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Antimicrobial Agents and Chemotherapy, July 2001, p. 1955-1963, Vol. 45, No. 7
0066-4804/01/$04.00+0   DOI: 10.1128/AAC.45.7.1955-1963.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.

Genetic Organization of the Chromosome Region Surrounding mecA in Clinical Staphylococcal Strains: Role of IS431-Mediated mecI Deletion in Expression of Resistance in mecA-Carrying, Low-Level Methicillin- Resistant Staphylococcus haemolyticus

Yuki Katayama, Teruyo Ito, and Keiichi Hiramatsu*

Department of Bacteriology, Faculty of Medicine, Juntendo University, Tokyo, Japan

Received 5 September 2000/Returned for modification 26 December 2000/Accepted 6 April 2001

We report on the structural diversity of mecA gene complexes carried by 38 methicillin-resistant Staphylococcus aureus and 91 methicillin-resistant coagulase-negative Staphylococcus strains of seven different species with a special reference to its correlation with phenotypic expression of methicillin resistance. The most prevalent and widely disseminated mec complex had the structure mecI-mecR1-mecA-IS431R (or IS431mec), designated the class A mecA gene complex. In contrast, in S. haemolyticus, mecA was bracketed by two copies of IS431, forming the structure IS431L-mecA-IS431R. Of the 38 S. haemolyticus strains, 5 had low-level methicillin resistance (MIC, 1 to 4 mg/liter) and characteristic heterogeneous methicillin resistance as judged by population analysis. In these five strains, IS431L was located to the left of an intact mecI gene, forming the structure IS431L-class A mecA-gene complex. In other S. haemolyticus strains, IS431L was associated with the deletion of mecI and mecR1, forming the structure IS431L-Delta mecR1-mecA-IS431mec, designated the class C mecA gene complex. Mutants with the class C mecA gene complex were obtained in vitro by selecting strain SH621, containing the IS431L-class A mecA gene complex with low concentrations of methicillin (1 and 3 mg/liter). The mutants had intermediate level of methicillin resistance (MIC, 16 to 64 mg/liter). The mecA gene transcription was shown to be derepressed in a representative mutant strain, SH621-37. Our study indicated that the mecI-encoded repressor function is responsible for the low-level methicillin resistance of some S. haemolyticus clinical strains and that the IS431-mediated mecI gene deletion causes the expression of methicillin resistance through the derepression of mecA gene transcription.

* Corresponding author. Mailing address: Department of Bacteriology, Faculty of Medicine, Juntendo University, 2-1-1 Hongo, Bunkyo-ku, Tokyo, Japan 113-8421. Phone: 81-3-5802-1040. Fax: 81-3-5684-7830. E-mail: hiram{at}med.juntendo.ac.jp.

Antimicrobial Agents and Chemotherapy, July 2001, p. 1955-1963, Vol. 45, No. 7
0066-4804/01/$04.00+0   DOI: 10.1128/AAC.45.7.1955-1963.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.


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