The family simkaniaceae

Abstract

Simkaniaceae, a family that is distinguished phylogenetically within the order Chlamydiales, embraces the genera Simkania, Candidatus Fritschea, and at least a dozen additional lineages with varying depths of characterization. Like all Chlamydiae, Simkaniaceae are obligately intracellular bacteria found in association with eukaryotes, in which they replicate. Known natural eukaryotic hosts include humans, whiteflies, scale insects, and the benthic marine animal Xenoturbella. The effect of Simkaniaceae on hosts may be pathogenic, commensal, or mutualistic and they survive and replicate at temperatures ranging from 1.9 °C to 37 °C. The type strain S. negevensis (ATCC VR-1471), for example, has been shown experimentally to infect and replicate in human cells, insect cells, and Acanthamoeba polyphaga. Morphological characterization using transmission electron microscopy has been carried out for Simkania negevensis, Ca. Fritschea, and symbionts of Xenoturbella. These data show classic chlamydial replication involving elementary bodies (EBs) and reticulate bodies (RBs). However, unlike other RBs, those of S. negevensis are infectious. Chemotaxonomic properties have been examined by molecular analysis and inferred from the complete genome sequence of S. negevensis and partial sequence of Ca. Fritschea bemisiae. DNA sequence analysis is routinely used to identify Simkaniaceae. The full-length rRNA genes of all Simkaniaceae have 16S or 23S rDNA sequences at least 90 % identical to those of S. negevensis and at least 80 % identical to sequence accession number NR-036834.1 (16S) or U76710.2 (23S). Algorithms applied to Chlamydiales 16S rRNA sequences show that S. negevensis branches deeply relative to other chlamydial type strains. Within Simkaniaceae, full-length rRNA gene sequences are available for S. negevensis, Ca. Fritschea, symbionts of Xenoturbella, and other lineages. Partial rRNA sequences are available for other clades. Unlike all other Chlamydiae and most other bacterial lineages, three Simkaniaceae species have a group I intron in the I-CpaI target site of the 23S rRNA gene (position 1931, Escherichia coli numbering). Many proteins coded in the S. negevensis genome and an exonuclease coded in these introns show significant similarities to coding sequences in cyanobacteria, chloroplasts, and mitochondria.