having the highest numbers of antibiotic resistance mechanisms accompanied by the highest resistance rates amongst anaerobic bacteria. Species of the Bacteroidaceae have displayed increasing resistance to antimicrobial agents such as cefoxitin, clindamycin, metronidazole, carbapenems, & fluoroquinolones. Resistance Reservoirs: Bacteroides species accumulate a variety of antibiotic/antimicrobial resistance genes as they reside in the gastrointestinal tract. This allows the genetic transfer of these genes to other Bacteroides species and possibly other more virulent bacteria leading to an overall increase in multi-drug resistance. This is exacerbated by the tendency of resistance genes to be relatively stable even without the presence of the antibiotic. Epidemiology and pathogenesis The B. fragilis group is the most commonly isolated Bacteroidaceae in anaerobic infections, especially those that originate from the gastrointestinal microbiota. B. fragilis is the most prevalent organism in the B. fragilis group, accounting for 41% to 78% of the isolates of the group. These organisms are resistant to penicillin by virtue of production of beta-lactamase, and by other unknown factors. This group was formerly classified as subspecies of B. fragilis (i.e. B. f. ssp. fragilis, B. f. ssp. distasonis, B. f. ssp. ovatus, B. f. ssp. thetaiotaomicron, and B. f. ssp. vulgatus). They have been reclassified into distinct species on the basis of DNA homology studies. B. fragilis (formerly known as B. f. ssp. fragilis) is often recovered from blood, pleural fluid, peritoneal fluid, wounds, and brain abscesses. Although the B. fragilis group is the most common species found in clinical specimens, it is the least common Bacteroides present in fecal microbiota, comprising only 0.5% of the bacteria present in stool. Their pathogenicity partly results from their ability to produce capsular polysaccharide, which is protective against phagocytosis and stimulates abscess formation. Bacteroides fragilis is involved in 90% of anaerobic peritoneal infections. It also causes bacteremia associated with intra-abdominal infections, peritonitis and abscesses following rupture of viscus, and subcutaneous abscesses or burns near the anus. Though it is gram negative, it has an altered LPS and does not cause endotoxic shock. Untreated B. fragilis infections have a 60% mortality rate. See also List of oncogenic bacteria Infectious causes of cancer Pathogenic bacteria References External links Bacteroides references in Baron's Medical Microbiology (online at the NCBI bookshelf). Type strain of Bacteroides fragilis at BacDive - the Bacterial Diversity Metadatabase Bacteroidia Gram-negative bacteria Medically important anaerobes Gut flora bacteria