Antibiotic-resistant Staphylococcus aureus isolated from milk in the Mafikeng Area, North West province, South Africa

Abstract

The aim of this study was to isolate Staphylococcus aureus from samples of cow's milk obtained from different farm settings and to determine their antibiotic susceptibility patterns. Gram staining, oxidase, catalase, DNase, haemolysis and the MASTASTAPH^TM rapid agglutination tests were employed for bacterial identification. A total of 28 milk samples were collected and screened for the presence of S. aureus. All the samples were contaminated with S. aureus. A total of 240 S. aureus isolates were obtained during this study. The levels of contamination with S. aureus were higher in milk obtained from the communal farms in Lokaleng and Mogosane (24.6% and 35.4%, respectively)compared to the commercial farms in Rooigrond and Molelwane (17.9% and 22.1%, respectively). A large percentage of the S. aureus isolates (39% - 100%) from both communal farms was resistant to methicillin (MT), ampicillin (AP), penicillin G (PG), sulphamethoxazole (Smx), oxytetracycline (OT), erythromycin (E), nitrofurantoin (NI) and streptomycin (S), but not vancomycin (V). An even higher percentage (64.2% - 100%) of the isolates from both commercial farms was resistant to sulphamethoxazole and nitrofurantoin. A comparably smaller percentage (3.4% - 4.7%) of the isolates from both communal farms was resistant to vancomycin, but all isolates from commercial farm milk were susceptible to this drug. The predominant multiple antibiotic resistant phenotypes for isolates from the commercial farms were AP-Smx-NI and MT-AP-PG-OT-Smx-NI for Rooigrond and Molelwane farms, respectively, while those for isolates from the communal farms were MT-AP-PG-Smx-E-NI-S and MT-AP-PG-OT-Smx-NI-S for Lokaleng and Mogosane, respectively. When comparing the percentage of antibiotic resistance, a significant positive correlation was observed between the isolates from the commercial farms (r = 0.966, p < 0.01). S. aureus is normally resident in humans, therefore, the S. aureus present in the cows may have resulted from transmission between the two species, emphasising the need to improve sanitary conditions in the milking environment.