Enterobacter spp., Serratia marcescens, Citrobacter freundii, and Morganella morganii are characterized by chromosomally encoded AmpC ␤-lactamases and possess the ability to develop resistance upon exposure to broad-spectrum cephalosporins. To determine the incidences of the emergence of resistance during antimicrobial therapy for infections caused by these organisms and the effect of the emergence of resistance on patient outcomes, all patients who were admitted to the Asan Medical Center (Seoul, Republic of Korea) from January 2005 to June 2006 and whose clinical specimens yielded Enterobacter spp., S. marcescens, C. freundii, or M. morganii were monitored prospectively. The main end point was the emergence of resistance during antimicrobial therapy. A total of 732 patients with infections were included for analysis. The overall incidence of the emergence of antimicrobial resistance during antimicrobial therapy was 1.9% (14/732). Resistance to broadspectrum cephalosporins, cefepime, extended-spectrum penicillin, carbapenem, fluoroquinolones, and aminoglycosides emerged during treatment in 5.0% (11/218), 0% (0/20), 2.0% (2/100), 0% (0/226), 0% (0/153), and 1.1% (1/89) of patients, respectively. The emergence of resistance to broad-spectrum cephalosporins occurred more often in Enterobacter spp. (8.3%, 10/121) than in C. freundii (2.6%, 1/39), S. marcescens (0%, 0/37), or M. morganii (0%, 0/21). Biliary tract infection associated with malignant bile duct invasion was significantly associated with the emergence of resistance to broad-spectrum cephalosporins (P ‫؍‬ 0.024 at a significance level of 0.042, by use of the Bonferroni correction). Only 1 of the 14 patients whose isolates developed resistance during antimicrobial therapy died. The emergence of resistance was more frequently associated with broadspectrum cephalosporins than with the other antimicrobial agents tested, especially in Enterobacter spp. However, the emergence of resistance was associated with a low risk of mortality.Enterobacter spp., Serratia marcescens, Citrobacter freundii, and Morganella morganii have emerged as major causes of nosocomial infections caused by gram-negative bacteria. According to recent data from the SENTRY antimicrobial resistance surveillance program, Enterobacter spp., Serratia spp., Citrobacter spp., and M. morganii ranked 4th, 6th, 11th, and 12th, respectively, among the gram-negative organisms that cause bloodstream infections (3). These organisms are characterized by inducible resistance mediated by the chromosomal AmpC ␤-lactamase (12). This type of resistance can emerge rapidly during antimicrobial therapy (2, 9, 10), thus limiting the choice of antimicrobial agents that can be used to treat infections caused by these organisms.A landmark study by Chow et al. showed the emergence of resistance to broad-spectrum cephalosporins in 19% of Enterobacter blood isolates from patients receiving antimicrobial agents of this class (2). Chow et al. suggested that it may be prudent to avoid treatment with broad-spectrum cephal...

BackgroundViral pathogens have not generally been regarded as important causes of severe hospital-acquired pneumonia (HAP), except in patients with hematologic malignancy or transplant recipients. We investigated the role and distribution of viruses in adult with severe HAP who required intensive care.MethodsFrom March 2010 to February 2012, adult patients with severe HAP required admission to the intensive care unit (ICU), 28-bed medical ICU in a tertiary care hospital, were prospectively enrolled. Respiratory viruses were detected using multiplex reverse-transcription polymerase chain reaction and/or shell vial culture.ResultsA total of 262 patients were enrolled and 107 patients (40.8%) underwent bronchoscopic BAL for etiologic diagnosis. One hundred and fifty-six patients (59.5%) had bacterial infections and 59 patients (22.5%) had viral infections. Viruses were detected in BAL fluid specimens of 37 patients (62.7%, 37/59). The most commonly identified viruses were respiratory syncytial virus and parainfluenza virus (both 27.1%, 16/59), followed by rhinovirus (25.4%, 15/59), and influenza virus (16.9%, 10/59). Twenty-one patients (8.0%, 21/262) had bacterial-viral coinfections and Staphylococcus aureus was the most commonly coexisting bacteria (n = 10). Viral infection in non-immunocompromised patients was not uncommon (11.1%, 16/143), although it was not as frequent as that in immunocompromised patients (36.4%, 43/119). Non-immunocompromised patients were significantly older than immunocompromised patients and had significantly higher rates of underlying chronic obstructive pulmonary disease, tuberculous destroyed lung and chronic kidney disease. The 28 day mortalities of patients with bacterial infections, viral infections and bacterial-viral coinfections were not significantly different (29.5%, 35.6% and 19.0%, respectively; p = 0.321).ConclusionsViral pathogens are not uncommon in adult patients with severe HAP who required ICU admission. Since viral pathogens may cause severe HAP and could be a potential source of viral transmission, further investigation is required to delineate the role of viral pathogens in severe HAP.