Characterisation of the koala biovar of Chlamydia pneumoniae of four gene loci – ompAVD4, ompB, 16S rRNA, groESL spacer region
Wardrop, S, Fowler, A, O’Callaghan, P, Giffard, P & Timms, P 1999, Systematic and Applied Microbiology, vol. 22, pp. 22-27.
While there is significant genetic diversity in Chlamydia pecorum strains that infect koalas, little is known about genetic diversity of C. pneumoniae. DNA sequences of four gene fragments were found to be different between koala, human and horse C. pneumoniae biovars. Two gene loci, ompAVD4 and ompB, were identical among all isolates from koalas. This study, for the first time, reports on a respiratory infection associated with C. pneumoniae in a captive koala population.
Ten C. pneumoniae isolates were obtained from common infection sites (eye, urogenital or nasal) of either free-ranging or captive koalas. The isolates were then sequenced for four gene fragments of interest: ompAVD4, ompB, 16S rRNA and groESL intergenic region. Gene loci for ompAVD4 and ompB were 100% identical among all isolates. A biovar refers to a bacterial strain that is physiologically or biochemically distinct from other strains. In this study, it was found that there is a difference at all four gene loci analysed between koala, human and horse biovars of C. pneumoniae: 16SrRNA (up to 1.2% difference), groESL (up to 0.3%), ompB (up to 0.7%) and ompAVD4 (up to 9%). Thus, the koala biovar is genetically distinct from others at all loci tested. The levels of infection of C. pneumoniae in koalas are usually lower than C. pecorum. Presently, it is difficult to determine which bacterium causes certain types of disease in koalas as C. pecorum and C. pneumoniae infections often co-occur. This study has found an association between a respiratory infection and C. pneumoniae in a captive population of koalas, though it is unclear how commonly respiratory disease occurs in free-ranging populations.
Isolates of C. pneumoniae from different koala populations show ompAVD4 gene fragment conservation, and this lack of genetic diversity is similar to human C. pneumoniae biovars, suggesting C. pneumoniae has a host preference. ompAVD4, which in part codes for major outer membrane protein (MOMP), is not genetically diverse in many animals including guinea pigs and felines. This could be because MOMP is not involved in mounting an immune response in the host or that a different target, other than MOMP, is used by the host’s immune system to detect C. pneumoniae infection. Lack of genetic change at ompAVD4 means the immune system does not raise antibodies against it to detect infection. Indeed, several other studies have shown MOMP does not raise a strong antibody response.
The gene locus of C. pneumoniae’s ompAVD4 is highly conserved within this species and its protein elicits a poor immune response. Further study is therefore required to determine if MOMP is useful in the development of C. pneumoniae diagnostics and vaccines.
Summarised by Alexandra Selivanova
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