Bacteroides spp. anaerobically cultured in blood agar medium.
Bacteroides is a genus of Gram-negative, rod-shaped bacteria. Bacteroides species are non-endospore-forming, anaerobes, and may either be motile or non-motile, depending on the species. The DNA base composition is 40-48% GC. Unusual in bacterial organisms, Bacteroides membranes contain sphingolipids. They also contain meso-diaminopimelic acid in their peptidoglycan layer.
Bacteroides are normally commensal, making up the most substantial portion of the mammalian gastrointestinal flora, where they play a fundamental role in processing of complex molecules to simpler ones in the host intestine. As many as 1010-1011 cells per gram of human feces have been reported. They can use simple sugars when available, but the main source of energy is polysaccharides from plant sources.
Bacteroides species also benefit their host by excluding potential pathogens from colonizing the gut. Some species (B. fragilis, for example) are opportunistic human pathogens, causing infections of the peritoneal cavity, gastrointestinal surgery, and appendicitis via abscess formation, inhibiting phagocytosis, and inactivating beta-lactam antibiotics. Although Bacteroides species are anaerobic, they are aerotolerant and thus can survive in the abdominal cavity.
Bacteroides are generally resistant to a wide variety of antibiotics — beta-lactams, aminoglycosides, and recently many species have acquired resistance to erythromycin and tetracycline. This high level of antibiotic resistance has prompted concerns that Bacteroides species may become a reservoir for resistance in other, more highly pathogenic bacterial strains.
Bacteroides fragilis is an obligate anaerobe of the gut. It is involved in 90% of anaerobic peritoneal infections. B. fragilis is generally susceptible to metronidazole, carbapenems, and beta-lactam/beta-lactamase inhibitor combinations (e.g., Unasyn, Zosyn). The bacteria has inherent high-level resistance to penicillin. Clindamycin is no longer recommended as the first-line agent for B. fragilis due to emerging high-level resistance (>30% in some reports).
- 1. Monotherapy
- Preferred regimen (1): Imipenem
- Preferred regimen (2): Ertapenem
- Preferred regimen (3): Meropenem
- Preferred regimen (4): Doripenem 0.5-1.0 g IV q6h
- Preferred regimen (5): Piperacillin-tazobactam 3.375 g IV q6h
- Preferred regimen (6): Ampicillin-sulbactam 1-2 g IV q6h
- Preferred regimen (7): Tigecycline 100 mg IV THEN 50 mg IV q12h
- 2. Combination therapy
This image depicts a Petri dish culture plate containing a medium of blood agar to which menadione had been added. The dish had been inoculated with Prevotella melaninogenica, formerly known as Bacteroides melaninogenicus bacteria, which was incubated at a temperature of 35°C for a five day period. From Public Health Image Library (PHIL). 
This photomicrograph of an unknown sample revealed the presence of a mixed infection, which involved Gram-positive Peptostreptococcus anaerobius and P. asaccharolyticus (formerly Peptococcus asaccharolyticus), and Gram-negative Prevotella melaninogenica (formerly Bacteroides melaninogenicus), anaerobic bacteria. From Public Health Image Library (PHIL). 
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