Trends in antimicrobial susceptibility in UK centres: the MYSTIC Programme (1997-2002).

Trends in antimicrobial susceptibilities in three UK centres participating in the MYSTIC Programme were examined from 1997 to 2002. Isolates were tested using standard methodology to determine the susceptibility breakpoints of meropenem and several other antimicrobial agents including imipenem, ceftazidime, piperacillin/tazobactam, ciprofloxacin and gentamicin. Data are grouped in 2-year blocks. The carbapenems were the most active agents tested against the Enterobacteriaceae (99-100% and 98-100% susceptibility to meropenem and imipenem, respectively) and non-fermenters, including Pseudomonas spp. and Acinetobacter spp. With the exception of susceptibility to ciprofloxacin, which decreased among Enterobacteriaceae at the end of the 6-year period, all antibiotics tested retained their levels of activity. The proportion of extended-spectrum beta-lactamase (ESBL)- and AmpC-producing Enterobacteriaceae increased during the study (4.8% and 11.3% in 1997-1998; 7.4% and 16.7% in 2001-2002, respectively). Both meropenem and imipenem retained their potency against these ESBL- and AmpC-producing isolates (100% for all time periods). All the other antimicrobial agents tested had much lower susceptibility against these resistant isolates and this decreased further over the 6-year period, with the exception of tazobactam, which maintained its low levels. Although all antibiotics tested retained acceptable activity, the carbapenems remained the most active antimicrobial agents against Gram-negative bacteria, including ESBL- and AmpC-producing isolates.