title: Molecular characterization of MRSA isolates bracketing the current EUCAST ceftaroline-susceptible breakpoint for Staphylococcus aureus: the role of PBP2a in the activity of ceftaroline.
abstract: OBJECTIVE
  The objectives of this study were to characterize contemporary MRSA isolates and understand the prevalence and impact of sequence variability in PBP2a on ceftaroline susceptibility.
  
  
  METHODS
  A total of 184 MRSA isolates collected from 28 countries were collected and characterized.
  
  
  RESULTS
  WT PBP2a proteins were found in MRSA distributed evenly over the ceftaroline MIC range of 0.5-2 mg/L (n=56). PBP2a variations found in 124 isolates fell into two categories: (i) 12 isolates contained a substitution in the transpeptidase pocket located in the penicillin-binding domain and exhibited significantly decreased ceftaroline susceptibility (typically 8 mg/L); and (ii) isolates with substitutions in the non-penicillin-binding domain (nPBD) in a region proposed to be functionally important for cell wall biogenesis. The majority (71%) of isolates containing only nPBD variations were inhibited by 2 mg/L ceftaroline, 23% by ≤1 mg/L and 6% by 4 mg/L. These data suggest that the WT MRSA distribution extends beyond the current EUCAST and CLSI susceptible breakpoints and includes isolates inhibited by 2 mg/L ceftaroline. SCCmec type IV was the predominant type in the ceftaroline-susceptible population (68%), whereas it only represented 6% of the non-susceptible population. The variations of MLST lineages were fewer among the non-susceptible group.
  
  
  CONCLUSIONS
  This study suggests that MRSA populations with a WT PBP2a and those with nPBD variations overlap significantly and that PBP2a sequence-independent factors contribute to ceftaroline susceptibility. Whereas characterization of isolates with a ceftaroline MIC of 2 mg/L enriched for isolates with nPBD variations, it was not a discrete population. In contrast, the rare isolates containing a substitution in the transpeptidase-binding pocket were readily differentiated.
authors.affiliation: Infection Innovative Medicines Unit, AstraZeneca R&D Boston, Waltham, MA 02451, USA.
authors.forename: Sushmita D
authors.initials: SD
authors.lastname: Lahiri
authors.affiliation: Infection Innovative Medicines Unit, AstraZeneca R&D Boston, Waltham, MA 02451, USA.
authors.forename: Robert E
authors.initials: RE
authors.lastname: McLaughlin
authors.affiliation: Infection Innovative Medicines Unit, AstraZeneca R&D Boston, Waltham, MA 02451, USA.
authors.forename: James D
authors.initials: JD
authors.lastname: Whiteaker
authors.affiliation: Infection Innovative Medicines Unit, AstraZeneca R&D Boston, Waltham, MA 02451, USA.
authors.forename: Jane E
authors.initials: JE
authors.lastname: Ambler
authors.affiliation: Infection Innovative Medicines Unit, AstraZeneca R&D Boston, Waltham, MA 02451, USA ralm@macrolidepharma.com.
authors.forename: Richard A
authors.initials: RA
authors.lastname: Alm
mesh_terms: D000900:Anti-Bacterial Agents / Q000494:pharmacology*; D001426:Bacterial Proteins / Q000235:genetics*; D002511:Cephalosporins / Q000494:pharmacology*; D014644:Genetic Variation; D005838:Genotype; D006801:Humans; D055624:Methicillin-Resistant Staphylococcus aureus / Q000187:drug effects* / Q000235:genetics* / Q000302:isolation & purification; D008826:Microbial Sensitivity Tests; D046915:Penicillin-Binding Proteins / Q000235:genetics*; D013203:Staphylococcal Infections / Q000382:microbiology; D000097583:Ceftaroline
chemical_list: D000900:Anti-Bacterial Agents; D001426:Bacterial Proteins; D002511:Cephalosporins; D046915:Penicillin-Binding Proteins; D000097583:Ceftaroline; C487789:mecA protein, Staphylococcus aureus
keywords: methicillin-resistant Staphylococcus aureus; penicillin-binding proteins; resistance