Abstract
From October 1988 to January 1992, 9 strains of Pseudomonas aeruginosa carrying a transferable plasmid encoding imipenem-hydrolyzing β-lactamase (pI-approx. 9.5) were isolated from 9 different patients hospitalized in the neurosurgical ward of Toyama City Hospital. Six strains were isolated in 1991, and the amount of antipseudomonal agents (especially imipenem) used in the neurosurgical ward increased considerably during the same year. All strains were resistant to imipenem, carbenicillin and antipseudomonal cephems such as ceftazidime, cefsulodin, cefpirome, etc.; 4 strains of were susceptible to piperacillin and 5 to aztreonam. The imipenem-hydrolyzing β-lactamase appeared to be zinc-dependent class B β-lactamase with a molecular weigh of 28, 000, pI of 9.5 and broad substrate specificities similar to those of the metallo-β-lactamase from P. aeruginosa (Watanabe et al., 1991) and Serratia marcescens TN 9106 (Osano et al., 1994). The gene encoding imipenem-hydrolyzing β-lactamase was located on the transferable plasmid of each isolate. The manner of production of these enzymes was constitutive. Both low-level imipenem-and high-level cephem-resistance were cotransferred to P. aeruginosa with the production of metallo-β-lactamase, but piperacillin-, aztreonam-, and high-level imipenem-resistance were not. Esherichia coli harboring recombinant plasmid encoding imipenem-hydrolyzing β-lactamase exhibited a pattern of resistance similar to that of transconjugants of P. aeruginosa. Production of chromosomal cephalosporinase in piperacillin-resistant strains was derepressed, and production of outer membrain protein of D2 was diminished in highly imipenem resistant strains. The serotype of all 9 strains was type B, but only 3 strains of them had the same serotype, pyocin-type and phage type. Above results suggest that the continuous isolation of imipenem-and cephem-resistant P. aeruginosa producing metallo-β-lactamase was related to the use of the antipseudomonal β-lactam agents such as imipenem.
