| Summary: | Fluoroquinolones (FQ) are commonly used in dogs with bacterial skin infections. Their use as first choice, along with the increased incidence of FQ-resistance, represents a risk to animal and public health. Our study determined minimum inhibitory (MIC) and bactericidal (MBC) concentrations of five FQs in <i>Staphylococcus aureus</i>, <i>Staphylococcus pseudintermedius,</i> and <i>Escherichia coli,</i> together with FQ-resistance mechanisms. MICs, efflux pump (EP) overexpression and MBCs were measured in 249 skin infection isolates following CLSI guidelines (CLSI VET01-A4, CLSI M26-A). Chromosomal and plasmid-mediated resistance genes were investigated after DNA extraction and sequencing. FQ-resistance was detected in 10% of methicillin-susceptible (MS), 90% of methicillin-resistant (MR) staphylococci and in 36% of <i>E. coli</i>. Bactericidal effect was observed except in 50% of MRSA/P for ciprofloxacin and in 20% of MRSPs for enrofloxacin. Highest MICs were associated with double mutation in <i>gyrA</i> (Ser83Leu + Asp87Asn), efflux pumps and three PMQR genes in <i>E. coli</i>, and <i>grlA</i> (Ser80Phe + Glu84Lys) in <i>S. aureus</i>. EP overexpression was high among <i>E. coli</i> (96%), low in <i>S. aureus</i> (1%) and absent in <i>S. pseudintermedius</i>. Pradofloxacin and moxifloxacin showed low MICs with bactericidal effect. Since in vitro FQ resistance was associated with MR, FQ use should be prudently guided by susceptibility testing.
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