Analysis of antibiotic resistance phenotypes and molecular variants in antibiotic resistance genes among Mycoplasma synoviae isolates collected between 2018 and 2025.

INTRODUCTION

Antibiotic treatment represents one of the available control strategies for managing clinical outbreaks in flocks infected with Mycoplasma synoviae (MS). Therefore, elucidating the antibiotic susceptibility profiles of MS isolates and monitoring the evolutionary trends of resistance-associated genetic mutations are of critical importance.

METHODS

In present study, the minimum inhibitory concentrations (MICs) of 12 commonly used antimicrobial agents were evaluated for 35 MS isolates collected between 2018 and 2025 using the broth microdilution method. In addition, the molecular variation characteristics of the quinolone resistance-determining region (QRDR) and macrolide resistance-associated genes were analyzed in the same 35 MS strains.

RESULTS

All MS isolates exhibited low minimum inhibitory MIC values for tylvalosin (MIC₉₀ = 0.25 μg/mL) and valnemulin (MIC₉₀ = 0.0078 μg/mL), indicating high in vitro susceptibility to both agents. In contrast, the majority of isolates showed markedly elevated MICs for ciprofloxacin (0.25-32 μg/mL), enrofloxacin (0.5-16 μg/mL), and tilmicosin (0.125-8 μg/mL), suggesting reduced susceptibility or emerging resistance to these antimicrobials. The MIC values for the other ten drugs remained relatively low, indicating preserved efficacy. Two amino acid substitutions Asn345Ser in GyrB and Thr85Ile in ParC led to high level resistance to fluoroquinolones. In domain II of the 23S rRNA gene, nucleotide substitutions G778A and G915C, together with substitution G2761C in domain V, were contributed to the elevated MICs for macrolides, tetracyclines, and cephalosporins.

CONCLUSION

The results of this study would deepen veterinarians' understanding of resistance mechanisms and provide scientifically validated data to guide the rational use of existing antimicrobials, as well as to support the development of novel therapeutics against MS infection.