Unusual form of oxacillin resistance in methicillin-resistant Staphylococcus aureus clinical strains.
The mechanisms by which there is differential expression of resistance to oxacillin within the populations of a single strain remains to be fully understood. The purpose of this study was to evaluate and characterize 25 GOA48 methicillin-resistant Staphylococcus aureus (MRSA) oxacillin-susceptible mecA-positive strains, which were obtained by screening consecutively 832 S. aureus isolates. These 25 isolates (3% of the total strains investigated) were uniformly detected by extending the 24-h oxacillin agar screen plate to 48 h (namely, GOA48-MRSA). Twenty-two isolates tested positive for penicillin-binding protein 2a, whereas the remaining 3 isolates were inconsistently mecA positive. Inconsistent detection of mecA by polymerase chain reaction (PCR) in the mentioned 3 isolates was investigated by colony hybridization using a mecA probe (> or = 80% of colonies hybridized poorly to the probe). A PCR product that amplified the empty SCCmec insertion site (attB), present only if the element was excised, resulted positive in all 3 isolates before oxacillin exposure, whereas integrated elements were positive only for oxacillin-grown isolates. The remaining 22 strains did not reveal excision demonstrating stable mecA. We concluded that resistance to beta-lactams in MRSA-positive mecA strains susceptible to oxacillin is associated to an extreme heterogeneous expression of resistance combined in some cases to oxacillin SCCmec excision.
Department of Internal Medicine, Medical College of Virginia Campus of Virginia Commonwealth University, Richmond, VA 23298-0049, USA.
Betty A
BA
Forbes
Karina
K
Bombicino
Konrad
K
Plata
Arabela
A
Cuirolo
Dawn
D
Webber
Connie L
CL
Bender
Adriana E
AE
Rosato
D000900:Anti-Bacterial Agents / Q000494:pharmacology*; D001287:Attachment Sites, Microbiological; D001426:Bacterial Proteins / Q000235:genetics; D015169:Colony Count, Microbial; D004269:DNA, Bacterial / Q000235:genetics; D006801:Humans; D016106:Methicillin Resistance; D008826:Microbial Sensitivity Tests / Q000379:methods; D050296:Microbial Viability; D009693:Nucleic Acid Hybridization / Q000379:methods; D015345:Oligonucleotide Probes / Q000235:genetics; D010068:Oxacillin / Q000494:pharmacology*; D046915:Penicillin-Binding Proteins / Q000235:genetics; D016133:Polymerase Chain Reaction; D013203:Staphylococcal Infections / Q000382:microbiology; D013211:Staphylococcus aureus / Q000187:drug effects* / Q000302:isolation & purification; D018440:beta-Lactam Resistance
D000900:Anti-Bacterial Agents; D001426:Bacterial Proteins; D004269:DNA, Bacterial; D015345:Oligonucleotide Probes; D046915:Penicillin-Binding Proteins; C487789:mecA protein, Staphylococcus aureus; D010068:Oxacillin