CA-MRSA infections are increasing worldwide and are often caused by geographic predominating strain types, i.e. USA300-ST8-IV in the US and CC80-IV in Europe. Few nationwide studies have been performed characterizing isolates causing CA-MRSA infections. In this study national data of CA-MRSA obtained in a 9 yrs. period was investigated. The aim was to determine bacterial characteristics of MRSA causing community associated (CA) infections in Denmark.

Prospective collection, storage and testing of all Danish MRSA isolates from1999-2007 along with evaluation of clinical information obtained from discharge summaries and general practitioners notes.CA-MRSA infections were identified and all isolates were susceptibility tested by disc diffusion and typed by a combination of PFGE, spa and multi locus sequence typing (MLST) and annotated to clonal complex (CC).

CA-MRSA constituted 784/2170 (36.1%) of all MRSA infections in the study period. Numerically CA-MRSA increased from 11 in 1999 to 182 in 2007, corresponding to an increase in incidence from 0.21 to 3.34/100,000 inhabitants. Bacterial isolates could be ordered into 13 MLST-CCs, dominated by CC80-IV, CC8-IV, CC5-IV and CC30-IV. Antimicrobial susceptibility patterns were correlated to CCs.

CA-MRSA constituted an increasing number of Danish MRSA infections. Isolates were dominated by international renowned CA-MRSA clones dominated by CC80-IV but with an increasing diversity observed in the study period. Low level resistance to antimicrobials was predominantly seen in isolates belonging to CC30-IV, whereas most CC80-IV isolates were multi-resistant.