IMIPENEM-RESISTANT PSEUDOMONAS AERUGINOSA

Abstract

Seventy imipenem-resistant clinical isolates of Pseudomonas aeruginosa were examined for resistance to various β-lactam antibiotics. The 70 strains were grouped on the basis of their resistance to imipenem and ceftazidime. Group A (20 strains) was resistant to 12.5μg/ml imipenem and sensitive to 1.5μg/ml or less of ceftazidime. Group B (49 strains) was resistant to 12.5 to 25μg/ml imipenem and 12.5 to 100μg/ml ceftazidime, Group C (1 strain) was resistant to 25μg/ml imipenem and ≥200μg/ml ceftazidime. The levels of β-lactamase activities to cephaloridine produced by group A, B and C strains were <0.05U/mg protein, <0.01 to 0.85U/mg protein, and 0.61U/mg protein, respectively. The β-lactamases of group A strains were inducible only by imipenem, while those of group B strains could also be induced by other β-lactam antibiotics. Group C strain, GN17203, produced a β-lactamase that could hydrolyze imipenem. This β-lactamase was encoded by the 31-Mda plasmid, pMS350, belonging to incompatibility group P-9. The plasmid conferred resistance to β-lactams, gentamicin and sulfonamide, and was transferable via conjugation to P. aeruginosa, but not to Escherichia coli. The molecular weight of the purified enzyme was estimated to be 28, 000, and the isoelectric point was 9.0. The enzyme showed a broad substrate profile, hydrolyzing imipenem, oxyiminocephalosporins, 7-methoxycephalosporins, and penicillins. The enzyme activity was inhibited by EDTA, iodine, p-chloromercuribenzoate, CuSO₄, and HgCl₂, but not by clavulanic acid or sulbactam.