Difference in virulence between Staphylococcus aureus isolates causing gangrenous 2 mastitis versus subclinical mastitis in a dairy sheep flock 3 4

Difference in virulence between Staphylococcus aureus isolates causing gangrenous 2 mastitis versus subclinical mastitis in a dairy sheep flock 3 4 Eric VAUTOR Abstract – Staphylococcus aureus mastitis in dairy sheep ranges from subclinical mastitis to 22 lethal gangrenous mastitis. Neither the S. aureus virulence factors nor the host-factors or the 23 epidemiological events contributing to the different outcomes are known. In a field study in a 24 dairy sheep farm over 21 months, sixteen natural isolates of S. aureus were collected from six 25 subclinical mastitis cases, one lethal gangrenous mastitis case, nasal carriage from eight ewes, 26 and one isolate from ambient air in the milking room. A genomic comparison of two strains, 27 one responsible for subclinical mastitis and one for lethal gangrenous mastitis, was performed 28 using multi-strain DNA microarrays. Multiple typing techniques (pulsed-field-gel-29 electrophoresis, multiple-locus variable-number, single-nucleotide polymorphisms, randomly 30 amplified polymorphic DNA, spa typing and sas typing) were used to characterise the 31 remaining isolates and to follow the persistence of the gangrenous isolate in ewes' nares. Our 32 results showed that the two strains were genetically closely related and they shared 3 615 33 identical predicted open reading frames (ORF). However, the gangrenous mastitis isolate 34 carried variant versions of several genes (sdrD, clfA-B, sasA, sasB, sasD, sasI, and splE) and 35 was missing fnbB and a prophage. The typing results showed that this gangrenous strain 36 emerged after the initial subclinical mastitis screening, but then persisted in the flock in the 37 nares of four ewes. Although we cannot dismiss the role of host susceptibility in the clinical 38 events in this flock, our data support the hypothesis that S. aureus populations had evolved in 39 the sheep flock and that S. aureus genetic variations could have contributed to enhanced 40 virulence. 41 42 subclinical mastitis / gangrenous mastitis / dairy sheep / Staphylococcus aureus / 43 microarray 44 45