Structure–functional analysis of the Dictyoglomus cell envelope

Several closely related strains of the thermophilic bacterium Dictyoglomus have been isolated from various hot springs on the Philippine archipelago. These strains as well as Dictyoglomus thermophilum H-6-12 were analyzed in view of the structure–functional relationships of the cell envelopes. All envelopes of Dictyoglomus strains show several peculiar features that are apparently either unique for the genus or common for other phylogenetically related Thermotogales. The filamentous cells develop pili at the cell poles, guided by large columnar protein assemblies that traverse the periplasm. Filamentous protein complexes span the periplasmic space at the longitudinal sides of the cell. By the end of the exponential growth phase, Dictyoglomus strains form multicellular aggregates (“rotund bodies”) inside a compartment surrounded by a single, continuous outer envelope. The formation of these rotund bodies which are also found in some other deeply branching thermophilic phyla (Thermotoga, Thermus) was studied in detail. The transition between unicellular and multicellular lifestyle can be explained by the partial detachment of a protoplast from the outer envelope during cell division. When the outer envelope is partially detached from the protoplast, mechanical forces generated by protoplast elongation may drive cell rearrangement of daughter cells inside the compartment. During the following rounds of cell division, the overall shape of the compartment changes from spindle-like to globular geometry. Analysis of subcellular fractions of Dictyoglomus cells shows that glucan hydrolases are associated with the compartment. This feature is discussed in view of the multicellular life style of Dictyoglomus.