Bacteroides: Classification, virulence factors and biochemical tests




Characteristics of Bacteroides

  • Gram negative anaerobic rod
  • Shape: Pleuromorphic
  • Size: (0.5-1.5)µm wide and (2-6)µm long
  • Non motile except B. polypragmatus, B. xylanolyticus
  • Non capsulated except fragilis
  • Non spore forming
  • Habitat: Norma flora of gastrointestinal tract, mouth, skin, nasopharyns, Upper respiratory tract, vagina
  • Opportunistic human pathogen

Classification of Bacteroides

  • on the basis of medical importance bacteriodes is classified into two group

1. Bacteroides fragilis group:

  • Examples:
    • B.fragilis
    • B. distasonis
    • B. ovatus,
    • B. thetaiotaomicron
    • B. vulgatus
    • B. idgatus
    • B. uniformis
    • B. tiariabilisleggerthii
    • B. splanchnicus
  • these are commensals of GI tract

2. Bacteroides melaninogenicus group

  • Examples:
    • B. melaninogenicus sub spp intermedius
    • B. ruminicola
  • Now the name has changed to Prevotella melaningenica
  • these are Norma flora of URT, GIT, vagina

Pathogenesis:

1. Mode of transmission:

  • Displacement of their normal habitat

2. Virulence factors of Bacteroides

  • i) Capsule:- Helps in attachment, and resist phagocytosis and complement mediated lysis.
  • ii) Fimbriae: helps in attachment
  • iii) Endotoxin (LPS):- stimulates leucocytes
  • iv) Short chain Fatty acids (succinic acid):- resist phagocytosis and intracellular killing
  • v) Enzymes- protect from Oxygen toxicity
    • Catalase and Superoxide dismutase: These enzymes inactivates H2O2 and release super oxide free radicals there by protect anaerobes when exposed to oxygen
    • Protease
    • Collagenase
    • Phospholipase
    • Neuraminidase
    • Heparinase
    • Haemolysin
    • Fibrinolysin
    • Gluconidase

Clinical manifestation of Bacteroides:

  • Genitourinary tract infection
  • Appendicitis
  • Bacteriaemia
  • Endocarditis, pericarditis, vascular graft infection
  • Meningitis
  • Septic artheritis
  • Bed sore
  • Inflammation of viscent angina ( throat)
  • Skin and soft tissue infection
  • Upper respiratory infection

Laboratory diagnosis of Bacteroides

Samples: pus, exudates, biopsy

1. Microscopy:

  • gram negative, non motile, non sporing pleuromorphic rod

2. Culture:

i) Blood Agar (BA):

  • Kanamycin or neomycin Blood agar selective for Anaerobes
  • Inculates anaerobically at 37C for 48 hours
  • fragilis Form non haemolytic grey colony of 1-3 mm diameter
  • melanogenicus form black brown haemolytic colony in 3-5 days

ii) Bactrroides Bile Esculin Agar (BBE):

  • hydrolyses esculin,
  • colony surrounded by dark zone

3. Biochemical tests for fragilis

  • i) Esculin hydrolyses: positive (+)
  • ii) Indole: negative (-)
  • iii) Urease: Negative (-)
  • iv) Catalase: Positive (+)
  • v) Oxidase : variable (+/-)
  • vi) Fermentative: glucose +, lactose +, maltose +, sucose + rhamnose -, Arabinose -, salicin -, trehalose –
  • vii) Bile resistant: grow in 20% bile
  • viii) Thioglycolate with bile: Positive (+)
  • ix) Beta lactamase production: fragilis is resistant to penicillin

4. Serology

5. PCR

Treatment of Bacteroides:

  1. Metronidazole: drug of choice
  2. Cephalosporin
  3. Chloramphenicol

Bacteroides: Classification, virulence factors and biochemical tests