Veterinary macrolides against Streptococcus agalactiae: antibacterial basis, resistance determinants, and PK/PD implications.

Streptococcus agalactiae remains an important pathogen causing bovine mastitis, with measurable impacts on herd productivity and milk quality. Veterinary macrolides retain antibacterial activity against a substantial proportion of S. agalactiae isolates, as evidenced by in vitro susceptibility data across multiple geographic regions, although resistance determinants such as erm(B) are prevalent. Pharmacokinetic data indicate that macrolides can achieve significant mammary-site exposure, including prolonged milk persistence and favorable tissue distribution, providing a pharmacological basis for potential efficacy. However, the interpretation of susceptibility is limited by the lack of validated mastitis-specific breakpoints, variable genotype-phenotype concordance, regulatory restrictions, milk residue concerns, and insufficient pathogen-specific clinical outcome data. This review integrates the antibacterial basis, resistance determinants, susceptibility interpretation, regulatory constraints, mammary-site pharmacokinetic/pharmacodynamic (PK/PD) considerations, biofilm potential, and antimicrobial stewardship implications for veterinary macrolides against bovine S. agalactiae. Overall, macrolides demonstrate biological plausibility and partial microbiological support, but their clinical utility in treating bovine mastitis remains incompletely defined. Further studies integrating mastitis-specific PK/PD targets, pathogen-stratified clinical outcomes, and longitudinal resistance surveillance are warranted to better define the practical role of veterinary macrolides in bovine S. agalactiae mastitis.