Methicillin Resistance Does Not Impact Disease Severity in S. aureus Necrotizing Soft Tissues Infection

Author(s):
Alexis L. Woods ; Erin C. Howell M.D.; Amy H. Kaji M.D. Ph.D.; Jessica A. Keeley M.D.; Dennis KIM; Angela L. Neville M.D.

Background:

Staphylococcus aureus is one of the most common pathogens identified in necrotizing soft tissue infection (NSTI) with the incidence increasing over the last few decades. The objective of this study was to evaluate if there were differences in presentation or increased disease severity associated with Methicillin Resistant S. aureus (MRSA) compared to Methicillin Sensitive S. aureus (MSSA) NSTI.

Methods:

Retrospective review of all adult patients (age > 18) with NSTI at our county funded, academic medical center over a seven-year period (2008-2015). Patients with positive wound cultures for MSSA or MRSA were compared.

Results:

Of the 228 patients with an NSTI, 81 patients (35.5%) had S. aureus in their wound cultures; 54 (23.7%) had MSSA and 27 (11.8%) MRSA. Forty-six (20.1%) of the NSTIs were monomicrobial S. aureus, with MSSA identified as a single agent in 25 (10.9%) and MRSA in 21 (9.2%).

Patients with MSSA were more likely to have polymicrobial infections (53.7% vs 22.2%, p=0.007), diabetes mellitus (74.1% vs 50.0%, p=0.03), higher admission glucose levels (335 versus 171 mg/dL, p=0.04), and higher white blood counts (WBC) (18.5 versus 13.6 K/mm³, p=0.02). In a sub-analysis of monomicrobial MSSA versus MRSA NSTI, the only trend to remain was a higher WBC in MSSA group (18.8 vs. 13.5 K/mm³, p=0.03). Patients with monomicrobial MRSA infections were more likely to have an NSTI in the pelvic/perineum region (38.1% vs 8.0%, p=0.03). No other differences in admitting vitals, laboratory values, physical exam findings, or outcomes (see table) were identified between monomicrobial isolates.

Conclusions:

Staphylococcus aureus is involved in a large percentage of NSTI and was the single causative agent in 20% of NSTI in this series. MSSA and MRSA are clinically indistinguishable, though MSSA was found more frequently in polymicrobial infections. Methicillin resistance does not portend increased disease severity in NSTI due to S. aureus.