Why non-aureus Staphylococcus (NAS) isolated from bovine milk should be a concern for the rise of superbugs

Abstract

Non-aureus Staphylococci (NAS) are most often reported as a group, but there are currently over 70 recognized species. Some species are becoming increasingly antibiotic-resistant, mainly by acquiring resistance genes. Therefore, knowing the genotypic profile of these species provides valuable information about the pathogenic potential of these microorganisms, which in turn facilitates more effective therapeutic approaches against these pathogens. This work aimed to analyze the genetic profile of 14 methicillin-resistant Staphylococcus non-aureus spp. (MRNAS) isolated from milk cows with low and high somatic cell count (SCC), and dairy cows with clinical mastitis. The isolates were selected after identifying the mecA gene using conventional PCR from a previous analysis. Then, using whole genome sequencing (WGS), the genomes were analyzed for the presence of resistance genes and virulence genes, identification of the SCCmec type, presence of mobile genetic elements (MGEs) such as bacteriophages and plasmids, sequence type (ST) and phylogenetic analysis. The results showed the presence of twenty-three acquired resistance genes and twenty-nine virulence genes. The SCCmec type was identified in 9 (64 %) of the 14 genomes analyzed. Plasmids and bacteriophages responsible for transferring resistance genes were also identified, containing genes such as those for resistance to streptomycin, erythromycin, lantibiotics, trimethoprim, and lincosamide. The phylogenetic tree showed three distinct clades, having a diverse number of STs between the genomes, which, combined with WGS, allowed the efficient typing of the NAS genomes. These results show that the S. epidermidis species was more pathogenic than the others analyzed. Furthermore, all NAS genomes presented virulence and resistance genes, in addition to MGEs responsible for the horizontal transfer of these genes between species, which is very worrying. Although studies indicate NAS as a secondary pathogen of mastitis, its control is necessary due to the large number of virulence and resistance genes found in this analysis; and the presence of MGEs responsible for gene transfer between species. In addition, milk can be an agent of transmission for these resistant microorganisms to consumers, thus contributing to the increase in antimicrobial resistance in animals and humans.