Resistance in Enterobacteriaceae: Results of a multicenter surveillance study, 1995-2000

OBJECTIVES: To assess changes over time in susceptibility of Enterobacteriaceae from patients in ICUs, compare susceptibility rates of isolates from patients in ICUs with those from inpatients outside ICUs, and explore phenotypic patterns of cross-resistance and co-resistance. DESIGN: From 1995 to 2000, centers participating in the ICU Surveillance Study tested 100 to 200 consecutive nosocomial gram-negative bacilli by broth microdilution. SETTING: Each year, 42 to 97 U.S. hospitals tested isolates. RESULTS: In all years, imipenem was the most potent agent tested, followed by amikacin and ertapenem. Extended-spectrum beta-lactam, and monobactam agents had good activity against Escherichia coli and Klebsiella species, but limited activity against Enterobacter species. Susceptibility to imipenem and amikacin did not fluctuate during the analysis period, whereas susceptibility to ceftazidime, ceftriaxone, and ciprofloxacin decreased 2% to 5%. The decline was most pronounced for susceptibility of Escherichia coli to ciprofloxacin: 98.7% of ICU isolates were susceptible in 1995 versus 93.2% in 2000. Susceptibility of ICU isolates was lower than that of non-ICU isolates, except for ciprofloxacin, for which the reverse was true. Cross-resistance was common among extended-spectrum cephalosporins and penicillins, but uncommon between imipenem and ertapenem. Only imipenem and ertapenem remained highly active against Enterobacteriaceae with a phenotype suggesting possible production of an extended-spectrum beta-lactamase and those with a phenotype suggesting possible Amp C hyperproduction. CONCLUSIONS: Imipenem was the most active agent against nosocomial Enterobacteriaceae. Susceptibility to ciprofloxacin decreased from 1995 to 2000, particularly in. Escherichia coli, and, in contrast to other agents, was lower among non-ICU isolates.