Susceptibility of mastitis-causing pathogens (Escherichia coli and Staphylococcus aureus) to disinfectants used as...

Mastitis is one of the most important diseases for dairy farming and Escherichia coli and Staphylococcus aureus are among the most common mastitis-causing pathogens. Aiming to prevent mastitis, pre- and post-milking disinfectant dips are used, but this constant exposure can contribute to the emergence of antimicrobial resistant strains. Thus, this study aimed to determine the in vitro susceptibility of mastitis-causing S. aureus and E. coli to disinfectants used as teat dipping. The disinfectants tested were hydrogen peroxide, sodium hypochlorite, chlorhexidine digluconate, lactic acid, quaternary ammonium, and iodine. Susceptibility was assessed through broth microdilution to obtain minimal inhibitory concentration (MIC). In general, all E. coli and S. aureus strains were inhibited with lower concentrations of chlorhexidine digluconate, hydrogen peroxide, iodine, lactic acid, and quaternary ammonium than those used in field. Regarding sodium hypochlorite, 80.77 % (42/52) of the E. coli isolates were not susceptible to the concentration recommended for use as teat dipping, while 34.5 % (138/400) of the S. aureus had the MIC equal or higher than this same concentration. In addition, an increase in the MIC according to the year of isolation of the pathogens was observed, with the latest isolates being more tolerant to all disinfectants, with exception of chlorhexidine digluconate. These results strongly suggest that continuous exposure to disinfectants can lead to the selection of less susceptible strains in dairy farming, which is a major issue for animal and public health.