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Antimicrobial Agents and Chemotherapy, January 2004, p. 285-296, Vol. 48, No. 1
0066-4804/04/$08.00+0     DOI: 10.1128/AAC.48.1.285-296.2004
Copyright © 2004, American Society for Microbiology. All Rights Reserved.

Local Variants of Staphylococcal Cassette Chromosome mec in Sporadic Methicillin-Resistant Staphylococcus aureus and Methicillin-Resistant Coagulase-Negative Staphylococci: Evidence of Horizontal Gene Transfer?

Anne-Merethe Hanssen,* Gry Kjeldsen, and Johanna U. Ericson Sollid*

Department of Microbiology and Virology, Institute of Medical Biology, University of Tromsø, Tromsø, Norway

Received 8 April 2003/ Returned for modification 7 July 2003/ Accepted 23 September 2003

The mecA gene in Staphylococcus aureus is located on the genetic element staphylococcal cassette chromosome (SCC). Different SCCmecs have been classified according to their putative recombinase genes (ccrA and ccrB) and overall genetic composition. Clinical isolates of coagulase-negative staphylococci (CoNS; n = 39) and S. aureus (n = 20) from Norway, India, Italy, Finland, the United States, and the United Kingdom were analyzed by pulsed-field gel electrophoresis, which showed that most isolates were genetically unrelated. Cluster analyses of 16S rRNA gene and pta sequences confirmed the traditional biochemical species identification. The mecI, mecR1, mecA, and ccrAB genes were detected by PCRs, identifying 19 out of 20 S. aureus and 17 out of 39 CoNS isolates as carriers of one of the three published ccrAB pairs. New variants of SCCmec were identified, as well as CoNS isolates containing ccrAB genes without the mec locus. ccrAB and mec PCRs were verified by hybridization. Sequence alignments of ccrAB genes showed a high level of diversity between the ccrAB alleles from different isolates, i.e., 94 to 100% and 95 to 100% homology for ccrAB1 and ccrAB2, respectively. All of the ccrAB3 genes identified were identical. Genetically unique and sporadic methicillin-resistant S. aureus (MRSA) contained local variants of ccrAB gene pairs identical to those found in MR-CoNS but different from those in MRSA from other regions. Allelic variants of ccrAB in isolates from the same geographic region showed sequence conservation independent of species. The species-independent sequence conservation found suggests that there is a closer genetic relationship between ccrAB2 in Norwegian staphylococci than between ccrAB2 sequences in international MRSA and Norwegian MRSA. This might indicate that different staphylococcal species acquire these genes locally by horizontal gene transfer.

* Corresponding author. Mailing address for Anne-Merethe Hanssen: Department of Microbiology and Virology, Faculty of Medicine, Institute for Medical Biology, University of Tromsø, N-9037 Tromsø, Norway. Phone: 47 77 64 57 52. Fax: 47 77 64 53 50. E-mail: annemh{at}fagmed.uit.no. Present address for Johanna U. E. Sollid: Institute of Medicine, University of Bergen, Haukeland Hospital, N-5021 Bergen, Norway. Phone: 47 55 97 30 75. Fax: 47 55 97 29 50. E-mail: johanna{at}fagmed.uit.no.

Antimicrobial Agents and Chemotherapy, January 2004, p. 285-296, Vol. 48, No. 1
0066-4804/04/$08.00+0     DOI: 10.1128/AAC.48.1.285-296.2004
Copyright © 2004, American Society for Microbiology. All Rights Reserved.



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