Genome expansion in bacteria: the curios case of Chlamydia trachomatis

Recent findings indicated that a correlation between genomic % AT and genome size within strains of microbial species was predominantly associated with the uptake of foreign DNA. One species however, Chlamydia trachomatis, defied any explanation. In the present study 79 fully sequenced C. trachomatis genomes, representing ocular- (nine strains), urogenital- (36 strains) and lymphogranuloma venereum strains (LGV, 22 strains), in three pathogroups, in addition to 12 laboratory isolates, were scrutinized with the intent of elucidating the positive correlation between genomic AT content and genome size.

The average size difference between the strains of each pathogroup was largely explained by the incorporation of genetic fragments. These fragments were slightly more AT rich than their corresponding host genomes, but not enough to justify the difference in AT content between the strains of the smaller genomes lacking the fragments. In addition, a genetic region predominantly found in the ocular strains, which had the largest genomes, was on average more GC rich than the host genomes of the urogenital strains (58.64% AT vs. 58.69% AT), which had the second largest genomes, implying that the foreign genetic regions cannot alone explain the association between genome size and AT content in C. trachomatis. 23,492 SNPs were identified for all 79 genomes, and although the SNPs were on average slightly GC rich (~47% AT), a significant association was found between genome-wide SNP AT content, for each pathogroup, and genome size (p < 0.001, R (2) = 0.86) in the C. trachomatis strains.

The correlation between genome size and AT content, with respect to the C. trachomatis pathogroups, was explained by the incorporation of genetic fragments unique to the ocular and/or urogenital strains into the LGV- and urogential strains in addition to the genome-wide SNP AT content differences between the three pathogroups.