Recent antimicrobial susceptibility of Lawsonia intracellularis field isolates from pigs with proliferative hemorrhagic enteropathy in Korea

The study was performed to investigate the antimicrobial susceptibility of L. intracellularis field isolates from Korean pig farms. The novel four field isolates successfully obtained from clinical cases were prepared in IEC-18 cells to conduct the in vitro antimicrobial susceptibility testing (AST) by determining minimum inhibitory concentrations (MIC)s; either intracellular (InMIC) or extracellular MICs (ExMIC). The final MICs were assessed by counting the number of heavily infected cells (HICs;> 30 bacteria per cell) using an immunoperoxidase monolayer assay. Enrofloxacin (0.125 to 0.25 μg/mL: InMIC and 2 to 16 μg/mL; ExMIC) presented the most notable antimicrobial susceptibility, and marbofloxacin (0.25 to 0.5 μg/mL and 4 to 32 μg/mL) followed. Colistin (0.125 to 2 μg/mL and 2 to 4 μg/mL) presented a susceptibility followed by tylvalosin (0.5 to 1 μg/mL and 2 to 4 μg/mL). Florfenicol and lincomycin had the weakest susceptibility and amoxicillin, penicillin G, chlortetracycline, oxytetracycline, tiamulin, tilmicosin and tylosin displayed weak susceptibility. Dividing cells in culture and strict environmental conditions to isolate and cultivate L. intracellularis has limited the maintenance of this bacteria in vitro to only a few laboratories. Four isolates were successfully maintained in our laboratory, which enabled us to determine the antimicrobial susceptibility profile. In addition, the results could be one of the contributions in clinical fields.