OUTER-MEMBRANE PROTEIN-A GENE SEQUENCING DEMONSTRATES THE POLYPHYLETIC NATURE OF KOALA CHLAMYDIA-PECORUM ISOLATES

Chlamydia are considered to be the most important pathogen of koalas i n which they cause ocular and urogenital infections. As recently as 19 96 it was realised that koala chlamydial infections do not belong to t he species Chlamydia psittaci but instead should be reassigned to the species C. pecorum and C. pneumoniae. We have used DNA sequence analys is of parr of the chlamydial major outer membrane protein gene, ompA V D4, to compare 15 koala C. pecorum isolates. Unexpectedly, we found th at the koala isolates did not cluster as a single branch in the C. pec orum tree, but instead were represented by five genetically very disti nct genotypes. Two of the genotypes (which contained five koala isolat es each) were koala-specific whereas one genotype contained a single k oala isolate plus three sheep and two cattle isolates. For all five ko ala genotypes, their nearest relatives were not other koala genotypes, but sheep, cattle or pig isolates. It may be inferred from our data t hat C. pecorum strains infecting koalas do not form a monophyletic gro up with respect to other C. pecorum strains, and therefore the model w hich states that there was a single acquisition of a C. pecorum infect ion by a koala and that all C. pecorum strains now infecting koalas ar e descended from that founding strain is unlikely to be correct. The m ost plausible model is that koalas have obtained C. pecorum infections as a result of a series of cross-species transmission events, possibl y from pigs and/or ruminants.