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USA600 Methicillin–Resistant Staphylococcus aureus in ICU Patients With Pneumonia: A Case-Control Study Design to Evaluate Epidemiology and Outcomes of an Emerging Strain

Arshad, Samia MPH*; Shoyinka, Adenike MD*; Perri, Mary-Beth MT*; Moreno, Daniela MS*; Jacobsen, Gordon MS*; Zervos, Marcus J. MD*†and On behalf of the IMPACT-HAP Study Group

Infectious Diseases in Clinical Practice: March 2014 - Volume 22 - Issue 2 - p 85–88
doi: 10.1097/IPC.0b013e318298b81c
Original Articles

Background Methicillin-resistant Staphylococcus aureus (MRSA) infections are associated with severe necrotizing syndromes and high mortality, accounting for 20% to 40% of all hospital-acquired pneumonia and ventilator-associated pneumonia. There is insufficient data on comparative outcomes by specific strain characteristics in MRSA isolates in patients with pneumonia.

Methods Clinical outcomes of patients with USA600 (ST45) MRSA strain were compared with other sequence types in MRSA pneumonia from 5 different hospitals in a retrospective case-control study. Patients were identified through review of microbiology laboratory records and through the Improving Medicine through Pathway Assessment of Critical Therapy in Hospital-Acquired Pneumonia database. Pulsed-field gel electrophoresis of SmaI-digested genomic DNA was performed on all isolates using a CHEF-DR III (BioRad). Pulsed-field gel electrophoresis patterns were compared using BioNumerics software (Applied Maths).

Results Two hundred fifty-one consecutive patients with MRSA pneumonia were evaluable for an all-cause 28-day mortality and 14-day failure outcome. Prevalence of USA600 was 8% (21). Laboratory characteristics of USA600 isolates were 100% (21) Panton-Valentine leukocidin toxin negative, agr I type, 67% (14) heteroresistant to vancomycin, and higher rates of SCCmec type II versus SCCmec type IVa, 95% versus 5%. Twenty-eight–day mortality rates were USA600 (52.4%), USA100 (ST5; 29%), and USA300 (ST8; 32%); and 14-day failure rates were USA600 (50%), USA100 (37%), and USA300 (27%), respectively.

Conclusions This is the first comparative observational study of its kind, with evidence of a much higher failure rate (>50%) within patients with USA600 MRSA pneumonia. The high mortality rate in this subset warrants further investigation of factors of this emerging strain that may predict mortality and failure outcomes.

From the *Division of Infectious Diseases, Henry Ford Health System, Detroit, MI and †Wayne State University School of Medicine, Detroit, MI.

Correspondence to: Samia Arshad, MPH, Division of Infectious Diseases, Henry Ford Health System, 2799 West Grand Blvd, CFP 314, Detroit, MI 48202. E-mail:

All authors have contributed to the design, data collection, analysis, preparation, and critical revision of the manuscript. MJZ takes responsibility for the integrity of the work as a whole, from inception to publication.

The preliminary results of this study were presented as A.Shoyinka, S. Arshad, D. Moreno, M. Perri, A. Chen, M. Zervos, 2012. Comparative Outcomes in ST45/USA600 Versus Other CDC Strain Types in MRSA Pneumonia (abstract number 1361) at the 2012 ECCMID Annual Conference, London, UK; oral presentation.

This study was supported by Pfizer Inc., US Medical. The University of Louisville Foundation was responsible for project oversight and distribution of funds to participating institutions.

Samia Arshad receives research funding from Forest Research Institute Pfizer, and Merck. Marcus J. Zervos receives research funding from Astellas, Cubist, and Pfizer; speaker’s honoraria from Astellas, Cubist, and Pfizer; and is a member of scientific Advisory Board or a consultant of Cubist and Astellas. The remaining authors have no funding or conflicts of interest to disclose.

© 2014 by Lippincott Williams & Wilkins.