Prevalence and antimicrobial susceptibility profiles of Staphylococcus aureus from cow’s milk, nasal and environmental swabs in selected dairy farms in Morogoro, Tanzania

Abstract

Staphylococcus aureus is a microorganism that is present as a commensal on the skin, the nose and mucous membranes of healthy humans and animals. However, it is also an opportunistic pathogen that can cause multiple infectious diseases of diverse severity. A cross-sectional study was carried out to determine the prevalence and antimicrobial resistance profiles of S. aureus from samples of cow’s milk, farm environmental and cattle nasal swabs from three dairy farms in Morogoro. A total of 377 samples; raw milk (n = 100), nasal swabs (n=133) and environmental swabs (n = 144) were screened for the presence of S. aureus. California mastitis test (CMT) was used to establish the status of mastitis. Gram staining, oxidase, catalase, DNase, haemolysis and coagulase tests were employed for bacterial identification. Antimicrobial susceptibility testing was conducted using the Kirby- Bauer disk diffusion assay. Out of 200 cows (800 quarters milk) CMT screened for mastitis, (12.5%) quarters from 80 cows tested positive for subclinical mastitis, while twelve (1.5%) quarters from seven (3.5%) cows tested positive for clinical mastitis.Overall, 97 (25.7%) of 377 samples were positive for S. aureus, which were detected in 49.0%, 11.6%, and 40.0% of the milk, environment and nasal samples, respectively.Antimicrobial susceptibility testing revealed resistance to: Ampicillin (67.4%), Cefoxitin (14.8%), Erythromycin (21.1%), Gentamycin (3.2%), Oxacillin (37.9%), Tetracycline (55.8%), Trimethoprime-Sulfamethoxazole (29.5%) and Vancomycin(9.5%). 28.4% (n=27) of all isolates were resistant to Oxacillin and/or Cefoxitin, and therefore classified as Methicillin-Resistant Staphylococcus aureus (MRSA). 63%of the MRSA isolates originated from milk, 22.2% from nares and 14.8% from environmental samples. Over a half of all isolates were classified as multidrug resistant;of these 43.2% (n=41), 6.3% (n=6) and 1.1% (n=1) were simultaneously resistant to three, four and five antimicrobial agents, respectively. Taken together, this study revealed the prevalence of multidrug resistant S. aureus in cow’s milk, nares and farm environment. Our findings also confirm the presence of livestock-associated MRSA, and thus underline the importance of applying biosecurity measures and good hygiene practices to prevent MRSA spread at the farm level and throughout the food production chain.

Keywords: Staphylococcus aureus, Antimicrobial resistance, MRSA, Livestock environment