Article Summary
Archaea, 1 (2001)
© 2001 Heron PublishingVictoria, Canada
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The ultrastructure of Ignicoccus: Evidence for a novel outer membrane and for intracellular vesicle budding in an archaeon

REINHARD RACHEL,1,2 IRITH WYSCHKONY,1 SABINE RIEHL 1,3 and HARALD HUBER 1

1 Lehrstuhl für Mikrobiologie und Archaeenzentrum, Universität Regensburg, Universitätsstr. 31, D-93053 Regensburg, Germany
2 Author to whom correspondence should be addresssed ([email protected])
3 Present address: Aventis Behring GmbH, Emil-von-Behring-Strasse 76, D-35041 Marburg, Germany

Received July 9, 2001; Accepted August 13, 2001; Published online September 21, 2001

Summary
A novel genus of hyperthermophilic, strictly chemolithotrophic archaea, Ignicoccus, has been described recently, with (so far) three isolates in pure culture. Cells were prepared for ultrastructural investigation by cultivation in cellulose capillaries and processing by high-pressure freezing, freeze-substitution and embedding in Epon. Cells prepared in accordance with this protocol consistently showed a novel cell envelope structure previously unknown among the Archaea: a cytoplasmic membrane; a periplasmic space with a variable width of 20 to 400 nm, containing membrane-bound vesicles; and an outer sheath, approximately 10 nm wide, resembling the outer membrane of Gram-negative bacteria. This sheath contained three types of particles: numerous tightly, irregularly packed single particles, about 8 nm in diameter; pores with a diameter of 24 nm, surrounded by tiny particles, arranged on a ring with a diameter of 130 nm; and clusters of up to eight particles, each particle 12 nm in diameter. Freeze-etched cells exhibited a smooth surface, without a regular pattern, with frequent fracture planes through the outer sheath, indicating the presence of an outer membrane and the absence of an S-layer. The study illustrates the novel complex architecture of the cell envelope of Ignicoccus as well as the importance of elaborate preparation procedures for ultrastructural investigations.

Keywords
cell envelope, cellulose capillary tubes, cryo-fixation, freeze-substitution, hyperthermophilic archaea, membrane vesicles, periplasm.