Non-aureus staphylococci and mammaliicocci isolated from dairy cow milk and their relationships with the presence of mastitis: a retrospective investigation

Non- aureus staphylococci and mammaliicocci (NASM) have been associated with bovine mastitis and increased milk somatic cell count (SCC) but their relationships with mammary gland health at the species level are not completely understood. The implementation of MALDI-TOF MS in milk microbiology is generating large and dependable datasets with the potential of providing useful information in this respect. We present the retrospective analysis of 17,213 milk samples sent to our laboratory in 2021–2022, including 13,146 quarter samples from cows with subclinical (SCM) or clinical mastitis (CM) from 104 farms, and 4,067 composite herd survey (HS) samples from 21 farms. NASM were isolated from 21.12% of SCM, 11.49% of CM, and 15.59% of HS milk samples. The three most frequently identified NASM in SCM milk were Staphylococcus chromogenes (33.33%), S. haemolyticus (26.07%), and S. epidermidis (10.65%); these were significantly associated with the presence of SCM (p < 0.05) together with S. microti and S. hyicus . The three most frequently identified NASM in CM milk were S. chromogenes (31.69%), S. haemolyticus (21.42%), and Mammaliicoccus sciuri (18.38%); however, no significant associations were found between NASM species and CM. The three most frequently identified NASM in HS milk were S. chromogenes (44.49%), S. epidermidis (17.84%), and S. haemolyticus (17.23%), with S. chromogenes being isolated in all the farms sending HS milk (100%). In conclusion, this retrospective study contributes to expanding our knowledge on the epidemiology of NASM at the species level and provides further insights on their relationships with mammary gland health in modern dairy farms.