A prospective observational study was conducted to identify factors associated with bloodstream infections (BSIs) caused by integron-carrying Enterobacteriaceae and to evaluate the clinical significance of integron carriage. Consecutive patients with Enterobacteriaceae BSIs were identified and followed up until discharge or death. Identification of blood isolates and susceptibility testing were performed by the Wider I automated system. int-1-specific PCR, conserved-segment PCR, and DNA sequencing were used to determine the presence, length, and content of integrons. The relatedness among the isolates was examined by pulsed-field gel electrophoresis. Two hundred fifty episodes of Enterobacteriaceae BSI occurred in 233 patients; 109 (43.6%) were nosocomial, 82 (32.8%) were community acquired, and 59 (23.6%) were health care associated. Integrons were detected in 11 (13.4%) community-acquired, 24 (40.7%) health care-associated, and 46 (42.2%) nosocomial isolates. Integron-carrying organisms were more likely to exhibit resistance to three or more classes of antimicrobials (odds ratio [OR], 9.84; 95% confidence interval [95% CI], 5.31 to 18.23; P < 0.001) or to produce extended-spectrum -lactamases (OR, 5.75; 95% CI, 2.38 to 13.89; P < 0.001) or a VIM-type metallo--lactamase (P, 0.003). Inter-or intraspecies integron transfer and cross-transmission of integroncarrying clones were observed. Use of cotrimoxazole (OR, 4.77; 95% CI, 1.81 to 12.54; P < 0.001) and a nosocomial or other health care setting (OR, 3.07; 95% CI, 1.30 to 7.22; P, 0.01) were independently associated with BSIs caused by integron-carrying Enterobacteriaceae. Patients with a nonurinary source of bacteremia (OR, 9.46; 95% CI, 2.77 to 32.32; P < 0.001) and a Pitt bacteremia score of >4 (OR, 23.36; 95% CI, 7.97 to 68.44; P < 0.001) had a significantly higher 14-day mortality rate, whereas integron carriage did not affect clinical outcomes. These findings may have implications affecting antibiotic policies and infection control measures.The pattern of life-threatening infections is changing over time, along with our clinical practices and antibiotic usage. In recent years, infections caused by multidrug-resistant gramnegative bacilli have increasingly been recognized as important causes of morbidity and mortality among hospitalized patients (19,27). Antimicrobial resistance may develop through mutations in chromosomal DNA or through acquisition of resistance genes carried by plasmids or transposons. A substantial proportion of these resistance genes in gram-negative bacilli are packaged as discrete small mobile units into DNA structures called integrons (3,4,14,25,28). These genetic structures operate as a general gene-capture system and provide a powerful mechanism for the acquisition and dissemination of antimicrobial resistance genes. Integrons possess three essential components in the 5Ј-conserved segment (5Ј CS), including an int gene, encoding an integrase; a specific recombination site (attI site); and a promoter that directs transcription of ...