Phenotypic Support for the Phylogenetic Separateness of Chlamydia versus Chlamydophila

Detectable glycogen is produced by Chlamydia but not by Chlamydophila. The fastidious intracellular life style of bacteria belonging to the Chlamydiales prevents the characterization of most other physiological, biochemical and chemotaxonomic traits that are readily studied in free-living bacteria. The most commonly identified phenotypic chlamydial trait used by diagnosticians and researchers is serotype. Most chlamydial isolations have produced single serovars, rather than mixed populations, and most naturally infected host species carry only a limited subset of chlamydial species. This suggests that most Chlamydiaceae species have specific host or disease associations and so Chlamydiaceae species names were derived from a particular disease or host associated with each group. Species are ecologically distinct populations whose members typically compete for the same niche. Among obligate parasites, host specificity can be a natural outcome of this process, taking on the form of differences in virulence, morphology, propagation and chronic maintenance. However, apparent host specificity also may be an artefact of selective testing, selective diagnostic reagents or limited access to new hosts. Thus, although chlamydiae seem to be host-specific, they will try to grow in any cells available. Nearly all species have been found to grow in the yolk sacs of embryonating eggs; a human isolate of C. pneumoniae has been grown in amoebae; ovine and bovine C. pecorum strains and human Ch. trachomatis strains grow in guinea-pigs and mice; avian C. psittaci grow in mammals and tortoises. The condition of the host or the presence of other bacteria or viruses can also play a role in chlamydial disease. So, although long-term adaptation between chlamydiae and animal species has contributed to the evolution of ecologically distinct populations, disease and host, per se, are not definitive markers for purposes of classification.

Genetic traits conclusively distinguish these Chlamydiaceae species. The separation of Chlamydia and Chlamydophila is supported by differences in genome size (they are approximately 1.1 and 1.2 Mbp, respectively) and sequence analysis of rRNAs, ompA (which expresses the major outer-membrane protein, MOMP), kdtA (KDO-transferase), groEL (chaperonin), cysteine-rich lipoprotein, and 60-kDa cysteine-rich protein (R.M. Bush and K.D.E Everett, unpublished data). The large genetic differences among the Chlamydiaceae species and their nearest relatives makes it reasonable to suppose that chlamydiae radiated from single-celled eukaryotes into multicelled hosts. This model predicts that intense selective pressure in animal hosts has influenced the evolution of the Chlamydiaceae, and is congruent with the large DNA–DNA (deoxyribonucleic acid) reassociation differences that separate the nine Chlamydiaceae species. The phenotypes of some Chlamydiaceae groups have been exhaustively studied and are well known (see below).