Bacteroides

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The page below is a sample from the LabCE course Medically Important Anaerobes. Access the complete course and earn ASCLS P.A.C.E.-approved continuing education credits by subscribing online.

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Bacteroides

Significant Bacteroides species include:
  • B. fragilis - most frequently isolated
  • B. ovatus
  • B. thetaiotaomicron - pigmented on KV agar
Things to remember:
  • Bacteroides spp.are the most commonly encountered anaerobes in clinical specimens.
  • Bacteroides spp. are included in the normal microbiota of the gut.
  • Bacteroides spp.are more virulent and resistant to antimicrobials than most of the other anaerobes.
  • Always look for Bacteroides fragilis group in mixed cultures, as it is usually the significant pathogen.
  • Nomenclature notes: Bacteroides ureolyticus, known for pitting agar, has been renamed Campylobacter ureolyticus; B. vulgatus has been renamed Phoecaeicola vulgatus.
Figure 13. Centers for Disease Control and Prevention. (1974). Under a magnification of 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. [Image]. https://phil.cdc.gov/Details.aspx?pid=11198.
Figure 14. Centers for Disease Control and Prevention. (1972). TwoBacteroides fragilis subsp. fragilisbacterial colonies, which had been cultivated on blood agar medium, for 48-hours. [Image]. https://phil.cdc.gov/Details.aspx?pid=10811.

Figure 13. Gram stain of Bacteroides
Figure 14. Bacteroides fragilis growth on blood agar after 48 hours of anaerobic incubation