Etiology and antimicrobial resistance of subclinical mastitis pathogens Staphylococcus aureus, Streptococcus spp. and Enterococcus spp. in sheep milk

The present study investigates the prevalence and etiology of subclinical mastitis in Țurcana sheep flocks located in south-western Romania. Milchtest and California Mastitis Test (CMT), were used for the detection of subclinical mastitis. A number of 360 milk samples across three lactation phases were analyzed. Subclinical mastitis was identified in 146 (40.6%) of sheep milk samples. Only milk samples (n = 146) collected from sheep diagnosed with subclinical mastitis were considered for microbiological analyses. Thus, Staphylococcus aureus (129/146; 88.4%) was identified as the predominant pathogen followed by Streptococcus spp. (14/146; 9.6%) Enterococcus spp. (7/146; 4.8%), Pseudomonas aeruginosa (6/146; 4.1%) and Klebsiella pneumoniae (5/146; 3.4%). Antimicrobial susceptibility of isolated strains of Staphylococcus aureus, Streptococcus spp. and Enterococcus spp. was also tested. The results showed varying resistance patterns depending on the farm and microorganism. In heard A the highest resistance of Staphylococcus aureus was identified to polymyxin B (79.7%) followed by novobiocin (73.4%) and tetracycline (45.3%); Streptococcus spp. to streptomycin (100%), novobiocin (100%), tetracycline (100%) and Enterococcus spp. to tylosin (100%), streptomycin (75.0%) and tetracycline (75.0%). In herd B, Staphylococcus aureus showed high resistance rates to polymyxin B (86.3%), novobiocin (74.5%) and cloxacillin (39.2%); Streptococcus spp. strains to streptomycin (100%), novobiocin (100%), tetracycline (100%) and Enterococcus spp. to gentamicin (100%), streptomycin (66.7%) and erythromycin (66.7%). The obtained preliminary results indicate a potential risk that future treatments of sheep mastitis may not be effective, and demonstrated that raw unpasteurized milk can harbor antimicrobial resistance pathogens posing a threat to public health. However, further investigations involving molecular research on resistance genes are required to draw comprehensive conclusions.