Susceptibility testing

The minimum inhibitory concentration (MIC) determinations were performed according to the broth microdilution method described in the Clinical and Laboratory Standard Institute guidelines (CLSI, 2013). Briefly, serial 2-fold dilutions of FFC or TAP in cation-adjusted Mueller-Hinton II broth (MHIIB; Difco) or MHIIB supplemented with 5% sheep blood for S. suis were prepared in a 96-well U bottom microtiter plate (Nunc, ThermoScientific). Bacteria at final concentration of 5 × 10⁵ colony-forming unit (CFU)/mL were inoculated into each well and grown at 37°C for 18 h. The plates were assessed by eye. The MIC was determined to be the lowest concentration with no visible clumps. E. coli (ATCC 25922) was used as a reference strain for the standardization of antibiotic susceptibility. All MIC assays in this study were done at least in triplicate.