Ultrastructure of purple membrane and cell wall of Halobacterium halobium

Freeze-substitution studies of bacteria

Typically, models of bacterial structure combine biochemical data obtained from bulk analyses of cell populations with electron microscopic observation of individual cells. Recent development of a battery of cryotechniques specific for biological electron microscopy have begun to supercede routine procedures such as conventional thin sectioning. One of these cryotechniques, freeze-substitution, combines the advantages of ultrarapid freezing with standard microtomy methods. This technique is particularly well suited to the examination of bacterial structure and has yielded additional ultrastructural information consistent with biochemical data but often challenging models of cell structure obtained from conventional microscopical methods. In addition to retaining more accurately the spatial distribution of cell components, freeze-substitution has been successfully combined with immunochemical labelling techniques and has enabled identification and localization of specific molecules both within the cell and on the cell surface. In this review, I describe current ideas on bacterial ultrastructure, modified in accordance with new data obtained from recent freeze-substitution studies.

Photoreceptor disk membranes of Lampetra japonica

With the aim of characterizing photoreceptor outer segments and obtaining in situ observation of macromolecular variations due to cell types as well as adaption, we counted the number of outer segment disk membranes using electron micrographs of ultrathin sections as well as intramembrane particles on the complementary replicas of the retina of Lampetra japonica. Long photoreceptor cells (LPCs, cone-type cells) numbered 10,000/mm2 in the central as well as peripheral regions, while short ones (SPCs, rod-type cells) numbered 30,000/mm2 in the same regions. The LPC outer segment exhibited 306 disks on average during the light cycle versus 364 during the dark cycle. 12.0% of the LPC disks during the light cycle versus 13.4% during the dark cycle represented the "open" disks. The SPC outer segment exhibited 470 disks on average during the light cycle versus 507 during the dark cycle. 11.1% of the SPC disks during the light cycle versus 13.6% during the dark cycle represented the "open" disks. The LPC disk membrane contained 44.3 particles/0.01 microns 2 during the light cycle versus 39.5 particles during the dark cycle, 95% of which were derived from the protoplasmic fracture (PF) face. The SPCs contained 36.0 particles/0.01 micron 2 during the light cycle versus 43.6 during the dark cycle, 90% of which were derived from the PF-face. The present findings contradict the frequently cited hypothesis that an "open" disk, retaining continuity with the plasmalemma, is preserved characteristically into later stages by the cone outer segment. The significance of the intramembrane particles for the activity of the photoreceptor membrane is discussed.

Ultrastructure of square bacteria from a brine pool in Southern Sinai

The square bacterium discovered by Walsby (Nature [London] 283:69-71, 1980) has been shown to possess the ultrastructural features of a typical halophile. The cell wall is comprised of regularly arranged subunits demonstrated by thin sectioning, shadowed replicas, and negative staining. Optical diffraction confirms the existence of both hexagonal and tetragonal arrangements of the cell wall subunits and also of different lattice constants and suggests a mixed population of bacteria.

Single bacteriorhodopsin molecules revealed on both surfaces of freeze-dried and heavy metal-decorated purple membranes

The flat sheets of the purple membrane from Halobacterium halobium contain only a single protein (bacteriorhodopsin) arranged in a hexagonal lattice. After freeze-drying at -80 degrees C (a method that is superior to air-drying), shadowing with tantalum/tungsten, and image processing, structural details on both surfaces are portrayed in the range of 2 nm. One surface is rough and lattice lines are clearly visible, whereas the other is smooth and the hexagonal order seems to be absent. The optical diffraction patterns, however, indicate a hexagonal lattice for both surfaces. In addition, these diffraction patterns are characteristic and easily distinguished. The orientation of the two surfaces was identified by silver decoration: partial condensation of silver on purple membranes enabled the smooth surface to be identified as the plasmatic and the rough surface as the exoplasmic surface. After image processing, the exoplasmic surface shows a triplet structure which exactly fits the projected structure determined by Unwin and Henderson (1975. Nature(Lond.). 257:28-32) at molecular resolution, whereas, on the plasmatic surface, four image details per unit cell are visible. Three of them match the arrangement of bacteriorhodopsin, whereas the fourth must be located over a lipidic array. Summarizing these results, it is possible to show the part of each single bacteriorhodopsin protein that is present in the surfaces of the purple membrane. By "shadowing" the membranes perpendicularly, we prove that these components of the surfaces are mainly portrayed by a decoration effect of the tantalum/tungsten condensate.