Chlamydia pneumoniae is an intracellular bacterial pathogen with an extremely diverse host range (humans, amphibians, reptiles and marsupials). C. pneumoniae exposure is widespread in humans, with sero-prevalence studies reporting 50% infection levels by age 20 and reaching 80% in the elderly. In humans, C. pneumoniae infections can range from asymptomatic to severe respiratory disease, including pneumonia. Less common presentations include bronchitis, pharyngitis, laryngitis and sinusitis, making up 5% of cases. In addition to respiratory infections in humans, C. pneumoniae has also been associated with atherosclerosis and stroke, myocarditis, multiple sclerosis and Alzheimer’s disease.
Despite the widespread prevalence of C. pneumoniae in humans, all isolates studied to date are extremely similar at the DNA level. Four C. pneumoniae human isolates have had their full genome sequenced. Genomic comparisons revealed a highly conserved (>99.9%) gene order and organisation, with few deletions and less than 300 single nucleotide polymorphisms (SNPs) distinguishing the isolates. This near clonality of C. pneumoniae human isolates that are temporally and geographically separate, has been taken to indicate that human infections are a relatively recent event and that the efficient respiratory spread of the agent explains how 60–80% of adults worldwide have been infected at least once in their lifetime .
Researchers selected 23 target genes in this organism to investigate genetic diversity: seven of these had been lost or gained by C. pneumoniae, a further six were conserved, four were polymorphic, and six were truncated or length polymorphic in one strain or the other. The results highlights that C. pneumoniae animal isolates are much more genetically diverse than C. pneumoniae human isolates, and have crossed the host barrier to humans on at least two occasions. This study provides new insights into the evolution of this complex pathogen.
Chlamydia pneumoniae Is Genetically Diverse in Animals and Appears to Have Crossed the Host Barrier to Humans on (At Least) Two Occasions. 2010 PLoS Pathog 6(5): e1000903. doi:10.1371/journal.ppat.1000903