Antimicrobial resistance (AMR) poses an imminent threat to global public health, driven in part by the widespread use of antimicrobials in both humans and animals. Within the dairy cattle industry, Gram-negative coliforms such as Escherichia coli and Klebsiella pneumoniae stand out as major causative agents of clinical mastitis. These same bacterial species are frequently associated with severe infections in humans, including bloodstream and urinary tract infections, and contribute significantly to the alarming surge in antimicrobial-resistant bacterial infections worldwide. Additionally, mastitis-causing coliforms often carry AMR genes akin to those found in hospital-acquired strains, notably the extended-spectrum beta-lactamase genes. This raises concerns regarding the potential transmission of resistant bacteria and AMR from mastitis cases in dairy cattle to humans. In this narrative review, we explore the distinctive characteristics of antimicrobial-resistant E. coli and Klebsiella spp. strains implicated in clinical mastitis and human infections. We focus on the molecular mechanisms underlying AMR in these bacterial populations and critically evaluate the potential for interspecies transmission. Despite some degree of similarity observed in sequence types and mobile genetic elements between strains found in humans and cows, the existing literature does not provide conclusive evidence to assert that coliforms responsible for mastitis in cows pose a direct threat to human health. Finally, we also scrutinize the existing literature, identifying gaps and limitations, and propose avenues for future research to address these pressing challenges comprehensively.

Staphylococcus aureus is a mastitis pathogen that compromises cow health and causes significant economic losses in the dairy industry. High antimicrobial resistance and biofilm formation by S. aureus limit the efficacy of conventional treatments. This study evaluated the potential of polyhexamethylene biguanide nanoparticles (PHMB NPs) against mastitis-causing S. aureus. PHMB NPs showed low toxicity to bovine mammary epithelial cells (MAC-T cells) at concentrations up to four times higher than the minimum inhibitory concentration (1 µg/mL) against S. aureus. In Experiment 1, PHMB NPs significantly reduced biofilm formation by S. aureus by 50% at concentrations ≥1 µg/mL, though they showed limited efficacy against preformed biofilms. In Experiment 2, using an excised teat model, PHMB NPs reduced S. aureus concentrations by 37.57% compared to conventional disinfectants (chlorhexidine gluconate, povidone–iodine, and sodium dichloroisocyanurate), though limited by short contact time. These findings highlight the potential of PHMB NPs for the control of S. aureus growth and biofilm formation.

Antimicrobial resistance (AMR) is a global concern partly driven by excessive antimicrobial use (AMU) in animal production. Clinical mastitis (CM) is a leading cause of AMU in dairy farms, highlighting its importance from a public-health perspective. This observational study aimed to evaluate the use of antimicrobial susceptibility testing as a criterion for selecting antimicrobials for the treatment of mild and moderate CM in dairy cows. To this end, the association between in vitro antimicrobial susceptibility test results of mastitis-causing pathogens and in vivo bacteriological cure (BC) was assessed. A total of 340 cows with mild or moderate CM from 14 commercial dairy herds in Brazil were enrolled in the study. Isolates from the pretreatment milk samples were subjected to antimicrobial susceptibility testing (AST) using agar disk diffusion. Susceptibility results and cow-level variables were included in a generalized linear mixed model (GLMM) to evaluate their association with BC. No significant association was found between the antimicrobial susceptibility results and BC of the mastitis cases. In conclusion, although in vitro susceptibility testing remains an important tool for AMR surveillance, the findings of this study suggest that its utility in guiding antimicrobial treatment decisions in CM cases is limited.