Proceedings of the British Mastitis Conference (2003) Garstang, p 121-123 Institute for Animal Health
E.M. Smith, L.E. Green, G.F. Medley, H.E. Bird and C.G. Dowson
Department of Biological Sciences, University of Warwick, Coventry CV4 7AL
SUMMARY
Multilocus sequence typing was used to type a collection of 191
Staphylococcus aureus isolates collected from a Somerset organic dairy farm.
The longitudinal sampling regimen employed in this study showed that two closely related strains predominated on the farm throughout the sampling period, the more dominant strain however, changed over time.
Strain typing also showed that isolates of S. aureus display a level of host specificity, and that virulence may also be linked to a strain’s underlying genetic make-up.
INTRODUCTION
In the early 1990s environmental S. aureus isolates were detected on a dairy farm (1, 2), leading the investigators to conclude that S. aureus was
‘ubiquitous in the dairy farm environment’ (2). However, little strain typing was carried out and disease severity was not investigated.
Little is known about the potential effects of environmental reservoirs of S. aureus in this country, or about the numbers and distribution of strains of
S. aureus on and between farms, and the effects these may have on disease severity. A collection of isolates was made from a Somerset organic dairy farm experiencing an outbreak of S. aureus mastitis to help answer some of these questions.
MATERIALS & METHODS
Monthly quarter milk samples were taken from 26 cows over a ten-month period. Dairy cow environment samples were also collected to determine the on-farm S. aureus population.
Speciation of isolates were confirmed by polymerase chain reaction (PCR) and all confirmed S. aureus isolates were then typed using multilocus sequence typing (MLST).
A selection of S. aureus isolates was donated by Dr. A. Bradley (Bristol
University) for comparison with the isolates collected in this study.
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Proceedings of the British Mastitis Conference (2003) Garstang, p 121-123 Institute for Animal Health
RESULTS
Sample collection
A total of 2185 samples were collected, including 959 (43.9%) quarter milk samples. Of the 1681 samples analysed for S. aureus , 155 (9.2%) contained coagulase-positive staphylococci. These 155 samples yielded 196 isolates,
191 (97.4%) of which proved to be S. aureus .
Multilocus sequence typing (MLST)
All 191 isolates were typed using MLST, resulting in the discovery of two novel alleles and five new sequence types (ST). The strains donated by A.
Bradley yielded five novel alleles and one new ST.
DISCUSSION
While a number of strains of S. aureus were found on the farm, one or two
(in this case closely related) strains predominated. The results also indicate a transition at the herd level from one dominant strain to another over time, but further work is required to confirm this is the case.
The strain typing evidence also suggests that there is some difference between human and bovine infecting strains, including those isolated from farm workers, and between those causing clinical and sub-clinical disease.
CONCLUSIONS
Ø One or two strains predominated in the herd
Ø Human and bovine strains show host specificity
Ø Virulence seems to be linked to strain type
ACKNOWLEDGEMENTS
The authors would like to thank the owners and staff of the farm used in this study for their patience and hospitality. Also thanks to the staff of the
Molecular Biology Service at the University of Warwick for their expertise.
REFERENCES
1.
Matos, J.S., White, D.G., Harmon, R.J. and Langlois, B.E. (1991)
Isolation of Staphylococcus aureus from sites other than the lactating mammary gland. J. Dairy Sci.
74 : 1544-1549.
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Proceedings of the British Mastitis Conference (2003) Garstang, p 121-123 Institute for Animal Health
2.
Roberson, J.R., Fox, L.K., Hancock, D.D., Gay, J.M. and Besser, T.E.
(1994) Ecology of Staphylococcus aureus isolated from various sites on dairy farms. J. Dairy Sci.
77 : 3354-3364.
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