Bovine Mastitis Therapy at a Crossroads: Pharmacokinetic Barriers, Biofilms, Antimicrobial Resistance, and Emerging Solutions.

Bovine mastitis remains a major challenge in dairy production despite extensive antimicrobial use, with therapeutic failure increasingly attributed to factors beyond classical antimicrobial resistance (AMR). Growing evidence indicates that treatment inefficacy arises from the combined effects of pharmacokinetic/pharmacodynamic (PK/PD) constraints, biofilm-mediated tolerance, intracellular persistence, and the adaptive capacity of mastitis pathogens. Intramammary therapy is particularly limited by poor tissue penetration, episodic drug elimination via milk flow, and inactivation by milk components, frequently resulting in subtherapeutic exposure at the site of infection. These limitations are amplified in chronic and subclinical mastitis, where biofilms and intracellular reservoirs reduce antimicrobial susceptibility and promote relapse and resistance selection. This narrative review integrates current knowledge on pharmacokinetic and pharmacodynamic (PK/PD) barriers, microbial survival strategies, and antimicrobial resistance (AMR) mechanisms that underlie treatment failure in bovine mastitis. It critically evaluates conventional antimicrobial therapies alongside emerging approaches, including antimicrobial peptides, bacteriophages and endolysins, nanoparticle-based delivery systems, immunomodulators, CRISPR-guided antimicrobials, and drug repurposing strategies. Overall, available evidence highlights the potential of these approaches to enhance therapeutic durability, particularly in settings where biofilm formation, intracellular persistence, and resistance limit conventional treatment efficacy. By mapping research coverage across mastitis phenotypes and therapeutic outcomes, this review identifies key gaps in long-term efficacy and resistance mitigation and underscores the need for PK/PD-guided, biofilm-aware, and resistance-conscious strategies to support durable next-generation mastitis management.