Open Access Highly Accessed Open Badges Research article

Progression from new methicillin-resistant Staphylococcus aureus colonisation to infection: an observational study in a hospital cohort

Michelle ND Balm12, Andrew A Lover3, Sharon Salmon1, Paul A Tambyah45 and Dale A Fisher145*

Author Affiliations

1 Infection Control Team, National University Hospital, Singapore, Singapore

2 Microbiology, Department of Laboratory Medicine, National University Hospital, Singapore, Singapore

3 Infectious Diseases Programme, Saw Swee Hock School of Public Health, National University of Singapore, Singapore, Singapore

4 Division of Infectious Diseases, University Medicine Cluster, National University Hospital, Singapore, Singapore

5 Department of Medicine, Yong Loo Lin School of Medicine, National University of Singapore, Singapore, Singapore

For all author emails, please log on.

BMC Infectious Diseases 2013, 13:491  doi:10.1186/1471-2334-13-491

Published: 22 October 2013



Patients newly colonised with methicillin-resistant Staphylococcus aureus (MRSA) are at higher risk of clinical MRSA infection. At present, there are limited data on the duration or magnitude of this risk in a hospital population with a known time of MRSA acquisition.


A retrospective cohort study of 909 adult patients known to have newly identified MRSA colonisation during admission to National University Hospital, Singapore between 1 July 2007 and 30 June 2011 was undertaken. Patients were excluded if they had history of previous MRSA colonisation or infection, or if they had been a hospital inpatient in the preceding 12 months. Data were collected on the development of MRSA infection requiring hospitalisation up to 30 June 2012.


Of 840 patients newly colonised with MRSA as identified on active surveillance and not clinical specimens, 546 were men (65.0%) and the median age was 65 years (range 18–103 years). Median follow up was 24 months (range 0 –64 months, 85.1% followed >6 months). Clinical infection occurred in 121 patients (14.4%) with median time to infection of 22 days (95% CI 14–31). Overall 71.9% (87/121) of infected patients developed infection within 60 days of the date MRSA colonisation was detected. However, 17/121 patients (14.0%) developed clinical infection more than six months after documented MRSA acquisition. The most common sites of clinical infection were skin and soft tissue (49/121, 40.5%, 95% CI 31.7-49.8), respiratory tract (37/121, 30.6%, 95% CI 22.5-39.6) and bone and joint infections (14/121, 11.6%, 95% CI 6.5-18.7). Thirteen patients (13/121, 10.7%, 95% CI 5.8-17.7) had bacteraemias, of which six (5.0% 95% CI 1.8-10.5) were primary and seven (5.7%, 95% CI 2.3-11.6) were secondary to infection at other sites. Crude mortality at 30 days and six months was higher in patients with MRSA infection than colonisation alone (aOR 5.49, 95% CI 2.75-10.95, p<0.001 and aOR 2.94, 95% CI 1.78-4.85, p<0.001 respectively).


Risk of clinical infection is highest soon after MRSA acquisition. Prevention of MRSA acquisition in hospital will have significant impact on morbidity and mortality for patients.

MRSA; Colonisation; Hospital-associated infection