A small Acinetobacter plasmid carrying the tet39 tetracycline resistance determinant

mediate ceftaroline resistance (Table 1). Two isolates (ASARM167 and A38) belonging to ST22 (epidemic MRSA-15) had an E239K substitution in PBP2a. ASARM167 was isolated from a patient with bacteraemia at Cambridge University Hospitals NHS Foundation Trust (CUH) in 2008 and A38 was isolated from a canine wound infection in 2006 treated in Wiltshire, southwest England. 8 Phylogenetic analysis of these two isolates based on core genome SNPs placed them in different clades separated by .120 SNPs (data not shown), indicating that the E239K mutation arose independently in these two isolates. The third isolate (ASARM130) had the N146K substitution in PBP2a, belonged to ST241 (CC8) and was isolated from a patient with bacteraemia at CUH in 2007. This isolate was also noted to have an N204K substitution, which has not been reported previously in isolates with the N146K substitution. The effect of these PBP2a substitutions on the ceftaroline resistance phenotype was evaluated for these three isolates using the disc diffusion assay based on EUCAST guidelines 10 and the Etest (bioMérieux, Lyon, France) according to the manufacturer's instructions. Two isolates were susceptible and one was resistant to cef-taroline by disc diffusion, but all three isolates were susceptible by Etest (Table 1). Although isolates with the N146K substitution have been reported previously to have an MIC of 0.5 mg/L (susceptible), all previously reported isolates with E239K had an MIC of ≥2 mg/L (resistant). 2,4 The lack of association between a resistant phenotype and the N146K substitution indicates that secondary chromosomal mutations are likely to be involved, as reported previously. 4,7 The three study isolates were cultured before the clinical introduction of ceftaroline into clinical practice in the USA in 2010 and Europe in 2012, demonstrating that these are natural variants of PBP2a that occur (albeit at low prevalence) even without pressure from ceftaroline use. All previously reported isolates with PBP2a substitutions mediating ceftaroline resistance belonged to CC5 and CC8, which has led to the suggestion that these two lineages might be more prone to such mutations. Our findings suggest that they probably occur in multiple MRSA lineages including the pandemic CC22 lineage, which is important in many parts of the world including Australia and the Middle East and is the dominant MRSA lineage in the UK and much of Europe. References 1 Otero LH, Rojas-Altuve A, Llarrull LI et al. How allosteric control of Staphylococcus aureus penicillin binding protein 2a enables methicillin resistance …