A morphological study of Halobacterium halobium and its lysis in media of low salt concentration

Abstract

The reported absence of a cell wall in halobacteria cannot be confirmed. Improved fixation techniques clearly show a cell wall-like structure on the surface of these cells. A stepwise reduction of the salt concentration causes the release of cell wall material before the cell membrane begins to disintegrate. The cell membrane breaks up into fragments of variable but rather small size, which are clearly different from a 4S component reported by others to be the major breakdown product of the cell membrane. It appears more likely that the 4S component arises from the dissolution of the cell wall. A residue of large membranous sheets remains even after prolonged exposure of halobacteria envelopes to distilled water. The lipids in these sheets do not differ significantly from the lipids in the lysed part of the cell membrane. The sheets, however, contain a purple-colored substance, which is not present in the lysed part. The easily sedimentable residue that remains after lysis of the cells or envelopes in distilled water also contains "intracytoplasmic membranes" with unusual structural characteristics. They can also be identified in sections through intact bacteria or envelope preparations. Their function is at present unknown but seems to be related to the formation of gas vacuoles in these organisms.