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Applied and Environmental Microbiology, December 2005, p. 8352-8361, Vol. 71, No. 12
0099-2240/05/$08.00+0     doi:10.1128/AEM.71.12.8352-8361.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.

Genetic Variation among Staphylococcus aureus Strains from Norwegian Bulk Milk

H. J. Jørgensen,1* T. Mørk,2 D. A. Caugant,3 A. Kearns,4 and L. M. Rørvik1,5

Department for Feed and Food Hygiene,1 Department for Animal Health, National Veterinary Institute,2 Division of Infectious Disease Control, Norwegian Institute of Public Health,3 Department of Food Safety and Infection Biology, Norwegian School of Veterinary Science, Oslo, Norway,5 Laboratory of HealthCare Associated Infection, Health Protection Agency, London, United Kingdom4

Received 25 May 2005/ Accepted 27 August 2005

Strains of Staphylococcus aureus obtained from bovine (n = 117) and caprine (n = 114) bulk milk were characterized and compared with S. aureus strains from raw-milk products (n = 27), bovine mastitis specimens (n = 9), and human blood cultures (n = 39). All isolates were typed by pulsed-field gel electrophoresis (PFGE). In addition, subsets of isolates were characterized using multilocus sequence typing (MLST), multiplex PCR (m-PCR) for genes encoding nine of the staphylococcal enterotoxins (SE), and the cloverleaf method for penicillin resistance. A variety of genotypes were observed, and greater genetic diversity was found among bovine than caprine bulk milk isolates. Certain genotypes, with a wide geographic distribution, were common to bovine and caprine bulk milk and may represent ruminant-specialized S. aureus. Isolates with genotypes indistinguishable from those of strains from ruminant mastitis were frequently found in bulk milk, and strains with genotypes indistinguishable from those from bulk milk were observed in raw-milk products. This indicates that S. aureus from infected udders may contaminate bulk milk and, subsequently, raw-milk products. Human blood culture isolates were diverse and differed from isolates from other sources. Genotyping by PFGE, MLST, and m-PCR for SE genes largely corresponded. In general, isolates with indistinguishable PFGE banding patterns had the same SE gene profile and isolates with identical SE gene profiles were placed together in PFGE clusters. Phylogenetic analyses agreed with the division of MLST sequence types into clonal complexes, and isolates within the same clonal complex had the same SE gene profile. Furthermore, isolates within PFGE clusters generally belonged to the same clonal complex.

* Corresponding author. Mailing address: Department of Food and Feed Hygiene, National Veterinary Institute, P.O. Box 8156 Dep., 0033 Oslo, Norway. Phone: 47 23216000. Fax: 47 23216202. E-mail: hannah.jorgensen{at}vetinst.no.

Applied and Environmental Microbiology, December 2005, p. 8352-8361, Vol. 71, No. 12
0099-2240/05/$08.00+0     doi:10.1128/AEM.71.12.8352-8361.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.



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