Antimicrobial susceptibility profiles (MIC and MMC) against Mycoplasma gallisepticum and M. synoviae isolated from chicken farms in Thailand

Mycoplasma gallisepticum (MG) and M. synoviae (MS) are important pathogens causing significant economic losses in poultry production. Disease prevention and control strategies include vaccination, antimicrobial therapy, and strict biosecurity measures. The present study provides, for the first time, both minimum inhibitory concentration (MIC) and minimum mycoplasmacidal concentration (MMC) values as antimicrobial susceptibility profiles of Thai MG and MS field isolates. This study aimed to evaluate the susceptibility of five MG and ten MS isolates to various antimycoplasma drugs: doxycycline, lincomycin-spectinomycin, tiamulin, tilmicosin, tylosin, and tylvalosin, using a microdilution broth assay. For MG isolates, tilmicosin had the highest mean MIC (12.5 µg/mL), while tylvalosin presented the lowest MIC (0.0488 µg/mL). For MS isolates, lincomycin-spectinomycin showed the highest MIC (0.2148 µg/mL), and tylvalosin performed the lowest (0.0488 µg/mL). This study also demonstrates the MMC values, showing tilmicosin had the highest MMC (12.5 µg/mL) for MG field isolates, while doxycycline had the highest MMC (0.3320 µg/mL) for MS field isolates. Tylvalosin consistently demonstrated the lowest MIC and MMC values for both MG and MS field isolates, suggesting its outstanding efficacy. Importantly, MMC values were introduced as a practical and insightful complement to MIC values by revealing viability of MG and MS field isolates at MIC levels of doxycycline (12 of 15 isolates), lincomycin-spectinomycin (7 of 15 isolates), tiamulin (1 of 15 isolates), tilmicosin (3 of 15 isolates), tylosin (4 of 15 isolates), and tylvalosin (2 of 15 isolates). These findings provide valuable insights for integrating MIC and MMC analysis into routine antimicrobial selection, contributing to more effective disease control, offering both growth inhibition and bactericidal effectiveness, and rational antimicrobial use in poultry farming, which could mitigate antimicrobial resistance, especially in poultry farms with intensive usage of antimicrobials.