A Perspective on S-Layer Research

It is almost forty years since Houwink (1953) demonstrated by electron microscopy a monolayer of macromolecules in a regular array, which we would now call an S-layer, on the external surface of the cell wall of an unidentified Spirillum. It was an elegant oddity for only a short time; within three years shadow-cast preparations displayed arrays on representatives of four more distinct genera. An ever increasing number of genera and species have been added to the list and continue to be recognized as having hexagonal (p6), tetragonal (p4), trimeric (p3), and oblique (p2) lattice forms; to these must be added some with layer upon layer, even with different symmetries expressed in each layer. Messner and Sleytr (1992) have tabulated from the literature more than 300 strains representing some 93 genera belonging to all the major phylogenetic groups of prokaryotes including Archaea. Forty years of research have generated a substantial body of knowledge which is available in several recent reviews on S-layers (Messner and Sleytr, 1992; Koval, 1988; Smit, 1986) as well as on the range of surface components of bacterial walls (Beveridge and Graham, 1991). More specialized areas of study include the self-assembly of units to form S-layers (Sleytr and Messner, 1989), participation of carbohydrates in the macromolecules (Messner and Sleytr, 1991), and the analysis of computer-enhanced micrographic images with generation of 3-D models (Hovmoller et al., 1988). The status of work in hand at the time of the previous workshop is published in a book (Sleytr et al., 1988) and this present volume is for current observations and concepts.