Prevalence and characterization of Staphylococcus aureus and methicillin-resistant Staphylococcus aureus isolated from bulk tank milk in Shandong dairy farms

Staphylococcus aureus (S. aureus) is one of the most common pathogens that cause mastitis in dairy cows. The emergence of methicillin-resistant S. aureus (MRSA) constitutes a serious public health concern due to its ability to colonize and infect humans and animals. In this study, the prevalence and antimicrobial resistance of S. aureus and MRSA were investigated from bulk tank milk (BTM) in Shandong dairy farms. A total of 418 pooled BTM samples from 418 different dairy farms were collected. The prevalence of S. aureus was 28.9% (121/418), while the MRSA prevalence was 0.7% (3/418). The strains exhibited high percentages of resistance to penicillin (74.4%) and erythromycin (58.7%), and 91.7% of the isolates were found to be resistant to at least one antimicrobial agent. Moreover, 55.4% (67/121) of isolates were multidrug-resistant (MDR). The most predominant virulence genes were sed (13.2%, 16/121) and sec (8.3%, 10/121), which encode Staphylococcal enterotoxins (sed and sec). Multilocus sequence typing (MLST) was employed to identify 18 different sequence types (STs). The predominant STs were ST50 (13.2%, 16/121) and ST398 (12.4%, 15/121), while ST97, ST2779, and ST3191 were observed in MRSA isolates. Staphylococcal protein A gene (spa) typing identified spa types t034 (12.4%, 15/121) and t189 (9.9%, 12/121) most frequently among S. aureus isolates, while t002, t267, and t437 were observed in MRSA isolates. The results of this study indicated that S. aureus isolates had a relatively high prevalence rate in BTM, and these isolates harbored multiple virulence and exhibited multiple antimicrobial resistance. Meanwhile, our results confirm the lurking threat of MRSA in raw milk and suggest the need for surveillance programs starting at the farm level.

Automated summary

The study revealed a relatively high prevalence of S. aureus in bulk tank milk from Shandong dairy farms, with the isolates carrying multiple virulence genes and exhibiting multiple antimicrobial resistance. The lurking threat of MRSA in raw milk emphasizes the need for surveillance programs starting at the farm level.