The planctomycetes: emerging models for microbial ecology, evolution and cell biology

Abstract

The planctomycetes are an unusual but widely distributed group of budding bacteria which are proving to be of increasing relevance to at least three major areas of research in microbiology: microbial ecology, molecular evolution and cell biology. They constitute one of the phylogenetically distinct major 'phyla' of the domain Bacteria defined using 16S rRNA sequence analysis (Woese, 1987), and form a group so deeply branching it has been proposed as a new bacterial order, Planctomycetales, and family, Planctomycetaceae (Schlesner & Stackebrandt, 1986). Cultured planctomycetes which have been analysed with respect to their cell wall chemistry display the unusual feature of not synthesizing the otherwise universal Bacterial cell wall peptidoglycan; this feature is shared only with the chlamydiae and mycoplasmas among the Bacteria (Konig et al., 1984; Liesack et al., 1986). Planctomycetes also exhibit some unusual molecular features, including short 5S rRNA and, in at least two species, unlinked rrn operon organization (Bomar et al., 1988; Liesack & Stackebrandt, 1989). Gemmata obscuriglobus, a budding bacterium isolated from a freshwater dam in Queensland, Australia (Franzmann & Skerman, 1984), is a member of the order Planctomycetales and is thus within the domain Bacteria, yet possesses a membrane-bounded nuclear body, an organelle hitherto known only in the Eucarya. The unique ultrastructure of this bacterium is of great potential significance to understanding the ways in which eukaryote cell organization may have evolved.