Bacteroides fragilis Group

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B. fragilis group is the most prevalent bacteriodaceae isolated. B. fragilis is the most prevalent organism in the B. fragilis group, accounting for 41% to 78% of the isolates of the group. However, it should be remembered that the other members of the group account for the rest of the B. fragilis group isolates. The relative distribution of the different B. fragilis group has important clinical implications in the management of infections involving anaerobic bacteria. This is because of the different antimicrobial susceptibility of various B. fragilis group members. Although members of fragilis group produce beta-lactamase and resist

Bacteroides Fragilis Group

penicillin, their susceptibility to cephalosporins is variable (2) but predictable. Other B.fragilis group also has variable resistance to penicillins and cephalosporins.

The B. fragilis group is the species of Bacteroidaceae that occur with greatest frequency in clinical specimens. These organisms are resistant to penicillin by virtue of production of beta-lactamase and by other unknown factors (55). This organism was formerly classified as subspecies of B. fragilis (i.e., ss. fragilis, ss. distasonis, ss. ovatus, ss. thetaiotaomicron, and ss. vulgatus). They have been reclassified into distinct species on the basis of DNA homology studies (1,56). B. fragilis (formerly known as B. fragilis ss. fragilis, one of the subspecies of B.fragilis) is the anaerobe most frequently isolated from infections (Fig. 2).

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 (57). Because of its presence in normal flora of the gastrointestinal tract, this organism is predominant in bacteremia associated with intra-abdominal infections (2,32), peritonitis and abscesses following rupture of viscus (18,19), and subcutaneous abscesses or burns near the anus (58,59). Although B. fragilis is not generally found as part of the normal oral flora, it can colonize the oral cavity of patients with poor oral hygiene or of those who previously received antimicrobial therapy, especially penicillin. Following the colonization of the oropharyngeal cavity, these organisms also can be recovered from infections that originate in this area such as aspiration pneumonia (38,60), lung abscess (60,61), chronic otitis media (14), brain abscess (37), and subcutaneous abscess or burns near the oral cavity (58,59).

B. fragilis can be recovered from infectious processes in the newborn. The newborn infant is at risk of developing these infections when born to a mother with amnionitis, experienced premature rupture of membranes, or acquire the infection during the newborn's passage through the birth canal, where B. fragilis is part of the normal flora (62). B. fragilis was recovered from newborns with aspiration pneumonia (63), bacteremia (11), omphalitis (64), and subcutaneous abscesses and occipital osteomylitis following fetal monitoring (65). Bilophila wadsworthia and Centipeda periodontii are new genuses and species found in abdominal and endodontic infections respectedly (66).

Prevotella oralis is part of the normal flora of the mouth and vagina. Unlike B. fragilis, however, strains of P. oralis generally are susceptible to penicillin and the cephalosporins, although more strains of P. oralis have shown resistance to these drugs. P. oralis almost never is found in pure culture in clinical infection. This organism can possess a capsule (67). It has been recovered from almost all types of respiratory tract and subcutaneous infections, including aspiration pneumonia (38), lung abscess (61), chronic otitis media (14), and sinusitis (15), and subcutaneous abscesses around the oral cavity (58).

Pigmented Prevotella and Porphyromonas require the presence of both hemin and vitamin Kj for growth. The requirement for vitamin Kj in vivo often is met by coexistence with organisms that are capable of supplying this need Pigmented Prevotella and Porphyromonas are part of the normal oral and vaginal flora and are the predominant anaerobic gram negative bacilli isolated from respiratory infections. These include aspiration pneumonia (38), lung abscess (61), chronic otitis media (14), and chronic sinusitis (15). These organisms have been recovered also from abscesses and burns around the oral cavity (58), human bites (68), paronychia (69), urinary tract infection (70), brain abscesses (37), and osteomyelitis (71). Also, they have been isolated from patients with bacteremia associated with infections of the upper respiratory tract (11). Pigmented Prevotella and Porphyromonas play a major role in the pathogenesis of periodontal disease (72) and periodontal abscesses (73).

Of the pigmented Prevotella and Porphyromonas, Porphyromonas asaccharolytica is generally the most frequent clinical isolate. Prevotella intermedia is identified less frequently, and Prevotella melaninogenica is the least common. The presence of capsular material suppresses phagocytosis and is therefore an important factor influencing the pathogenicity of the pigmented Prevotella and Porphyromonas (67,74,75). Porphyromonas gingivalis is very similar to P. asaccharolytica and only the production of phenylacetic acid by P. gingivalis will differentiate them (76). P. gingivalis is an important isolate in periodontitis (76).

Bacteroides ruminicola ss. brevis also has been recovered from these sites (38,61) as well as from peritonsillar abscesses (17), chronic sinusitis (15), mastoiditis (16), and peritonitis (18). B. ruminicola has recently been divided into Prevotella buccae and Prevotella oris according to their beta-glucosidase activity (76). P. oris strains are generally more resistant to penicillin than P. buccae.

B. bivia and B. disiens are important isolates in obstetrical and gynecological infections. They account for 9% and 1% of all anaerobic gram-negative bacilli isolates.

Bacteroides ureolyticus (formerly called Bacteroides corrodens and related to Campylobacter) characteristically forms small colonies with a zone around or under the colony that has been described as "pitting" of the agar: thus its former name "corrodens." B. ureolyticus is part of the normal flora of the mouth and has been isolated from blood cultures shortly after dental surgery, periodontal abscesses, aspiration pneumonia (38,60), and lung abscesses (60,61).

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