Gram negative Non Sporing Anaerobes


Historically, all short Gram-negative anaerobic rods of coccobacilli have been classified in the genus Bacteroides and longer rods with tapering end in the genus Fusobacterium. Recent applications of new technique to the Bacteroides have resulted in the definition of two additional genera; Porphyromonas and Prevotella. The genus Bacteroides is now restricted to species found among the normal gut flora. Prevotella contains saccharolytic oral and genitourinary species, including Pr. melaninogenicus, which produces a characteristic black-brown pigment. The genus Porphyromonas contains asaccharolytic-pigmented species, which form part of the normal mouth flora (P. gingivalis) and may be involved in endogenous infection within the oral cavity. Most important non-sporing anaerobe causing infection is Bacteroides fragilis although others are much more common (e.g. in gingivitis and other endogenous oral infections).
Bacteroides fragilis



  • Small
  • Pleomorphic
  • Gram-negative
  • Rod
  • Capable only of anaerobic respiration
  • Non-spore forming
  • Non-motile


  • Endogenous infection arising from contamination by gut contents or feces is most common route of acquisition.


  • Intra-abdominal sepsis
  • Liver abscesses
  • Aspiration pneumonia
  • Brain abscesses
  • Wound infections
  • Infections often mixed with aerobic and microaerophilic bacteria.

Laboratory Identification:

  • Grows on blood agar incubated anaerobically and in other media designed for isolation of anaerobes.
  • Plates may require up to 48 h. Incubation at 35 ºC for colonies to become visible.
  • Cultures have a foul odor due to the fatty acid end products of metabolism.
  • These can be used as identifying characteristics by analysis of culture supernates by gas-liquid chromatography (GLC).
  • The major products of Bacteroides are acetate and succinate.
  • Full identification in the diagnostic laboratory is based on biochemical tests and antibiogram.