Surveillance of antimicrobial resistance of maltose negative Staphylococcus aureus in South African dairy herds

Abstract

BackgroundThe discovery of antimicrobials in the 1930s was one of the greatest achievements in medicine. However, bacterial resistance to antimicrobials was already observed in the 1940s and has been reported since then in both human and veterinary medicine, including in dairy cows. Many years of monitoring milk samples in South Africa, has led to the identification of a new strain of Staphylococcus aureus (S. aureus), which is maltose negative and appears to be an emerging pathogen. In this study the differences in susceptibility to antimicrobials of this strain were evaluated over time, over different seasons, in different provinces, and according to somatic cell count (SCC) categories.Results A data set of 271 maltose negative S. aureus isolates, cultured from milk samples from 117 herds out of the estimated 2000 commercial dairy herds in South Africa between 2010 and 2017, was studied using the disc diffusion method. This analysis was done using the Clinical Laboratory Standards Institute (CLSI) breakpoints in order to compare using both the previously used system (intermediate category grouped with resistant) and more recent system, (intermediate category grouped with susceptible). The results between the previously used system and the more recent system analysis differed for tylosin, cefalonium, oxy-tetracycline and cloxacillin. Neither the analysis using the previous system nor the more recent system showed an effect of province for the maltose negative S. aureus. This was in contrast to the results for maltose positive S. aureus where differences between provinces were shown in a previous study. For the susceptibility testing of 57 maltose negative S. aureus and 57 maltose positive S. aureus the minimum inhibitory concentration (MIC) results for the maltose negative S. aureus confirmed the results of the disc diffusion method. ConclusionsThe maltose negative strains of S. aureus differed in general, in their antimicrobial resistance patterns over time, in comparison to maltose-positive S. aureus strains. MIC testing also indicated that more multi-resistant isolates were seen with the maltose negative S. aureus than in the maltose positive strains.