ABSTRACT
The traditional classiŠcation of chlamydiae was mainly based on host susceptibility and the biological properties of the pathogens.1,2 But in the past three decades, molecular phylogenetic investigations have revolutionized our view of the taxonomic relationships of microorganisms.3-5 Studies of phylogenetic relationships between the major outer membrane protein (ompA) genes of chlamydiae laid the groundwork for the classiŠcation based on genetic relatedness.6 Further phylogenetic analyses of other chlamydial genes, most notably the ribosomal RNA genes, demonstrated essentially identical evolutionary relationships among chlamydiae. A subsequent classiŠcation scheme, based on the sequence differences of the 16S and 23S ribosomal RNA genes, separated the family Chlamydiaceae into two genera, Chlamydia and Chlamydophila7-10 (Figure 46.1). The reclassiŠcation of Chlamydiaceae into two genera did not meet with wide approval from scientiŠc colleagues, public health workers, and funding agencies.11,12 Therefore, reclassiŠcation of Chlamydiaceae into a single genus Chlamydia has been proposed, containing nine species named by the current species epithets.12
