Antimicrobial resistance trends in medical centers using carbapenems: report of 1999 and 2000 results from the MYSTIC program (USA).

Trends in susceptibility to antimicrobials were assessed from United States participants using 4488 isolates in the MYSTIC Program, 1999 (10 centers) through 2000 (15 centers). Diverse types of hospitals (general service, university, cancer, federal, pediatric, cystic fibrosis) were enrolled from 13 states. In 2000, oxacillin-susceptible staphylococci were 100% susceptible to meropenem, imipenem, and cefepime; but only 88% of strains were susceptible to ceftazidime. Among Enterobacteriaceae, > or = 96% of Enterobacter spp., Citrobacter spp., and Serratia spp. were susceptible to meropenem, imipenem, and cefepime; but ceftazidime, ceftriaxone, and piperacillin/tazobactam had 5-20% resistance rates. Extended-spectrum beta-lactamase resistance rates in Klebsiella spp. and Escherichia coli (6-7% and 3-5%, respectively) were stable over 2 years. Acinetobacter spp. were 78-81% susceptible to carbapenems but only 63-72% susceptible to ciprofloxacin. Meropenem, tobramycin, and piperacillin/tazobactam were the most active against Pseudomonas aeruginosa, but ciprofloxacin inhibited only 74% in 2000. Overall, meropenem remained the most potent agent, especially against ceftazidime- or piperacillin/tazobactam-resistant strains.