Current methods used to classify strains, including biological, morphological, and DNA hybridization techniques and major outer membrane protein (ompl) gene analysis, can be imprecise or difficult to perform. To facilitate classification, 2.8-kb partial ribosomal DNA (rDNA) segments from a strain and a strain were amplified by PCR and sequenced. Subsequently, a 1, 320-bp region in this segment, including both the 16S/23S intergenic spacer (232 ± 11 bp) and domain I (620 ± 2 bp) of the 23S gene, was sequenced from 41 additional strains and from the chlamydia-like organisms sp. strains “Z” and “Zl.” When both parsimony and distance analyses were performed, these sequences were found to have variable regions that grouped the isolates into two lineages ( and non-) and nine distinct genotypic groups. The lineage included human, swine, and mouse-hamster groups. The non- lineage included , and abortion, avian, feline, and guinea pig groups. These nine groups were essentially equidistant from the genetic root and were congruent with groups identified previously by using DNA-DNA homology, genomic restriction endonuclease analysis, host specificity, tissue specificity, and/or disease production. Phylogenetic trees based on the intergenic spacer or on domain I were congruent with trees previously derived from sequences. DNA sequence analysis of either the intergenic spacer or domain I provides a rapid and reproducible method for identifying, grouping, and classifying chlamydial strains.


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