy with metronidazole (500mg i.v. or p.o. q6h). Because of limited or unknown penetration of beta-lactam/beta-lactamase inhibitors and imipenem, these agents cannot be recommended (84,120,121). Further, case reports of B. fragilis meningitis treated with chloramphenicol resulted in death or failure that required a change to metronidazole. Chloramphenicol failure is likely due to its bacteriostatic activity against B. fragilis. Some authors have measured bactericidal activity during therapy as a guide (32).Meropenem has been recently approved for treatment of meningitis. Efficacy against B. fragilis in the setting of meningitis is unknown.

Septic Arthritis

In the largest review of anaerobic septic arthritis, 20 out of 180 cases were caused by B. fragilis (34). Of the Bacteroides cases, most were from hematogenous spread, with the knee as the most common involved joint. One recent report described B. fragilis septic arthritis of the shoulder in a patient with Sickle cell disease (77). Prosthetic joints appear more susceptible to anaerobic infection, primarily with Peptostreptococci followed by B. fragilis (13,72).

Drug of Choice: Therapy for joint infections is directed at the B. fragilis group species and its susceptibility. Given the limited clinical experience, metronidazole (500mg i.v. or p.o q6h), carbapenems (imipenem 500mg i.v. q6h; meropenem 1g i.v. q8h), and beta-lactam/beta-lactamase inhibitor combinations (ampicillin/sulbactam 3g i.v. q6h; ticarcillin/clavulanate 3.1g i.v.q 8h;piperacillin/tazobactam 3.75 g i.v. q6h) are suitable empiric choices until results can be obtained from the laboratory. Duration of therapy is usually 4-6 weeks. Prosthetic joints usually require removal, followed by 4-6 weeks of antibiotic therapy. One case of B. fragilis septic arthritis in a patient with sickle cell disease was treated successfully with four weeks of metronidazole and chloramphenicol, while a second responded to metronidazole and ticarcillin/clavulanic acid (64,77).

Brain Abscess

B. fragilis is a frequently isolated pathogen as a component of a mixed infection in brain abscesses. Previous studies have reported Bacteroides to be isolated as frequently as 20-60% from culture, associated with a number of other pathogens (particularly Streptococci) (12,16,110). This high percentage represents the inclusion of organisms formerly classified as Bacteroides.

Drug of Choice: Historically, treatment of brain abscess has included penicillin (20-24 million units/d) and chloramphenicol (1-1.5 g i.v. q6h) as empiric therapy (110). However, while penicillin remains a mainstay of therapy, chloramphenicol has been largely replaced with metronidazole. Metronidazole (500mg i.v. or p.o q6h) provides several advantages, including its bactericidal activity and excellent concentration in brain abscess pus (2,59,100,110). It is not affected by steroids, and the superior outcomes may reflect degradation of chloramphenicol in pus. No randomized trials have been conducted to compare these two agents. Patients suspected to have Staphylococci as a component of the abscess should be treated with nafcillin in place of penicillin, along with metronidazole. Alternatives for penicillin allergic patients include cefotaxime (3 g i.v. q8h) or ceftriazone (2-4 g / day). Duration of therapy, when surgery is performed, is dependent upon clinical and radiologic response. Generally, 4-6 weeks of therapy is required. When surgical drainage is not performed, longer duration may be attempted (6-8 weeks). Although some patients respond to medical management alone, most require surgical therapy. Aspiration and drainage by steriotaxic C.T. guidance may suffice in conjunction with antibiotics for stable patients without progressive neurologic signs (93).


The site of infection is important when B. fragilis is suspected as part of a polymicrobial aerobic/anaerobic osteomyelitis. Trauma and direct spread from contiguous foci are the principle indicative factors for development of anaerobic osteomyelitis. B. fragilis is most frequently associated with osteomyelitis when peripheral vascular disease secondary to diabetes is present (4). Using careful culture techniques, 70% of diabetic associated osteomyelitis cases are polymicrobial, with B. fragilis as the second most common anaerobe isolated (128). One recent report describes vertebral osteomyelitis complicated by rupture of the aorta and a second vertebral osteomyelitis following a presumed bacteremia secondary to anal dilatation procedure (17,27).

Drug of Choice: Treatment requires identification of infecting organism from bone, and susceptibility testing of B. fragilis group isolates, when possible (13). Empiric therapy should be based upon knowledge of resistance of the Bacteroides spp, and coinfecting organisms. Agents that may be suitable as monotherapy, or in combination, include cefoxitin (2g i.v. q6h), clindamycin (600-900 mg i.v. q8h), metronidazole (500 mg q6-8h), beta-lactam/beta-lactamase inhibitors (ampicillin/sulbactam 3g i.v. q6h; ticarcillin/clavulanate 3.1g i.v. q 8h; piperacillin/tazobactam 3.75 g i.v. q6h) and carbapenems (imipenem, 500 mg i.v. q6h; meropenem 1g i.v. q8h). Moxifloxacin may be an alternative therapeutic choice, although data are not yet available. Therapy should continue for 4-6 weeks, although prolonged oral therapy including amoxicillin/clavulanate (500mg p.o. t.i.d. or 875 mg p.o. b.i.d.) may be required when vascular insufficiency is not corrected.

Underlying Diseases

Patients with intra-abdominal infections due to leakage of bowel contents into the peritoneum (as a result of perforation, erosion from cancer, post-surgical complications, or complication of endoscopy) are highly susceptible to this organism (>85%). Again, these are almost exclusively mixed infections. Patients with diabetes are also susceptible to mixed infections that include B. fragilis. More serious infections in this setting include necrotizing fasciitis and synergistic anaerobic infections. Typically less life threatening, but possibly limb threatening infections include diabetic foot ulcers. In the latter case, the role of B. fragilis in the infectious process is not well defined (42).

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The standard approach to infections involving B. fragilis typically includes both modalities of debridement and antibiotics. Hyperbaric oxygen treatment has been advocated as an adjunct for more serious infections involving B. fragilis group and other anaerobes. In animal experiments, hyperbaric oxygen has been reported to improve survival in mixed aerobic and anaerobic infections (118). However, despite its use, data supporting its role in the treatment of B. fragilis infections is not conclusive (55).


The high degree of association of abscess and B. fragilis infections typically requires follow-up radiographic imaging to monitor resolution of infectious process and the absence of new fluid collections (such as in intra-abdominal infections). Clinical follow-up would be the same as for most infections with resolution of fever and leukocytosis as sentinel indicators.


There are no vaccines currently available.


Prophylaxis for gastrointestinal surgical procedures is important in prevention of mixed infections including B. frgailis. Current recommendations include oral neomycin and erythromycin base (1g each at 1pm, 2pm and 11pm the day before an 8am operation) as first line agents for elective procedures, with cefoxitin (1 g IV) or cefazolin (1 g IV) + metronidazole (0.5g IV) as second line agents (68). Other general measures for prevention of infections involving B. fragilis include good skin care in diabetic and bed ridden patients to prevent the development of pressure ulcerations.


The role of B. fragilis has been well established for many types of infection. However, as a component of a polymicrobial infection, it is difficult to determine if the organism is always playing a dominant role. In the most serious infections involving B. fragilis, there appears to be good correlation between susceptibility test results and clinical response. However, it is more difficult to correlate specific susceptibility results with clinical outcomes in individual cases. The clinical outcome may be difficult to assess because of many influencing factors. Some patients may respond without antimicrobial therapy, or to inappropriate chemotherapy, with or without surgical intervention. The effects of surgical drainage, debridement, and other procedures are also important. Improper surgical management may result in poor clinical outcome even when antimicrobial therapy is adequate, and proper surgical management may result in a good outcome, despite inappropriate antimicrobial therapy (75). Recently, the correlation of poor microbiologic outcomes and a strong trend towards increased mortality was reported for patients with Bacteroides spp bacteremia resistant to antibiotics of which they were treated (78). This sentinel study now establishes the importance of antimicrobial susceptibility testing in the empiric and specific treatment of infections involving this group of organisms.

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Table 1. Comparative activity of antimicrobial agents against B. fragilis group species*

 B. fragilis B. distasonis B. thetaiotamicron
Antibiotic NCCLS breakpointsH (S, I) MIC50 MIC90 Range MIC50 MIC90 Range MIC50 MIC90 Range
Ticarcillin (32, 64) 32 128 NA 128 128 NA 64 128 NA
Piperacillin (32, 64) 16 256 NA 64 128 NA 32 128 NA
Cefoxitin (16, 32) 8 32 2 - 256 16 32 4 - 256 8 16 1 - 256
Cefotetan (16, 32) 8 64 2 - 256 128 256 4 - 256 64 256 1 - 256
Ceftizoxime (16, 32) 64 128 4 - 256 32 256 1 - 256 64 256 1 - 256
Imipenem (4, 8) 0.5 0.5 0.015 - 4 0.06 0.25 0.015-0.25 0.5 1 0.03 - 4
Meropenem (4, 8) 0.5 0.5 0.015 - 4 0.06 0.25 0.015-0.25 0.5 1 0.03 - 4
Amox /Clavulanate (4, 8) 1 4 0.25 - 4 2 32 0.125-32 1 2 0.125 - 4
Ampicillin/Sulbactam (8, 16) 2 8 NA 8 32 NA 4 16 NA
Piperacillin/Tazobactam (32, 64) 0.5 4 NA 8 32 NA 8 16 NA
Ticarcillin/Clavulanate (32, 64) 1 8 NA 16 64 NA 4 16 NA
Clindamycin (2, 4) 0.5 128 0.06-128 2 128 0.06-128 4 128 0.06-128
Chloram - phenicol& (8, 16) 1 8 NA 1 8 NA 1 8 NA
Metronidazole (8, 16) 0.5 0.5 0.25 - 1 0.5 0.5 0.25 - 1 0.5 1 0.25 - 1
Trovafloxacin (2, 4) 0.25 2 0.25 - 2 0.5 1 0.125 - 1 0.5 2 0.25 - 4
B. ovatus B. vulgatus
Antibiotic MIC50 MIC90 Range MIC50 MIC90 Range
Ticarcillin 64 128 NA 32 128 NA
Piperacillin 32 128 NA 64 128 NA
Cefoxitin 8 16 2 - 64 4 16 2 - 64
Cefotetan 64 128 2 - 128 4 128 2 - 256
Ceftizoxime 32 256 4 - 256 16 128 1 - 256
Imipenem 0.125 0.25 0.015-0.5 0.25 0.5 0.015-0.5
Meropenem 0.125 0.25 0.015-0.5 0.25 0.5 0.015-0.5
Amox /Clavulanate 2 8 0.25 - 16 2 8 0.25 - 8
Ampicillin/Sulbactam 4 16 NA 4 16 NA
Piperacillin/Tazobactam 8 16 NA 8 16 NA
Ticarcillin/Clavulanate 4 16 NA 2 8 NA
Clindamycin 2 128 0.06-128 0.06 128 0.03-128
Chloram - phenicol& 1 8 NA 1 8  NA
Metronidazole 0.5 1 0.25 - 1 0.5 0.5 0.25 - 1
Trovafloxacin 1 2 0.125 - 2 0.25 4 0.125 - 4


 *  Data based upon various published studies; H,  S, I; Susceptible and intermediate breakpoints in μg/ml; I,  Not available; &,  Drug tested only at screening dilutions

Table 2.  Comparative antimicrobial trials for treatment of intra-abdominal and pelvic infections.

Cefoxitin + amino glycoside vs. clindamycin + amino glycoside     (29,79,115)
Ceftizoxime vs. cefoxitin (7)
Piperacillin vs. cefoxitin (71)
Ampicillin/sulbactam vs. clindamycin + amino glycoside (108)
Imipenem vs. clindamycin + amino glycoside (98,105)
Aztreonam + clindamycin vs. clindamycin + amino glycoside (8,129)
Aztreonam + clindamycin vs. imipenem (26)
Ticarcillin/clavulanate vs. clindamycin amino glycoside (40)
Piperacillin/tazobactam vs. imipenem (9)
Piperacillin/tazobactam vs. clindamycin + amino glycoside (89)
Meropenem vs. clindamycin + tobramycin (22)
Meropenem vs. cefotaxime + metronidazole (63)
Piperacillin vs. cefoxitin (112)
Piperacillin vs. clindamycin + gentamicin (43)
Piperacillint/tazobactam vs. clindamycin + gentamicin (113)
Ampicillin/sulbactam vs. clindamycin (67)
Imipenem vs. clindamycin gentamicin (66)
Meropenem vs. clindamycin gentamicin (54)

What's New

Kalapila A, et al.  Multidrug-Resistant Bacteriodes fragilisi - Seattle, Washington, 2013.  Morbidity and Mortality Weekly Report (MMWR). 2013;62(34):694-696.

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Clinical Manifestation

Laboratory Diagnosis





Solomkin JS, Mazuski JE, Bradley JS, Rodvold KA, Goldstein EJC, Baron EJ, O'Neill PJ, Chow AW, Dellinger EP, Eachempati SR, Gorbach S, Hilfiker M, May AK, Nathens AB, Sawyer RG, Bartlett JG.  Diagnosis and Management of Complicated Intra-Abdominal Infection In Adults and Children: Guidelines by the Surgical Infection Society and the Infectious Disease Society of America.  Clin Infect Dis 2010;50:133-164.

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Snydman DR, Jacobus NV, et al. Lessons Learned from the Anaerobe Survey: Historical Perspective and Review of the Most Recent Data (2005-2007). Clin Infect Dis. 2010 Jan 1;50 Suppl 1:S26-33.

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Bacteroides species