Selective whole genome amplification as a tool to enrich specimens with low Treponema pallidum genomic DNA copies for whole genome sequencing
Downstream next generation sequencing of the syphilis spirochete Treponema pallidum subspecies pallidum (T. pallidum) is hindered by low bacterial loads and the overwhelming presence of background metagenomic DNA in clinical specimens. In this study, we investigated selective whole genome amplification (SWGA) utilizing Multiple Displacement Amplification (MDA) in conjunction with custom oligonucleotides with an increased specificity for the T. pallidum genome, and the capture and removal of CpG-methylated host DNA followed by MDA as enrichment methods to improve the yields of T. pallidum DNA in rabbit propagated isolates and lesion specimens from patients with primary and secondary syphilis. Sequencing was performed using the Illumina MiSeq v2 500 cycle or NovaSeq 6000 SP platform. These two enrichment methods led to 93-98% genome coverage at 5 reads/site in 5 clinical specimens from the United States and rabbit propagated isolates, containing >14 T. pallidum genomic copies/μl input for SWGA and >129 genomic copies/μl for CpG methylation capture with MDA. Variant analysis using sequencing data derived from SWGA-enriched specimens, showed that all 5 clinical strains had the A2058G mutation associated with azithromycin resistance. SWGA is a robust method that allows direct whole genome sequencing (WGS) of specimens containing very low numbers of T. pallidum, which have been challenging until now.