Abstract
Methicillin-resistant Staphylococcus aureus (MRSA) USA300 isolates have been recognized globally, not only in community but also in healthcare settings. USA300 isolates were initially resistant only to methicillin, but resistance to non-β-lactams has emerged with time. To evaluate the prevalence and antimicrobial susceptibility of USA300 isolates in Stockholm, we conducted a nine-year retrospective study. Of 5359 consecutive MRSA cases in Stockholm, isolates from 285 cases were USA300 strains according to the pulsed-field gel electrophoresis pattern. Of these cases, repeated isolates with altered antibiotic resistance patterns were observed in six individuals. Therefore, antimicrobial susceptibility testing was performed on totally 291 isolates. To study the phylogenetic relatedness of isolates in transmission events and genomic resistance traits, 35 isolates were further studied by whole genome sequencing (WGS). The incidence of MRSA was increased from 17.6 per 100,000 inhabitants in 2008 to 37.3 per 100,000 inhabitants in 2016, while the proportion of USA300 cases declined from 6.6% in 2008 to 2.6% in 2016. Among the USA300 isolates, 73.5% were community-associated, 21.3% healthcare-associated, and 5.2% had unknown acquisition. The highest resistance rate among non-β-lactams was found in erythromycin (86%), followed by fluoroquinolones (68–69%). 57% of the isolates were resistant to both erythromycin and fluoroquinolone. Simultaneous resistance to four non-β-lactam antibiotic classes was found in six isolates. Four isolates were susceptible to all non-β-lactam antibiotics. Ceftaroline, daptomycin, linezolid, mupirocin, rifampicin, teicoplanin, telavancin, trimethoprim-sulfamethoxazole and vancomycin retained full activity in the study. WGS analysis indicated that isolates from an outbreak were phylogenetically closely related. In conclusion, USA300 MRSA isolates in Stockholm have neither been limited to the community setting, nor remained susceptible to non-β-lactam agents. WGS is becoming a useful tool in tracing transmission events. The results herein provide the most up-to-date and comprehensive information regarding status of USA300 strains in this geographic area.
Highlights
Methicillin-resistant Staphylococcus aureus (MRSA) USA300 emerged first as communityassociated MRSA in the USA in the late 1990s [1, 2]
Patient data were reviewed to distinguish CA-MRSA strains from healthcare-associated MRSA (HA-MRSA) strains by epidemiological criteria
Strains that did not meet the definition of CA-MRSA were classified as HA-MRSA [9, 10]
Summary
Methicillin-resistant Staphylococcus aureus (MRSA) USA300 emerged first as communityassociated MRSA in the USA in the late 1990s [1, 2]. USA300 strains spread rapidly across the USA, and presently predominate in community, and in healthcare settings. It is likely that USA300 strains are endemic pathogens worldwide, and not imported strains [2]. The first USA300 MRSA case in Stockholm was observed in January 2004 in a patient who had acquired MRSA abroad. In the following years the USA300 strains were reported as community-associated [3]. As occurred in other countries, these strains are emerging as healthcare-associated MRSA (HA-MRSA) in Stockholm
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
