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.
Forbes
Betty A
BA
Department of Internal Medicine, Medical College of Virginia Campus of Virginia Commonwealth University, Richmond, VA 23298-0049, USA.
Bombicino
Karina
K
Plata
Konrad
K
Cuirolo
Arabela
A
Webber
Dawn
D
Bender
Connie L
CL
Rosato
Adriana E
AE
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