Expanding dynamics of the virulence-related gene variations in the toxigenic Vibrio cholerae serogroup O1

Duochun Wang,Jie Li,Xin Lu,Yujie Fang,Bo Pang,Biao Kan,Jialiang Xu,Zhenpeng Li

doi:10.1186/s12864-019-5725-y

Abstract

BackgroundToxigenic Vibrio cholerae serogroup O1 is the causative pathogen in the sixth and seventh cholera pandemics. Cholera toxin is the major virulent factor but other virulence and virulence-related factors play certain roles in the pathogenesis and survival in the host. Along with the evolution of the epidemic strains, the virulence-related genes also experience variation, gain and loss, and lead to genetic divergence in different strains.ResultsIn this study, we analyzed the virulence-related gene profiles in the toxigenic serogroup O1 strains isolated from 1923 to 2015, the genomes of which were publicly available. The virulence-related genes of the V. cholerae O1 strains were annotated based on the Virulence Factors Database (VFDB). An average of 230.1 virulence-related genes per strain were identified; significant differences in the average numbers were found between the classical and El Tor biotypes, and increasing trends in the number of virulence-related genes along with the isolation years were observed in the El Tor biotype strains. A total of 176 homologs of virulence-related genes were found from these strains, of which 25 belonged to the core genes, suggesting their conservative and necessary roles in V. cholerae pathogenesis. We described the diversities of the homologs by defining gene sequence type, and illustrated its association with gene duplication; we found that gene duplication clearly increased the complexity of the gene sequence types in the core virulence-related genes. In addition, we provided virulence-related gene profiles whose genetic characteristic depend on the isolation years from the view of gene gain and loss, variation, gene duplication and gene sequence type number.ConclusionsOur study reveals the comprehensive variation dynamics of the virulence-related genes in toxigenic V. cholerae serogroup O1 during epidemics. The increasing trend for the virulence-related genes may suggest the evolutional advantage of strains by gaining virulence-related genes with diverse functional categories.

Highlights

Toxigenic Vibrio cholerae serogroup O1 is the causative pathogen in the sixth and seventh cholera pandemics
If we remove the core homologs containing duplicate genes, the average number of genes contributing to a genotype for the core virulence-related genes increased to 155.8, which was 35.6% higher than the situation when core homologs containing duplicate genes not removed. These results suggest that gene duplication obviously increased the gene sequence types of core virulence-related genes
We analyzed the genomic changes of virulence-related genes with isolation years in V. cholerae toxigenic strains for the O1 serogroup from 1923 to 2005

Summary

Introduction

Toxigenic Vibrio cholerae serogroup O1 is the causative pathogen in the sixth and seventh cholera pandemics. The study indicated the past epidemics were attributable to a single expanded lineage, and described the periodicity of lineage introduction and the stable routes of cholera spread [8]. Another recent study has given an integrated view of V. cholerae in the Americas. The study analyzed the two of the largest cholera epidemics in modern history, and found that intercontinental introductions of seventh pandemic El Tor V. cholerae have caused the two epidemics, further consolidating the importance of local lineages [9]

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Conclusion