Results from the Meropenem Yearly Susceptibility Test Information Collection (MYSTIC) Programme: report of the 2001 data from 15 United States medical centres.

The Meropenem Yearly Susceptibility Test Information Collection (MYSTIC) Programme is an international surveillance network of more than 100 medical centres where meropenem is the primary therapeutic carbapenem. Institutions have been monitored since 1997 (1999 in United States (US)) using National Committee for Clinical Laboratory Standards (NCCLS) reference susceptibility methods to monitor in vitro activity of meropenem and selected other broad-spectrum antimicrobial agents. In 2001, a total of 2874 strains were processed from the 15 US medical centres. Molecular methods were associated with MIC methods as needed for defining epidemic spread of resistant strains. The meropenem MIC(90) values were 0.03 mg/l for Citrobacter spp., Escherichia coli and Klebsiella spp.; 0.06 mg/l for Proteus mirabilis and Serratia spp. and 0.12 mg/l for Enterobacter spp. This potency was 8-16-fold greater than that of imipenem and the meropenem spectrum of activity versus the Enterobacteriaceae was the broadest of all tested antimicrobial agents. Only piperacillin/tazobactam (MIC(9), 64 mg/l) and tobramycin (MIC(90), 4 mg/l) were active against more than 90.0% of Pseudomonas aeruginosa at the NCCLS susceptible breakpoint, and the carbapenems were the most active compounds against Acinetobacter spp. However, Acinetobacter spp. isolates were resistant to all of the antimicrobial agents tested and the molecular typing results suggested that they were epidemiologically related. Only ciprofloxacin and ceftazidime had significantly reduced activity against oxacillin-susceptible staphylococci (87.9-92.6% susceptible. These 2001 US MYSTIC Programme results demonstrated no significant decline in carbapenem activity or susceptibility rates compared with the previously monitored years (1999-2000). Most apparent were the decreasing susceptibility rates for ciprofloxacin and ceftazidime against staphylococci. Continued surveillance in these institutions appears warranted as sites of high potential emerging resistance risk.