A Retrospective Review of the Clinical Characteristics and Molecular Epidemiology of Methicillin Resistant Staphylococcus aureus (MRSA) Infections at a Large Academic Medical Center

Background: MRSA infections remain an ongoing problem in healthcare settings and in the community.

Objective: Review MRSA clinical characteristics, risk factors, and molecular epidemiology of MRSA cases at our institution to determine etiology and potential modifiable risk factors of infection.

Methods: Retrospective analysis with medical record review and repetitive element Polymerase Chain Reaction (rep-PCR: DiversiLab^TM, Biomerieux, NC) genotyping of a subset of MRSA isolates from patients at The Ohio State University Medical Center collected in January 2007 – October 2008.

Results: 141 subjects (77 male, 64 female) aged 18-93 years were evaluated. 116 isolates were obtained in the inpatient setting. Isolates included blood (n=70), skin/soft tissue (46, 33%), sputum (8, 6%), bronchoalveolar lavage [BAL (11, 8%)], bone (2, 1%), abscess (2, 1%), and other (2, 1%).  These isolates were classified into 3 categories: hospital-acquired (HA), community-acquired (CA), and hospital-acquired community onset (HACO). Patient outcomes included cure (n=29, 21%), death (19, 13%), failure/recurrent (29, 21%), or indeterminate (62, 44%).  Risk factors for infection included: history of MRSA infection (n=37, 26%), hemodialysis (18, 13%), surgery (58, 41%), residence in extended care facility (35, 25%), and presence of invasive devices (62, 44%) were also measured.  The majority of isolates were characterized as HACO (n=74, 53%). Those patients who died were more likely to have had MRSA bacteremia (n=9, 47%). Of those who were bacteremic and died, most were white males (n=6, 66%), had invasive devices present prior to infection (5, 56%), were previously hospitalized (8, 89%), had received hemodialysis (6, 67%), had cardiac prostheses (4, 44%), had diabetes (4, 44%), had renal failure (5, 56%), and had chronic lung disease (6, 67%).  All patients with poor outcomes had MRSA isolates that were resistant to several antibiotics.  Although all isolates were sensitive to vancomycin, most of those who died had vancomycin MIC’s ≤ 2 (n=18, 95%).  Twenty-three genotypes were found among the 141 isolates with 3 genotypes responsible for the majority of cases (n=70, 50%).  One genotype (n=14) was found mainly in blood isolates (7, 50%).  Another genotype (n=44) was found primarily in skin and soft tissue isolates (21, 48%).

Conclusions: In this study, factors associated with poor outcome included male sex, white race, underlying comorbidities, prior hospitalization, hemodialysis, and presence of cardiac prostheses.  Three genotype clusters were responsible for the majority of MRSA infections.  Among them, one genotype likely represents HA-MRSA, while another is likely representative of community-acquired MRSA.  Genotypic analysis allows for classification of patients into discrete subgroups with potentially unique risk factors.