ORIGINAL RESEARCH Prevalence of Multidrug-Resistant Pseudomonas aeruginosa and Carbapenem-Resistant Enterobacteriaceae Among Specimens From Hospitalized Patients With Pneumonia and Bloodstream Infections in the United States From 2000 to 2009 Marya D. Zilberberg, MD, MPH 1,2 *, Andrew F. Shorr, MD, MPH 3 1 EviMed Research Group, LLC, Goshen, Massachusetts; 2 Department of Public Health, University of Massachusetts, Amherst, Massachusetts; 3 Department of Medicine, Washington Hospital Center, Washington, DC. BACKGROUND: Antimicrobial resistance complicates anti- biotic selection. Pseudomonas aeruginosa (PA), common in pneumonia and blood stream infections (BSIs), is frequently resistant to multiple antimicrobial classes. Carbapenem- resistant Enterobacteriaceae (CRE) have emerged as a pathogen of concern over the past decade. OBJECTIVE: To determine the prevalence of CRE and multidrug-resistant PA (MDR-PA) in pneumonia and BSI hospitalizations. DESIGN: Survey of data from a nationally representative sample of microbiology laboratories in 217 hospitals in the United States. METHODS/SETTING: We examined Eurofins’ The Surveil- lance Network database from 2000 to 2009 to explore the proportion of all PA in pneumonia and BSI that is MDR. We performed the same analysis for CRE as a proportion of Enterobacteriaceae. We defined MDR-PA as any isolate resistant to 3 drug classes. Enterobacteriaceae were CRE if resistant to both a third generation cephalosporin and a carbapenem. RESULTS: We identified 205,526 PA (187,343 pneumonia; 18,183 BSI) and 95,566 Enterobacteriaceae specimens (58,810 pneumonia; 36,756 BSI). The prevalence of MDR- PA was 15-fold higher than CRE in both infection types (pneumonia: 22.0% MDR-PA vs 1.6% CRE; BSI: 14.7% MDR-PA vs 1.1% CRE). There was a net rise in MDR-PA as a proportion of all PA from 2000 to 2009 (BSI: 10.7%– 13.5%; pneumonia: 19.2%–21.7%). The CRE phenotype emerged in 2002 in both infection types, peaking in 2008 at 3.6% in BSI and 5.3% in pneumonia, and stabilized thereafter. CONCLUSIONS: Although CRE organisms have emerged as an important pathogen in BSI and pneumonia, MDR-PA remains more prevalent in the United States. Journal of Hospital Medicine 2013;8:559–563. V C 2013 Society of Hospital Medicine Administration of initially appropriate antimicrobial therapy represents a key determinant of outcome in patients with severe infection. 1–9 The variable patterns of antimicrobial resistance seen between and within healthcare institutions complicate the process of anti- biotic selection. Although much attention has histori- cally focused on Staphylococcus aureus, resistance among Gram-negative pathogens has emerged as a major challenge in the care of hospitalized, and partic- ularly critically ill, patients. 2,10,11 Multidrug, and more specifically carbapenem resistance, among such common organisms as Pseudomonas aeruginosa (PA) and Enterobacteriaceae represents a major treatment challenge. 2 A recent US-based surveillance study reported that a quarter of device-related infections in hospitalized patients were caused by carbapenem- resistant PA. 10 In addition to changes in resistance patterns seen among PA isolates, increasing rates of nonsusceptibil- ity have been described among Enterobacteriaceae. Resistance rates to third-generation cephalosporins in these pathogens have risen steadily since 1988, reach- ing 20% among Klebsiella pneumoniae and 5% among Escherichia coli isolates by 2004. 11 In response to this, clinicians have increasingly utilized carbapenems to treat patients with serious Gram- negative infections. However, the development of several types of carbapenemases by Enterobacteria- ceae has led to a greater prevalence of carbapenem- resistant Enterobacteriaceae species (CRE). 12–18 In fact, a recent report from the Centers for Disease Control and Prevention (CDC) documents a rapid rise in both the prevalence and extent of CRE in the United States. 19 These Gram-negative multidrug-resistant (MDR) organisms frequently cause serious infections including pneumonia and bloodstream infections (BSI). The fact that these conditions, if not addressed in a timely and appropriate manner, lead to high morbidity, *Address for correspondence and reprint requests: Marya Zilberberg, MD, EviMed Research Group, LLC, PO Box 303, Goshen, MA 01032; Telephone: 413-268-6381; Fax: 413-268-3416; E-mail: evimedgroup@gmail.com Revised: July 12, 2013; Accepted: July 23, 2013 2013 Society of Hospital Medicine DOI 10.1002/jhm.2080 Published online in Wiley Online Library (Wileyonlinelibrary.com). An Official Publication of the Society of Hospital Medicine Journal of Hospital Medicine Vol 8 | No 10 | October 2013 559