Diversity and antimicrobial resistance profile of bacteria isolated from sheep mastitis and bulk-tank milk in the east-centre region of Portugal

The microbiological quality of milk reflects both udder health and the subsequent contamination that can occur through direct and indirect reservoirs. This study aimed to identify bacterial diversity in milk samples from ewes with mastitis and in bulk-tank milk, and to investigate their antibiotic susceptibility/resistance profiles. A total of 232 milk samples were analysed and 22 bacterial species were isolated, with Staphylococcus aureus (20.3%) and S. epidermidis (18.6%) as the dominant species. The diversity of bacterial species isolated from the bulk-tank was higher and included 44 species, dominated by E. coli (20.6%), S. aureus (7.8%) and Hafnia alvei (7.2%). Antimicrobial susceptibility testing was performed on 217 bacterial isolates. E. coli isolated from individual milk samples revealed high rates of resistance to penicillins and tetracyclines, whereas S. aureus revealed high rates of resistance to aminoglycosides, polymyxin, sulfonamides and tetracycline, maintaining high levels of susceptibility to third generation cephalosporins, lincosamides, some macrolides, penicillins, phenicols and pleuromutilins. The resistance profile of bacteria isolated from the bulktank milk was more concerning than that of the milk of ewe with mastitis. These results point to the need to implement structural improvements on farms and raise awareness about strengthening biosecurity, medical prophylaxis and control of access to veterinary medicines to improve milk quality and reduce the use of antibiotics.