USA500 Methicillin-Resistant Staphylococcus aureus: An Evaluation of Clinical Virulence in Bacteremia Open Access

Melendez, Andre Gerardo (2014)

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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.

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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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