An outbreak in an intensive care unit of a strain of methicillin-resistant Staphylococcus aureus sequence type 239 associated with an increased rate of vascular access device-related bacteremia.
BACKGROUND
Patients in intensive care units are at high risk of developing methicillin-resistant Staphylococcus aureus (MRSA) bacteremia. We report an epidemiological and bacterial genomic analysis of a 2-year outbreak in an intensive care unit of a variant of MRSA sequence type 239 (hereafter designated TW).


METHODS
A cohort study was conducted to compare risk factors for MRSA bacteremia in patients who acquired TW versus patients who acquired non-TW strains of MRSA. Genetic analysis of TW was performed using multilocus sequence typing and microarray analysis.


RESULTS
Patients who acquired TW were more likely than patients who acquired non-TW strains of MRSA to have MRSA isolated from blood samples (47% vs. 13%; P<.001) and to have MRSA-positive vascular access device-sample cultures (59% vs. 26%; P<.001), but less likely to have MRSA isolated from screening swab samples (30% vs. 71%; P<.001). This increased rate of TW bacteremia was confined to the first week after acquisition of TW infection. Using Cox regression analysis, the adjusted hazard ratio for bacteremia with TW was 4.5 times that of non-TW strains of MRSA (95% confidence interval, 2.25-9.00; P<.001). Microarray analysis revealed that TW had accumulated all detectable mobile genetic elements that were variably expressed by other epidemic strains of MRSA sequence type 239 in the United Kingdom.


CONCLUSIONS
To our knowledge, this is the first report to provide direct evidence that strains of MRSA can differ in their ability to cause bacteremia. Further genetic and in vitro analysis of the TW strain may provide insight into the mechanism of vascular access device-related bacteremia in the intensive care unit environment.
Edgeworth
Jonathan D
JD
Department of Infection, Guy's and St. Thomas' National Health Service Foundation Trust, St. Thomas' Hospitals, London, SE1 7EH, UK. Jonathan.Edgeworth@gstt.nhs.uk
Yadegarfar
Ghasem
G
Pathak
Smriti
S
Batra
Rahul
R
Cockfield
Joshua D
JD
Wyncoll
Duncan
D
Beale
Richard
R
Lindsay
Jodi A
JA
D017677:Age Distribution; D000368:Aged; D016470:Bacteremia / Q000188:drug therapy / Q000453:epidemiology* / Q000382:microbiology; D002408:Catheters, Indwelling / Q000009:adverse effects* / Q000382:microbiology; D015331:Cohort Studies; D016001:Confidence Intervals; D003428:Cross Infection / Q000175:diagnosis / Q000188:drug therapy / Q000453:epidemiology / Q000382:microbiology; D004196:Disease Outbreaks; D005260:Female; D006801:Humans; D015994:Incidence; D007362:Intensive Care Units; D008297:Male; D016106:Methicillin Resistance; D008875:Middle Aged; D016133:Polymerase Chain Reaction; D011336:Probability; D016016:Proportional Hazards Models; D017678:Sex Distribution; D013203:Staphylococcal Infections / Q000175:diagnosis / Q000188:drug therapy / Q000453:epidemiology* / Q000382:microbiology; D013211:Staphylococcus aureus / Q000145:classification* / Q000187:drug effects