Abstract:
Antibiotic resistance has been reported since the 1940s in both human and veterinary
medicine. Many years of monitoring milk samples in South Africa led to identification of a novel
maltose-negative Staphylococcus aureus (S. aureus) strain, which appears to be an emerging pathogen.
In this study, the susceptibility of this strain to antibiotics was evaluated over time, during diverse
seasons in various provinces and according to somatic cell count (SCC) categories. A data set of
271 maltose-negative S. aureus isolates, from milk samples of 117 dairy herds, was examined using the
disk diffusion method, between 2010 and 2017. This study also compared the susceptibility testing of
57 maltose-negative and 57 maltose-positive S. aureus isolated from 38 farms, from three provinces
using minimum inhibitory concentration (MIC). The MIC results for the maltose-negative S. aureus
isolates showed highest resistance to ampicillin (100%) and penicillin (47.4) and lowest resistance
(1.8%) to azithromycin, ciprofloxacin and erythromycin. The maltose-negative S. aureus isolates
showed overall significantly increased antibiotic resistance compared to the maltose-positive strains,
as well as multidrug resistance. Producers and veterinarians should consider probability of cure of
such organisms (seemingly non-chronic) when adapting management and treatment, preventing
unnecessary culling.
