Abstract: The mutant prevention concentration (MPC) represents a threshold above which the selective proliferation of resistant mutants is expected to occur only rarely. Measurement of the minimal inhibitory concentration (MIC) and the MPC for 32 veterinary clinical strains of Streptococcus suis revealed that ofloxacin and enrofloxacin are more potent than ciprofloxacin and pefloxacin mesylate to restrict the selective enrichment of resistant mutants. Based on their potential for restricting the selection of resistant mutants, the four fluoroquinolones, in descending order, were found to be enrofloxacin > ofloxacin > ciprofloxacin > pefloxacin mesylate. For several compounds, all of the clinical isolates lacked a known resistance mutation in the quinolone resistance-determining region (QRDR) of gyrA, gyrB, parC and parE. Additionally, the isolates were tested for CCCP or reserpine-sensitive efflux. The MICs of ciprofloxacin, enrofloxacin, ofloxacin and pefloxacin mesylate for each single-step mutants were determined by agar dilution on MuellerHinton agar plates plus 5% sheep blood in the presence and absence of CCCP (40 mg·L^-1) or reserpine (50 mg·L^-1). As a result, isolates showed no reduction in MIC in the presence of CCCP and considered negative for CCCPsensitive efflux. Nevertheless, it revealed a four or morefold decrease of ciprofloxacin accumulation in the presence of reserpine in the ciprofloxacin first-step mutants compared to that in the parent resistant strain, and it′s considered that there is a reserpine-sensitive efflux to decrease the level of ciprofloxacin accumulation in single-step mutants.

Key words: fluoroquinolone, Streptococcus suis, MIC, MPC, single-step resistant mutants, active efflux pump