Abstract
Brachyspira (B.) hyodysenteriae is widespread globally, and can cause mucohaemorrhagic colitis (swine dysentery, SD) with severe economic impact in infected herds. Typical strains of B. hyodysenteriae are strongly haemolytic on blood agar, and the haemolytic activity is believed to contribute to virulence in vivo. However, recently there have been reports of atypical weakly haemolytic isolates of B. hyodysenteriae (whBh). In this study, 34 European whBh and 82 strongly haemolytic isolates were subjected to comparative genomic analysis. A phylogenetic tree constructed using core single nucleotide polymorphisms showed that the whBh formed a distinct sub-clade. All eight genes previously associated with haemolysis in B. hyodysenteriae were present in the whBh. No consistent patterns of amino acid substitutions for all whBh were found in these genes. In contrast, a genome region containing six coding sequences (CDSs) had consistent nucleotide sequence differences between strongly and whBh isolates. Two CDSs were predicted to encode ABC transporter proteins, and a TolC family protein, which may have a role in the export of haemolysins from B. hyodysenteriae. Another difference in this region was the presence of three CDSs in whBh that are pseudogenes in strongly haemolytic isolates. One of the intact CDSs from whBh encoded a predicted PadR-like transcriptional repressor that may play a role in repression of haemolysis functions. In summary, a sub-clade of whBh isolates has emerged in Europe, and several genomic differences, that potentially explain the weakly haemolytic phenotype, were identified. These markers may provide targets for discriminatory molecular tests needed in SD surveillance.
Highlights
Swine dysentery (SD) is a bacterial disease of pigs characterised by development of a severe mucohaemorrhagic colitis that mainly occurs in animals in the growing and finishing stages
The 39 isolates were subjected to whole genome sequencing, and all were identified as B. hyodysenteriae using Kraken k-mer analysis [34] (Additional file 1)
The genome analysis undertaken in this work verified that the studied weakly haemolytic isolates were B. hyodysenteriae, confirming the results obtained by PCR and MALDI-ToF
Summary
Swine dysentery (SD) is a bacterial disease of pigs characterised by development of a severe mucohaemorrhagic colitis that mainly occurs in animals in the growing and finishing stages. Studies have shown that a protein with haemolytic activity extracted from the supernatant of B. hyodysenteriae cultures can cause lesions resembling those seen in SD when placed in ligated ileal and colonic loops in germ-free pigs [5], and in the caecum in a murine model of SD [6]. These observations suggest that this haemolysin is involved in lesion formation, it does not exclude the possibility that other mechanisms are involved in aspects of the pathogenesis of SD. In other studies the extracted B. hyodysenteriae haemolysin has been described as having a wide variety of molecular masses depending on the purification methods used in the study [8]; some uncertainty remains about the precise identity of the haemolysin(s)
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