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

Objectives: 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.

Materials and methods: 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.

Results: 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.

Conclusions: 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.