Investigation of epidemiological links between patients harbouring identical, non-predominant MRSA genotypes as defined by double-locus sequence typing

Abstract number: P930

Senn L., Zanetti G., Bally F., Chuard C., Cometta A., Eisenring M.C., Kuhn G., Blanc D.S.

Background: Molecular epidemiology postulates that 2 isolates belong to the same chain of transmission if they are similar according to a highly discriminatory molecular typing method. This has been demonstrated in outbreaks, but was never studied in endemic situations. Epidemiological links cannot be established between isolates of meticillin-resistant Staphylococcus aureus (MRSA) that belong to endemic predominant genotypes. In contrast, small clusters of identical genotypes, some of which are subtypes of predominant genotypes, could be studied for epidemiological tracking.

Objective: To investigate epidemiological links between patients harbouring MRSA genotypes that belong to non-predominant typing clusters at a regional level.

Methods: We recently showed that double-locus sequence typing (DLST), a new typing strategy for MRSA that uses the repeat sequences of clfB and spa genes, was reproducible and discriminant enough to make this method a candidate of choice for epidemiological analyses. Our laboratory centralises the majority of the MRSA isolates from Western Switzerland. All nonrepetitive isolates received between 2005 and 2006 were typed by DLST.

Results: 1658 MRSA isolates were typed. 1118 (67%) belonged to 4 predominant genotypes, 216 (13%) to 54 small clusters of 2–12 patients, and 324 (20%) were unique. Strong epidemiological links (simultaneous hospitalisation in the same ward/hospital/long term care facility, or member of the same family) were found in 22/54 (41%) small clusters of non-predominant genotypes, totalising 60/216 patients (28%). In addition, weaker epidemiological links (hospitalisation during the same year in the same ward/hospital/long term care facility, or same place of residence) were found in 41/216 patients (19%). In the remaining 115/216 (53%) patients harbouring identical non-predominant genotypes, no epidemiological link could be detected.

Discussion: In our setting of patients harbouring non-predominant MRSA genotypes, epidemiological data could not explain the chain of transmission behind all isolates of the same genotype. One hypothesis is that our surveillance system was not able to detect the missing links. Another hypothesis is that some genotypes are stable over time, so that, as for predominant genotypes, epidemiological links can no longer be found.

Conclusions: Epidemiological tracking of MRSA isolates at a regional level cannot be inferred from molecular typing alone.

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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