Antimicrobial resistance trends of methicillin-resistant Staphylococcus aureus in Norway from 2008 to 2017

The purpose of this study was to analyse nationwide trends in antimicrobial susceptibility and molecular epidemiology of all MRSA strains in Norway over a 10-year period.

All cases of MRSA in Norway from 2008 to 2017 were included, limited to the first case per year per individual (n = 15 200). Strains were confirmed as MRSA with PCR and genotyped using spa-typing. Antimicrobial susceptibility data and epidemiological data were collected from the Norwegian MRSA reference laboratory and the Norwegian Surveillance System for Communicable Diseases, respectively.

Despite an increase in MRSA cases, antimicrobial resistance rates remained stable throughout the study period. The most common susceptibility profile of the MRSA strains was resistance to cefoxitin alone (41.4%), while co-resistance (58.6%) was observed most commonly for erythromycin (31.0%), tetracycline (25.3%) and ciprofloxacin/norfloxacin (21.6%). Notably, associations were identified between specific resistance patterns and clinical variables, including higher resistance rates among healthcare-associated MRSA, particularly in older adults and nursing home residents. Geographic associations were also observed, linking specific resistance profiles to strains acquired in North America, Africa and Asia.

The findings highlight a complex landscape of MRSA resistance in a low-prevalence country, characterized by a diverse genotypic population and stable longitudinal trends in resistance rates. However, the prevalence of co-resistance to one or more antibiotics among Norwegian MRSA strains was high and increasing. This study thus underscores the importance of continuous surveillance and adherence to antimicrobial guidelines.