Characterization of the novel clinical isolate X-4 containing a newtp0548sequence-type
Authors: Liu Dan , Tong Man-Li , Liu Li-Li , Lin Li-Rong , Lin Yu , Yang Tian-Ci 
Published: December 2019

Background

A noveltp0548sequence-type was identified in one clinical isolate (X-4) from a patient diagnosed with primary syphilis in Xiamen, China. To precisely define and characterize this new clinical isolate, we performed further genome-scale molecular analysis.

Methodology/Principal findings

The alignment of all publishedtp0548genotypes revealed that this new genotype had a unique nucleotide substitution G->T at position 167, and the letter “ao” was assigned to the genotype. Phylogenetic analysis showed that the “ao” genotype belonged to the SS14-like clade ofTreponema pallidum(TPA) strains. The genome of the X-4 isolate was then sequenced and analyzed, and the result of a multi-locus sequence analysis using a set of nine chromosomal loci showed that the X-4 isolate was clustered with a monophyletic group of TPA strains, which clearly identified the isolate as a TPA strain. Whole-genome phylogenetic analysis was subsequently conducted to corroborate the TPA strain classification of the X-4 isolate. And the isolate was genetically related to the SS14 strain, with 42 single nucleotide variations and 12 insertions/deletions. In addition, high intrastrain heterogeneity in the length of the poly G/C tracts was found in the TPAChi_0347 locus, which indicated that this gene was most likely involved in phase variation events. The first investigation of the length heterogeneity of the poly A/T tracts showed the variability of the ploy A/T was lower, and all the observed intrastrain variations fell within coding regions.

Conclusions/Significance

The study demonstrated the X-4 isolate was a TPA isolate containing a novel tp0548 sequence-type. The identification of intrastrain genetic heterogeneity at poly G/C tracts and poly A/T tracts of the isolate could provide a snapshot of the genes that potentially involved in genotype-phenotype variations. These findings provide an unequivocal characterization for better understanding the molecular variation of this emerging isolate.

Author summary

Three subspecies ofTreponema pallidum(pallidum, pertenue, and endemicum) are increasingly showing overlap in terms of transmission and clinical manifestations. We recently identified a noveltp0548genotype in the X-4 isolate, which was obtained from an adult male with genital lesions. The novel genotype contained a unique nucleotide substitution G->T at position 167 and belonged to the SS14-like clade of TPA strains, as determined by phylogenetic analysis. We conducted an in-depth exploration of the genome of the X-4 isolate using the pooled segment genome sequencing method followed by Illumina sequencing. Multi-locus sequence analysis of nine chromosomal loci demonstrated that the X-4 isolate was clustered within a monophyletic group of TPA strains, which identified the isolate as a TPA strain. Whole-genome phylogenetic analysis subsequently corroborated the TPA strain classification of the X-4 isolate and revealed that the isolate was very closely related to the SS14 strain, with 42 single nucleotide variations and 12 insertions/deletions. In addition, characterization of the intrastrain heterogeneity in the lengths of homopolymeric tracts in the X-4 isolate showed that the heterogeneity of the poly G/C tracts was greater than that of the poly A/T tracts, and high poly G/C tract diversity was observed in the TPAChi_0347 locus.