Streptococcus agalactiae causes high fish mortality and substantial economic losses in aquaculture systems. Therefore, evaluating the efficacy of antimicrobials is essential to support control strategies. Tylosin is a macrolide antibiotic widely used in veterinary medicine, that inhibits bacterial protein synthesis. The aim of this study was to assess the antimicrobial activity of tylosin against S. agalactiae.
The Minimum Inhibitory Concentration (MIC) of tylosin was determined for twelve S. agalactiae isolates using the broth microdilution method (128 – 0,0625μg.mL), following CLSI (2020) guidelines. The isolates were obtained from disease outbreaks in different Nile tilapia production systems in Brazil. Isolates were cultured at the Laboratory of Microbiology and Parasitology of Aquatic Organisms (LAPOA).
Quality control using Escherichia coli (ATCC 25922) and oxytetracycline yielded values within the expected range (0.5–2 µg/mL). Tylosin exhibited high antimicrobial activity, with an MIC of 0.5 µg/mL in 75% of the isolates tested. The remaining isolates showed MIC values ≥128 µg/mL, indicating variability in susceptibility among the isolates (Table 1).
Overall, ttylosin showed high activity against most clinical isolates of S. agalactiae from Nile tilapia, with a predominant MIC of 0.5 µg/mL, indicating its potential as an efficient therapeutic option for the control of this pathogen in aquaculture systems.