title: Development of homogeneous expression of resistance in methicillin-resistant Staphylococcus aureus clinical strains is functionally associated with a beta-lactam-mediated SOS response.
abstract: OBJECTIVE
  One of the main characteristics of methicillin-resistant Staphylococcus aureus (MRSA) from both hospitals and community is their heterogeneous expression of resistance. Recently, we reported new heterogeneous MRSA isolates phenotypically susceptible to oxacillin despite being mecA positive. These low-level mecA-mediated resistance MRSA strains are very heterogeneous in expression (HeR) and are likely to be clinically relevant since exposure of such isolates to beta-lactams can result in high-level homotypic resistance (HoR). We hypothesized that HeR to HoR selection in these clinically relevant strains may be determined by the pre-existence of a hypermutable population that favours its selection in the presence of oxacillin.
  
  
  METHODS
  Using established procedures, SA13011 HeR to HoR selection was performed by using subinhibitory concentrations of oxacillin and examined for mutability. Real-time RT-PCR and transcriptional profiling by DNA microarray were used to compare gene expression between both populations and related genetically modified SA13011 strain.
  
  
  RESULTS
  We found that HeR/HoR selection by oxacillin was associated with increased mutation rate and oxacillin-mediated SOS response. We determined increased expression of both mecA and SOS response lexA/recA regulators. Mutational inactivation of lexA repressor resulted in a significant decrease in both mutation rate and oxacillin resistance in the HoR cells. Complementation of the lexA mutant strain restored oxacillin resistance to the high levels observed in the corresponding HoR wild-type strain.
  
  
  CONCLUSIONS
  The present results support the notion that SOS response is mechanistically involved in generating mutations that, in addition to mecA induction, allow the selection of a highly oxacillin-resistant population.
authors.affiliation: Department of Internal Medicine, Division of Infectious Diseases, Virginia Commonwealth University, Richmond, VA 23298, USA.
authors.forename: Arabela
authors.initials: A
authors.lastname: Cuirolo
authors.forename: Konrad
authors.initials: K
authors.lastname: Plata
authors.forename: Adriana E
authors.initials: AE
authors.lastname: Rosato
mesh_terms: D000900:Anti-Bacterial Agents / Q000494:pharmacology*; D001426:Bacterial Proteins / Q000096:biosynthesis / Q000235:genetics; D045643:DNA Repair Enzymes / Q000096:biosynthesis / Q000235:genetics; D020869:Gene Expression Profiling; D006801:Humans; D016106:Methicillin Resistance; D055624:Methicillin-Resistant Staphylococcus aureus / Q000187:drug effects* / Q000502:physiology*; D009154:Mutation; D020411:Oligonucleotide Array Sequence Analysis; D046915:Penicillin-Binding Proteins; D020133:Reverse Transcriptase Polymerase Chain Reaction; D013014:SOS Response, Genetics; D013312:Stress, Physiological; D047090:beta-Lactams / Q000494:pharmacology*
chemical_list: D000900:Anti-Bacterial Agents; D001426:Bacterial Proteins; D046915:Penicillin-Binding Proteins; D047090:beta-Lactams; C487789:mecA protein, Staphylococcus aureus; D045643:DNA Repair Enzymes