Viruses are frequently found in the airway of patients with pneumonia requiring ICU admission and may cause severe forms of pneumonia. Patients with viral infection and bacterial infection had comparable mortality rates.

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...

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