Identification and characterization of a novel porin family highlights a major difference in the outer membrane of chlamydial symbionts and pathogens.

The Chlamydiae constitute an evolutionary well separated group of intracellular bacteria comprising important pathogens of humans as well as symbionts of protozoa. The amoeba symbiont Protochlamydia amoebophila lacks a homologue of the most abundant outer membrane protein of the Chlamydiaceae, the major outer membrane protein MOMP, highlighting a major difference between environmental chlamydiae and their pathogenic counterparts. We recently identified a novel family of putative porins encoded in the genome of P. amoebophila by in silico analysis. Two of these Protochlamydiaouter membrane proteins, PomS (pc1489) and PomT (pc1077), are highly abundant in outer membrane preparations of this organism. Here we show that all four members of this putative porin family are toxic when expressed in the heterologous host Escherichia coli. Immunofluorescence analysis using antibodies against heterologously expressed PomT and PomS purified directly from elementary bodies, respectively, demonstrated the location of both proteins in the outer membrane of P. amoebophila. The location of the most abundant protein PomS was further confirmed by immuno-transmission electron microscopy. We could show that pomS is transcribed, and the corresponding protein is present in the outer membrane throughout the complete developmental cycle, suggesting an essential role for P. amoebophila. Lipid bilayer measurements demonstrated that PomS functions as a porin with anion-selectivity and a pore size similar to the Chlamydiaceae MOMP. Taken together, our results suggest that PomS, possibly in concert with PomT and other members of this porin family, is the functional equivalent of MOMP in P. amoebophila. This work contributes to our understanding of the adaptations of symbiotic and pathogenic chlamydiae to their different eukaryotic hosts.

Emended description of the order Chlamydiales, proposal of Parachlamydiaceae fam. nov. and Simkaniaceae fam. nov., each containing one monotypic genus, revised taxonomy of the family Chlamydiaceae, including a new genus and five new species, and standards for the identification of organisms.

The current taxonomic classification of Chlamydia is based on limited phenotypic, morphologic and genetic criteria. This classification does not take into account recent analysis of the ribosomal operon or recently identified obligately intracellular organisms that have a chlamydia-like developmental cycle of replication. Neither does it provide a systematic rationale for identifying new strains. In this study, phylogenetic analyses of the 16S and 23S rRNA genes are presented with corroborating genetic and phenotypic information to show that the order Chlamydiales contains at least four distinct groups at the family level and that within the Chlamydiaceae are two distinct lineages which branch into nine separate clusters. In this report a reclassification of the order Chlamydiales and its current taxa is proposed. This proposal retains currently known strains with > 90% 16S rRNA identity in the family Chlamydiaceae and separates other chlamydia-like organisms that have 80-90% 16S rRNA relatedness to the Chlamydiaceae into new families. Chlamydiae that were previously described as 'Candidatus Parachlamydia acanthamoebae' Amann, Springer, Schönhuber, Ludwig, Schmid, Müller and Michel 1997, become members of Parachlamydiaceae fam. nov., Parachlamydia acanthamoebae gen. nov., sp. now. 'Simkania' strain Z becomes the founding member of Simkaniaceae fam. nov., Simkania negevensis gen. nov., sp. nov. The fourth group, which includes strain WSU 86-1044, was left unnamed. The Chlamydiaceae, which currently has only the genus Chlamydia, is divided into two genera, Chlamydia and Chlamydophila gen. nov. Two new species, Chlamydia muridarum sp. nov. and Chlamydia suis sp. nov., join Chlamydia trachomatis in the emended genus Chlamydia. Chlamydophila gen. nov. assimilates the current species, Chlamydia pecorum, Chlamydia pneumoniae and Chlamydia psittaci, to form Chlamydophila pecorum comb. nov., Chlamydophila pneumoniae comb. nov. and Chlamydophila psittaci comb. nov. Three new Chlamydophila species are derived from Chlamydia psittaci: Chlamydophila abortus gen. nov., sp. nov., Chlamydophila caviae gen. nov., sp. nov. and Chlamydophila felis gen. nov., sp. nov. Emended descriptions for the order Chlamydiales and for the family Chlamydiaceae are provided. These families, genera and species are readily distinguished by analysis of signature sequences in the 16S and 23S ribosomal genes.