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Journal of Bacteriology, April 2001, p. 2417-2424, Vol. 183, No. 8
Laboratory of Microbiology, The Rockefeller
University, New York, New York 10021,1 and
Molecular Genetics Unit, Instituto de Tecnologia
Química e Biológica, Universidade Nova de Lisboa, Oeiras,
Portugal2
Received 14 December 2000/Accepted 30 January 2001
Strains of methicillin-resistant Staphylococcus
aureus (MRSA) have become the most important causative agents
of hospital-acquired diseases worldwide. The genetic determinant of
resistance, mecA, is not a gene native to S.
aureus but was acquired from an extraspecies source by an
unknown mechanism. We recently identified a close homologue of this
gene in isolates of Staphylococcus sciuri, a taxonomically primitive staphylococcal species recovered most frequently from rodents and primitive mammals. In spite of the close
sequence similarity between the mecA homologue of
S. sciuri and the antibiotic resistance determinant
mecA of S. aureus, S. sciuri strains were found to be uniformly susceptible to
0021-9193/01/$04.00+0 DOI: 10.1128/JB.183.8.2417-2424.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.
Recruitment of the mecA Gene Homologue of
Staphylococcus sciuri into a Resistance Determinant
and Expression of the Resistant Phenotype in
Staphylococcus aureus
-lactam antibiotics. In an attempt to activate the apparently
"silent" mecA gene of S. sciuri, a
methicillin-resistant derivative, K1M200 (for which the MIC of
methicillin is 200 µg/ml), was obtained through stepwise exposure of
the parental strain S. sciuri K1 (methicillin MIC of 4 µg/ml) to increasing concentrations of methicillin. DNA sequencing of
the mecA homologue from K1M200 revealed the introduction
of a point mutation into the 
10 consensus of the promoter: the
replacement of a thymine residue at nucleotide 1577 in the susceptible
strain K1 by adenine in the resistant strain K1M200, which was
accompanied by a drastic increase in transcription rate and the
appearance of a new protein that reacted with monoclonal antibody
prepared against the penicillin-binding protein 2A (PBP2A), i.e., the
gene product of S. aureus mecA. Transduction of
mecA from K1M200 (cloned into a plasmid vector) into a
methicillin-susceptible S. aureus mutant resulted in a
significant increase of methicillin resistance (from a methicillin MIC
of 4 µg/ml to 12 and up to 50 µg/ml), the appearance of a
low-affinity PBP detectable by the fluorographic assay, and the
production of a protein that reacted in a Western blot with monoclonal
antibody to PBP2A. Antibiotic resistance and the protein products
disappeared upon removal of the plasmid-borne mecA
homologue. The observations support the proposition that the
mecA homologue ubiquitous in the antibiotic-susceptible animal species S. sciuri may be an evolutionary
precursor of the methicillin resistance gene mecA of the
pathogenic strains of MRSA.
*
Corresponding author. Mailing address: Laboratory of
Microbiology, The Rockefeller University, 1230 York Ave., New York, NY 10021. Phone: (212) 327-8278. Fax: (212) 327-8688. E-mail:
tomasz{at}mail.rockefeller.edu.
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