Mechanisms of beta-lactam resistance amongst Pseudomonas aeruginosa isolated in an Italian survey

The mechanisms of resistance to beta-lactam antibiotics in 325 isolates of Pseudomonas aeruginosa were examined. These isolates were selected because of their resistance to meropenem and imipenem (breakpoint, >4 mg/L), carben icillin (>128 mg/L), ceftazidime (>8 mg/L), piperacillin and ticarcillin/cl avulanate (>64 mg/L). The most frequent mechanism of resistance was beta-la ctamase-independent, so called 'intrinsic resistance', which was found in 1 83 isolates and was probably due to impermeability and/or efflux mechanisms . beta-Lactamase-mediated resistance was demonstrated in 111 strains (11.1% ). Derepression of Ambler Class C chromosomal beta-lactamase was detected i n 64 isolates, most of which were resistant to ceftazidime and piperacillin but susceptible to meropenem, whereas secondary plasmid-encoded beta-lacta mases were round in 34 isolates, all of them resistant to carboxypenicillin s and ureidopenicillins and susceptible to carbapenerms. Twelve strains sho wed more than one plasmid-encoded beta-lactamase plus derepression of chrom osomal Class C enzyme. Resistance to carbapenems was independent of resista nce to other beta-lactam antibiotics, indicating a different mechanism of r esistance, probably due to the loss of the D2 porin. In total, 32 strains w ere resistant to carbapenems: 24 only to imipenem;and eight to both imipene m and meropenem.