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Antimicrobial Agents and Chemotherapy, July 2007, p. 2514-2522, Vol. 51, No. 7
0066-4804/07/$08.00+0     doi:10.1128/AAC.00040-07
Copyright © 2007, American Society for Microbiology. All Rights Reserved.

Identification and Phenotypic Characterization of a β-Lactam-Dependent, Methicillin-Resistant Staphylococcus aureus Strain

Division of Infectious Diseases, Department of Internal Medicine, Virginia Commonwealth University, Richmond, Virginia 23298,¹ Foundation Hospital Saint Joseph, 185 rue Raymond Losserand, 75014, Paris, France,² Department of Molecular Biology, University of Gdansk, Kladki 24, 84-822 Gdansk, Poland³

Received 11 January 2007/ Returned for modification 26 March 2007/ Accepted 20 April 2007

Methicillin resistance in Staphylococcus aureus is primarily mediated by the acquired penicillin-binding protein PBP 2a, which is encoded by mecA. PBP 2a acts together with native PBP 2 to mediate oxacillin resistance by contributing complementary transpeptidase and transglycosylase activities, respectively. In this study, we have investigated a phenotype of β-lactam dependence in a clinical methicillin-resistant S. aureus strain (strain 2884D) obtained by in vitro selection with ceftobiprole. 28884D, which grew very poorly in blood agar, required the presence of the β-lactam antibiotics to grow. On the basis of this observation, we hypothesized that a gene or genes essential for growth were dependent on oxacillin induction. Identification and analysis of genes regulated by oxacillin were performed by both real-time reverse transcription-PCR and spotted microarray analysis. We found that mecA was constitutively expressed in strain 2884D and that the constitutive expression resulted from perturbations in the two systems involved in its regulation, i.e., MecI/MecR1 (staphylococcal chromosome cassette mec type I) and BlaI/BlaR1 (nonfunctional penicillinase operon). PBP 2 appeared to be poorly induced by oxacillin in 2884D. Further analysis of the PBP 2 two-component VraSR regulatory system showed that it was nonfunctional, accounting for the lack of response to oxacillin. Together, these results support the notion that limited PBP 2 availability may have led 2884D to become dependent on oxacillin-mediated mecA induction as a required survival mechanism.

Published ahead of print on 30 April 2007.