Discipline: mastitis; Keywords: antibiotic resistance, MIC, Staphylococcus aureus, mastitis, somatic cell count, seasons, regions.

The genus Staphylococcus comprises various species and strains which are pathogenic. The most clinically relevant staphylococci are the coagulase-positive Staphylococcus aureus, members of the S. intermedius group, and the non-aureus staphylococci. A characteristic of staphylococci is their ability to develop resistance to antibiotics (for example by mutations). In South Africa, S. aureus (maltose-positive) remains a challenge in udder health, even though this was reported as no longer the main challenge to udder health in most countries. Mastitis caused by S. aureus remains a problem in South Africa because it is resistant to most antibiotics and cannot be cured. A high proportion of cows are culled for this reason, having a negative economic impact on the dairy herd. The infected udder is considered the primary reservoir of S. aureus as the organism is mainly transmitted during milking. Another source is a proportion of heifers already infected with S. aureus entering the milking herd. This suggests routes of transmission in addition to the milking equipment and the milking parlour.

An understanding of S. aureus reservoirs and transmission is essential for effective control. Since 2005, an increasing number of coagulase-positive, maltose-negative staphylococci was isolated, later confirmed as maltose-negative S. aureus. These organisms did not cause the same level of udder damage than maltose-positive S. aureus and did not cause any chronic or repeat cases in the country. In more recent years, individual coagulase-positive and maltose-negative staphylococci indicated resistance, and an increase in somatic cell count (SCC) of infected udders. In the study by the authors cited below, this was taken further. The first and main objective was to investigate the retrospective antibiotic surveillance data of the unique maltose-negative S. aureus ST 2992 in various provinces, seasons and SCC categories over time. The second objective was to compare the results of the maltose-negative S. aureus, previously identified as an emerging pathogen, to those of maltose-positive S. aureus. 

A data set of 271 maltose-negative S. aureus isolates, from milk samples of 117 dairy herds, was examined between 2010 and 2017. The study also compared the susceptibility testing of 57 maltose-negative and 57 maltose-positive S. aureus isolated from 38 farms, from three provinces.

The 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. Antibiotic resistance trends over time differed between maltose-negative and maltose-positive S. aureus. Resistance of maltose-negative S. aureus indicated no significant differences amongst provinces, with limited differences amongst seasons for ampicillin, penicillin G and cephalexin. For cloxacillin and cephalonium specifically, high SCC corresponded with low antibiotic resistance, significantly different from low SCC to correspond with a high antibiotic resistance cephalonium. Furthermore, the results indicated more resistance for maltose-negative than for the maltose-positive S. aureus isolates to most of the antibiotics used.

In conclusion, the findings also showed resistance of the maltose-negative strain isolated from milk samples to antibiotics that are only used in human medicine, which implies a possible transfer from humans to animals and requires further studies under the “one health” approach. The fact that there are differences in antibiotic resistance profiles between the maltose-positive and maltose-negative S. aureus is useful information for both producers and veterinarians. This should provide different management and treatment protocols for the maltose-negative S. aureus which seems not to be a chronic intra-mammary infection, therefore preventing unnecessary culling.  


J. Karzis, I-M. Petzer, E. F. Donkin, V. Naidoo & E. M.C. Etter, 2020. Surveillance of Antibiotic Resistance of Maltose-Negative Staphylococcus aureus in South African Dairy Herds. Antibiotics 9, 616; doi:10.3390/antibiotics9090616