Abstract
Haemophilus parasuis is an economically important bacterial pathogen of swine. Extensive genetic and phenotypic heterogeneity among H. parasuis strains have been observed, which hinders the deciphering of the population structure and its association with clinical virulence. In this study, two highly divergent clades were defined according to iron–sulphur cluster regulator (iscR)-based phylogeny analysis of 148 isolates. Clear separation of serovars and potential virulence markers (PVMs) were observed between the two clades, which are indicative of independent evolution of the two lineages. Previously suggested virulence factors showed no correlation with clinical virulence, and were probably clade or serovar specific genes emerged during different stage of evolution. PVMs profiles varied widely among isolates in the same serovar. Higher strain diversity in respect of PVMs was found for isolates from multi-strain infected farms than those from single strain infected ones, which indicates that multi-strain infection in one farm may increase the frequency of gene transfer in H. parasuis. Systemic isolates were more frequently found in serovar 13 and serovar 12, while no correlation between clinical virulence and iscR-based phylogeny was observed. It shows that iscR is a reliable marker for studying population structure of H. parasuis, while other factors should be included to avoid the interference of gene exchange of iscR between isolates. The two lineages of H. parasuis may have undergone independent evolution, but show no difference in clinical virulence. Wide distribution of systemic isolates across the entire population poses new challenge for development of vaccine with better cross-protection. Our study provides new information for better deciphering the population structure of H. parasuis, which helps understanding the extreme diversity within this pathogenic bacterium.
Highlights
Haemophilus parasuis is a member of the family Pasteurellaceae, and is the causative agent of Glässer’s disease in swine
Phylogenetic sequence analysis of iron–sulphur cluster regulator (iscR) amino acid sequences showed that the isolates were divided into two deeply divergent clades, which were designated as clade I and clade II (Fig. 1)
Dominant sequence type (ST1 and ST6) in each clade were further aligned with iscR of other Gram-negative bacteria, and simultaneous amino acid substitutions at highly conserved residue 29, 68, 77 made clade I H. parasuis an evolutionarily distant lineage from this and other Gram-negative species (Fig. 2A)
Summary
Haemophilus parasuis is a member of the family Pasteurellaceae, and is the causative agent of Glässer’s disease in swine. H. parasuis could be a commensal organism of the upper respiratory tract of healthy pigs or pathogens causing systemic infection (Biberstein, Gunnarsson & Hurvell, 1977). Extensive heterogeneity among H. parasuis strains have been observed, and increasing efforts are made in deciphering the population structure and. Association between iscR-based phylogeny, serovars and potential virulence markers of Haemophilus parasuis. Strain classification of H. parasuis has been widely studied serotypically, and genotypically, since differentiation of non-virulent strains from virulent strains is essential for diagnosis and control of the disease (Kielstein & Rapp-Gabrielson, 1992; Turni & Blackall, 2005; Olvera, Calsamiglia & Aragon, 2006a; Moreno et al, 2016)
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