Abstract
Introduction: Methicillin-resistant Staphylococcus aureus (MRSA) is
the most common pathogen in healthcare-associated infections in the
US. USA500 and USA100 are the predominant molecular subtypes
causing healthcare-associated MRSA (HA-MRSA) infections. These
strains possess different microbiologic characteristics and several
in vitro and animal studies have suggested that USA500 is more
virulent than USA100. However, it is unknown whether individuals
with USA500 MRSA infections have worse outcomes compared to USA100.
The main objectives of this study were to identify the
epidemiologic, molecular, and clinical characteristics of USA500
bacteremia and determine whether bacteremia due to USA500 is
associated with greater attributable mortality compared to USA100.
Methods: Population based-surveillance for MRSA bacteremia was
conducted in the 8-county metropolitan Atlanta area from 2005-2011.
The analysis included patients with MRSA bloodstream infections due
to USA500 or USA100 strains. Bivariate analyses were performed to
compare characteristics of USA500 and USA100. Multivariable
logistic regression models were used to evaluate the association of
USA500 strain type and other factors predictive of attributable
mortality. Results: A total of 107 USA500 and 608 USA100 cases of
MRSA bacteremia were included in the study cohort. USA500 MRSA
cases were more likely to occur in blacks (72.4% vs 56.3%,
P=0.005), have HIV/AIDS (21.5% vs 2.6%, P<0.0001), and have
resistance to trimethoprim-sulfamethoxazole (90.7% vs 1.2%,
P<0.0001) compared to USA100. In multivariable analysis, USA500
was not associated with increased attributable mortality compared
to USA100 (aOR 0.57, 95% CI 0.26-1.22). Septic shock (aOR 7.36, 95%
CI 3.88-13.97), ICU admission prior to index culture (aOR 3.21, 95%
CI 1.39-7.40), age >55 years (aOR 1.99, 95% CI 1.11-3.53), and
central line-associated bloodstream infection (aOR 0.46, 95% CI
0.26-0.82) were factors associated with attributable mortality.
Conclusions: USA500 has a strong association with HIV/AIDS and
trimethoprim-sulfamethoxazole resistance compared to its HA-MRSA
counterpart, USA100. Contrary to in vitro and animal studies,
USA500 MRSA was not more virulent than USA100 in individuals with
bacteremia as measured by attributable mortality in this study
population.
Table of Contents
Introduction_________________________________________________________1-2
Background_________________________________________________________3-5
Methods____________________________________________________________6-14
Results_____________________________________________________________15-18
Discussion__________________________________________________________19-24
References__________________________________________________________25-27
Tables/Figures_______________________________________________________28-40
Appendix____________________________________________________________41-43
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