Close links between nosocomial MRSA and carriage MSSA isolates from the same location in Switzerland

Abstract number: P1421

Sakwinska O., Francioli P., Kuhn G., Moreillon P., Blanc D.S.

Staphylococcus aureus is a notorious nosocomial pathogen but it also harmlessly colonises a large fraction of healthy human population. Studies which simultaneously survey both carriage strains from healthy carriers and hospital-associated MRSA are rare. Such studies are, however, necessary to better understand evolution of this pathogen, and in particular MRSA. Resistance to meticillin in MRSA is conferred by the mecA gene situated on a mobile genetic element called SCCmec. It has been assumed that acquisitions of SCCmec by MSSA are relatively rare.

The nasal swabs were collected from 405 newly employed hospital personnel during their first medical check-up at the tertiary care hospital in Lausanne between October 2005 and July 2006. During 2005 and 2006, 1386 MRSA isolates from hospitalised patients of the same region were analysed. All isolates were typed by DLST, a newly developed, high resolution method based on sequencing of partial repeats sequences of clfB and spa. In addition, carriage MSSA isolates were typed with MLST.

130 out of 405 (32%) hospital employees were S. aureus carriers. Typing with DLST allowed distinguishing 115 unique DLST types and only 37 MLST types showing that DLST has superior resolution in comparison to MLST. Genetic diversity among carriage strains was extremely high, with majority of carriers colonised with a unique strain. In contrast, among hospital MRSA isolates nearly 50% were of a single DLST type. To our surprise, all locally predominant clones of MRSA were very closely related (single locus DLST type) to the MSSA counterparts encountered in carriers. What is more, the carriage isolates closely related to epidemic MRSA clones were also the most abundant ones.

It appears that the same genotypes make the most efficient colonisers as well as the most successful epidemic clones. This strongly indicates that epidemic behaviour of these strains is inherent to their genetic background independently of acquisition of resistance determinants.

Session Details

Date: 19/04/2008
Time: 00:00-00:00
Session name: 18th European Congress of Clinical Microbiology and Infectious Diseases
Location: Barcelona, Spain
Presentation type:
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