Comparative and Functional Genomic Analysis of Streptococcus Equi and Streptococcus Zooepidemicus : Identifying Novel Vaccine Targets

Heather, Zoe (2009). Comparative and Functional Genomic Analysis of Streptococcus Equi and Streptococcus Zooepidemicus : Identifying Novel Vaccine Targets. PhD thesis The Open University.

DOI: https://doi.org/10.21954/ou.ro.0000d627

Abstract

Streptococcus equi subspecies equi (S. equi) is a host-restricted pathogen of horses and the aetiological agent of strangles. Available evidence suggests that S. equi evolved from Streptococcus equi subspecies zooepidemicus (S. zooepidemicus), a versatile bacterium that is often isolated from the equine respiratory tract, but can cause opportunistic disease in horses and other animals. A comparison of the genomes of S. equi 4047 and S. zooepidemicus H70 and the screening of diverse S. equi and S. zooepidemicus strains uncovered the genetic events that have shaped the evolution of S. equi, and led to its emergence as a niche-adapted pathogen. This analysis provides evidence of functional loss, changes in the organisation and sequence of genes, and pathogenic specialisation through the acquisition of prophage encoding a phospholipase A2 toxin, and 4 superantigens, and an integrative conjugative element carrying a novel siderophore-like nonribosomal peptide synthetase (NRPS) system. The NRPS shares similarity with the yersiniabactin system found in the high pathogenicity island of Yersinia pestis and is the first of its kind to be identified in streptococci. As this genetic feature is absent from the S. zooepidemicus population, its gain is considered to have been a key event in the emergence of S. equi. Further work determined a role for the NRPS in iron acquisition, and through its heterologous reconstitution in Escherichia coli and/or the analysis of allelic replacement mutants in S. equi, identified biosynthetic genes, transporters involved in efflux and import of the NRPS product(s), salicylate as a substrate for the NRPS and its regulation by a novel iron-dependent IdeR-like repressor, using various in vitro growth assays, including sensitivity to streptonigrin and ⁵⁵Fe accumulation. Possible vaccine targets were identified in both subspecies and existing diagnostic tools were improved, which included the development of a quantitative PCR test for the detection of S. equi.