Morphology and cultivation

MORPHOLOGY AND CULTIVATION

Morphology

  • Rickettsiae are small, non-motile, non-capsulated pleomorphic, coccobacillary (0.3 – 0.6 x 0.8-2µm in size) forms existing as obligate intra cellular parasite.
  • Under the EM, the rickettsiae are seen to have a 3 layered cell wall and trilaminar plasma membrane, thus resembling Gram –ve bacteria.
  • They stain reasonably well with Giemsa, Castaneda, (bluish purple), Gimenez, Machiavello (deep red), and Leishman stains, but poorly with Gram’s stain.

Cultivation

  • Rickettsiae multiply by simple binary fission. They have cytochromes and their metabolic reactions are aerobic.
  • They possess many of the metabolic functions of bacteria but require exogenous co- factors from animal cells.
  • Rickettsiae can genrate their own energy, but they also depend on their host for some energy.
  • Rickettsiales require living cells for replication. They are readily cultivated in the yolk sac of developing chicken embryo (first shown by Cox), or in cell lines like mouse fibroblast, HeLa and HEp2.
  • Growth generally occurs in the cytoplasm of the infected cells or in some cases (spotted fever) in the nucleus.
  • E.canis can be propagated very well in dog monocytes culture.
  • Rochalimae quintana –the only rickettsiae which have the ability to grow on blood agar.
  • Guinea pig and Mice are useful for primary isolation.
Last modified: Monday, 4 June 2012, 5:50 AM