In vitro and in vivo susceptibility to cefalexin and amoxicillin/clavulanate in canine low-level methicillin-resistant Staphylococcus pseudintermedius

Abstract

Background: Methicillin-resistant Staphylococcus pseudintermedius (MRSP) lineages harbouring staphylococcal cassette chromosome (SCC) mec types IV, V and ψSCCmec57395 usually display low oxacillin MICs (0.5-2 mg/L). Objectives: To evaluate how oxacillin MICs correlate with PBP mutations and susceptibility to β-lactams approved for veterinary use. Methods: Associations between MICs and PBP mutations were investigated by broth microdilution, time-kill and genome sequence analyses in 117 canine MRSP strains harbouring these SCCmec types. Clinical outcome was retrospectively evaluated in 11 MRSP-infected dogs treated with β-lactams. Results: Low-level MRSP was defined by an oxacillin MIC <4 mg/L. Regardless of strain genotype, all low-level MRSP isolates (n = 89) were cefalexin susceptible, whereas no strains were amoxicillin/clavulanate susceptible according to clinical breakpoints. Exposure to 2× MIC of cefalexin resulted in complete killing within 8 h. High (≥4 mg/L) oxacillin MICs were associated with substitutions in native PBP2, PBP3, PBP4 and acquired PBP2a, one of which (V390M in PBP3) was statistically significant by multivariable modelling. Eight of 11 dogs responded to systemic therapy with first-generation cephalosporins (n = 4) or amoxicillin/clavulanate (n = 4) alone or with concurrent topical treatment, including 6 of 7 dogs infected with low-level MRSP. Conclusions: Oxacillin MIC variability in MRSP is influenced by mutations in multiple PBPs and correlates with cefalexin susceptibility. The expert rule recommending that strains with oxacillin MIC ≥0.5 mg/L are reported as resistant to all β-lactams should be reassessed based on these results, which are highly clinically relevant in light of the shortage of effective antimicrobials for systemic treatment of MRSP infections in veterinary medicine.