Streptococcus equi subsp. zooepidemicus in New Zealand horses

Abstract

Streptococcus equi subsp. zooepidemicus (S. zooepidemicus) is an opportunistic commensal bacterium of the horse mucosa causing disease in susceptible populations. This research sets out to enhance the understanding of S. zooepidemicus infections within the New Zealand horse population through strain-associated genomic and phenotypic characteristics. Retrospectively, S. zooepidemicus is the second most frequently isolated bacterium in New Zealand horses, being mainly associated with respiratory and urogenital tract infections. However, it also has the ability to colonise and or infect a diverse range of other anatomical regions of New Zealand horses. S. zooepidemicus isolates were shown to be genetically diverse via strain typing using either the single polymorphic gene, szp or standard multilocus sequence typing (MLST). Both typing schemes were shown to lack the discriminatory power needed to completely separate the isolates which was resolved by employing core genome MLST. Importantly, none of these typing methods could link a particular strain to disease status or anatomical region. However, when disease occurred the number of colonising strains tended to reduce to just the infective strain, indicating bacterial competition and strain fitness advantages. Factors available to S. zooepidemicus to colonise and persist in the horse include biofilm formation and cellular invasion with the majority of strains tested in vitro, able to form biofilms and all tested strains capable of invasion. Bioinformatic analysis showed that the pangenome of S. zooepidemicus is open, which demonstrates the ability of species to accessorise its genome enabling the species to diversify and exploit different hosts and environment. An average of 45 new genes are discovered with the addition of each new genome. Unfortunately, no single feature of the accessory genome analysed was able to differentiate the New Zealand horse S. zooepidemicus strains as commensals or pathogens or differentiate strains from respiratory disease or uterine infection. As a result, this study defines S. zooepidemicus as an opportunistic pathogen that can reside in the healthy equine population and infect susceptible hosts and are not anatomically, geographically or host bound.