Bacteroides

Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]

Overview

Bacteroides
Bacteroides spp. anaerobically cultured in blood agar medium.
Scientific classification
Kingdom: Bacteria Phylum: Bacteroidetes Class: Bacteroidetes Order: Bacteroidales Family: Bacteroidaceae Genus: Bacteroides Castellani & Chalmers 1919
Species
B. acidifaciens B. distasonis (reclassified as Parabacteroides distasonis) B. gracilis B. fragilis B. oris B. ovatus B. putredinis B. pyogenes B. stercoris B. suis B. tectus B. thetaiotaomicron B. vulgatus etc.

Overview

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.

Pathogenesis

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.

B. fragilis

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).

Treatment

Antimicrobial regimen

• Bacteroides fragilis

• 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

• Preferred regimen: Metronidazole 0.75-1.0 g IV q12h AND Cefotaxime 1.5-2 g IV q6h OR Aztreonam 1-2 g IV q8h OR Ceftriaxone 1 g IV q12h

Gallery

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  This Petri dish culture plate contained a medium of kanamycin menadione blood agar, which had been inoculated with an inoculum from a cervical swab culture specimen. The culture had given rise to pigmented colonies of Prevotella melaninogenica. From Public Health Image Library (PHIL).
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  Under a low magnification of 15x, this image depicts five colonies of Bacteroides variabilis bacteria. From Public Health Image Library (PHIL).
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  Under a low magnification of 15x, this image depicts five colonies of Bacteroides variabilis bacteria. From Public Health Image Library (PHIL).
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  Under a magnification of 956X, this Gram-stained photomicrograph depicts numbers of Gram-negative Bacteroides variabilis bacteria. From Public Health Image Library (PHIL).
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  Under a low magnification of 15x, this image depicts a single colony of Bacteroides terebrans bacteria. From Public Health Image Library (PHIL).
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  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).
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  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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  Under a magnification of 956X, this Gram-stained photomicrograph depicts numbers of Prevotella melaninogenica, formerly known as Bacteroides melaninogenicus, bacteria, which had been grown on thioglycollate medium for a 72 hour time period. From Public Health Image Library (PHIL).
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  This image depicts three Bacteroides bivius bacterial cultures, which had been grown on a blood agar plate (BAP) for a 48 hour time period. From Public Health Image Library (PHIL).
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  This digitally-colorized scanning electron micrograph (SEM) depicted a Gram-negative Sebaldella termitidis bacterium. From Public Health Image Library (PHIL).
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  This digitally-colorized scanning electron micrograph (SEM) depicted a small grouping of Gram-negative Sebaldella termitidis bacteria.From Public Health Image Library (PHIL).
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  This digitally-colorized scanning electron micrograph (SEM) depicted a small grouping of Gram-negative Sebaldella termitidis bacteria.From Public Health Image Library (PHIL).
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  This digitally-colorized scanning electron micrograph (SEM) depicted a Gram-negative Sebaldella termitidis bacteria, as it was about to enter the process of cell division. From Public Health Image Library (PHIL).
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  This digitally-colorized scanning electron micrograph (SEM) depicted numbers of Gram-negative Sebaldella termitidis bacteria. From Public Health Image Library (PHIL).
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  This digitally-colorized scanning electron micrograph (SEM) depicted two Gram-negative Sebaldella termitidis bacteria. From Public Health Image Library (PHIL).
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  This digitally-colorized scanning electron micrograph (SEM) depicted a small grouping of Gram-negative Sebaldella termitidis bacteria. From Public Health Image Library (PHIL).
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  This digitally-colorized scanning electron micrograph (SEM) depicted a Gram-negative Sebaldella termitidis bacterium, which was in the process of dividing into two separate organisms. From Public Health Image Library (PHIL).
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  Magnified 1000X, this photomicrograph depicted numbers of Gram-negative Bacteroides fragilis subsp. fragilis bacteria, which had been cultured on Schaedler agar medium, and processed using the Gram-stain method. From Public Health Image Library (PHIL).
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  his 1972 photograph revealed the morphology displayed by two colonies of Bacteroides hypermegas bacteria that had been grown on blood agar medium for 48 hours. From Public Health Image Library (PHIL).
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  This 1972 photograph depicts two Bacteroides fragilis subsp. fragilis bacterial cultures grown on blood agar medium for 48 hours. From Public Health Image Library (PHIL).
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  This micrograph depicts Bacteroides clostridiforme subsp. girans bacteria that had been cultured in thioglycollate broth medium for 48 hours. From Public Health Image Library (PHIL).
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  This micrograph depicts Bacteroides fragilis subsp. thetaiota bacteria that had been cultured in blood agar medium for 48 hours. From Public Health Image Library (PHIL).
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  This micrograph depicts Bacteroides biacutis bacteria cultured in blood agar medium for 48 hours. From Public Health Image Library (PHIL).
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  This micrograph depicts Bacteroides biacutis bacteria cultured in blood agar medium for 48 hours. From Public Health Image Library (PHIL).
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  This micrograph depicts Bacteroides fragilis ss. fragilis bacteria cultured in blood agar medium for 48 hours. From Public Health Image Library (PHIL).
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  This micrograph depicts Bacteroides fragilis ss. fragilis bacteria cultured in blood agar medium for 48 hours. From Public Health Image Library (PHIL).
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  This micrograph depicts Bacteroides fragilis subsp. distasonis bacteria cultured in blood agar medium for 48 hours. From Public Health Image Library (PHIL).
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  This photomicrograph shows Fusobacterium novum after being cultured in a thioglycollate medium for 48 hours. From Public Health Image Library (PHIL).
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  Magnified 1000X, this micrograph depicts Bacteroides fragilis subsp. fragilis bacteria that had been grown in Schaedler’s broth, and processed using the Gram-stain method. From Public Health Image Library (PHIL).
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  This photomicrograph shows the bacterium Bacteroides biacutus cultured in a thioglycollate medium for 48 hours. From Public Health Image Library (PHIL).

See also

• Flavobacterium
• Cytophaga

External links

• Bacteroides references in Baron's Medical Microbiology (online at the NCBI bookshelf).
• Bacteriodes in detail.