Non-aureus Staphylococci (CoNS) from clinical and environmental sources

Background: We have amassed a diverse collection of 400 non-aureus staphylococci (NAS), comprising isolates from clinical samples, healthy human volunteers and animals. NAS are implicated in certain healthcare-acquired infections but the burden of antimicrobial resistance (AMR) is understudied relative to S. aureus.Objectives: We aimed to test the antimicrobial susceptibility of our NAS strain collection to determine the prevalence of clinically relevant MICs.Methods: We subjected all isolates to broth dilution susceptibility testing using a multi-point inoculator against 8 antibiotics and made use of whole genome sequencing data on 316 isolates to assess the genetic mechanisms responsible for the observed levels of resistance.Results: Cefoxitin is used as a surrogate antibiotic for methicillin testing in S. aureus, as it stimulates expression of mecA. We observed 142 isolates with an MIC of at least 4 mg/L to cefoxitin, of which 54% (77/142) harboured an identifiable mecA gene. Differences in MICs against erythromycin were attributable to the presence of different resistance genes (msrA and ermC). In total, 48% (192/400) of isolates were multidrug resistant, displaying resistance to 3 or more of the antibiotics tested.Conclusions: The presence of MDR in NAS is a concern, with an increased likelihood of (1) MDR infections caused directly by NAS, and (2) these genes being passed on to other bacteria via horizontal gene transfer, both of which have clinical implications for treatment and management of patients