Clinical and bacteriological study of nosocomial infections due to Enterobacter aerogenes resistant to imipenem

Enterobacter aerogenes strains resistant to imipenem were isolated in 10 patients, 7 of whom had received imipenem-cilastatin. The strains were differentiated by biotype, antibiotype, and plasmid content. All of the strains overproduced a chromosomal cephalosporinase and lost a major outer membrane protein with a size of about 40 kDa. In 5 of the 10 patients, E. aerogenes strains resistant to extended-spectrum cephalosporin were isolated during the same stay. In three patients, the similarity between the imipenem-susceptible and -resistant strains suggests the occurrence of mutation and reversion in vivo. The combination imipenem-cilastatin has been critically important for use with multiresistant strains of Enterobacter spp., but its use increases the risk of selection of imipenem-resistant strains.