120 plus species including:


Bacteroides fragilis Group
Bacteroides caccae
Bacteroides distasonis
Bacteroides eggerthii
Bacteroides fragilis
Bacteroides merdae
Bacteroides ovatus
Bacteroides thetaiotaomicron
Bacteroides uniformis
Bacteroides vulgatus

Non-pigmented Group
Bacteroides capillosus
Bacteroides coagulans
Bacteroides forsythus
Bacteroides putredinis
Bacteroides splanchnicus
Bacteroides tectum
Bacteroides ureolyticus


Gram Stain: Negative.
Morphology: Pleomorphic rods. Cell morphology varies greatly within the family Bacteroidaceae. Some cells may be filamentous, pleomorphic, vacuolated or may stain irregularly. Cellular morphology is useful for differentiation of some species.
Size: 0.5-1.5 micrometers to 2.0-6.0 micrometers.
Motility: Both non-motile or motile.
Capsules: None.
Spores: None.
Other: Cellular morphology is useful for differentiation of some species.


Bacteroides fragilis have convex colonies that are otherwise similar to those of enteric gram-negative bacilli.


Characteristically fermentative; some strains can carry out anaerobic respiration; most species use sugars; others use peptones.



The predominant organism in the normal human large bowel.


These organisms are prevalent on mucosal surfaces and many produce virulence factors which contribute to pathogenesis. End products of anaerobic metabolism, such as butyrate and succinate, are cytotoxic. Anaerobic or mixed infections result from disruption of mucosal surfaces and introduction of anaerobes into a normally sterile sight. Predisposing factors include surgical procedures, trauma, peripheral vascular disease, carcinomas and prior infection with facultative organisms. Anaerobic infection is characterized by abscess formation and tissue destruction.

Isolated, either with other organisms or as the only organism present, from various types of human clinical specimens including appendicitis, peritonitis, heart valve infections, blood, rectal abscesses, pilonidal cysts, postsurgical wounds and lesions of the urogenital tract; occasionally isolated from the vagina. Although B. fragilis is the most common species of anaerobic bacteria isolated from human soft tissue infections and anaerobic bacteremia, it accounts for less than 1% of the normal human intestinal flora. (2)


For culture: Brucella Agar with H & K, Chocolate Agar, or Brain Heart Infusion (BHI) Agar.
For selective isolation: LKV Agar, BBE Agar, or Anaerobic PEA Agar.
For maintenance: Cooked Meat Medium, Thioglycollate Broth with Supplements, Brucella Agar with H & K, or Brain Heart Infusion (BHI) Agar.


Temperature: 35 degrees C.
Time: 48 hours (and up to 7 days).
Atmosphere: Anaerobic with 5% CO 2 .


1. Holt, J.G., et al. 1994. Bergey's Manual of Determinative Bacteriology , 9th ed. Williams & Wilkins, Baltimore, MD.

2. Holt, J.G., et al. 1986. Bergey's Manual of Systemic Bacteriology , Vol. I & II. Williams & Wilkins, Baltimore, MD.

3. The Oxoid Vade-Mecum of Microbiology . 1993. Unipath Ltd., Basingstoke, UK.

4. Murray, P.R., et al. 1995. Manual of Clinical Microbiology , 6th ed. American Society for Microbiology, Washington, D.C.

5. Internet: /Bacterial Database Search, February, 1998.