In 1996, Serratia marcescens KU3838 was isolated from the urine of a patient with a urinary tract infection at a hospital in northern Japan and was found to contain the plasmid pKU501. Previously, we determined that pKU501 carries bla IMP and the genes for TEM-1-type -lactamases as well as producing both types of -lactamases (H. Yano, A. Kuga, K. Irinoda, R. Okamoto, T. Kobayashi, and M. Inoue, J. Antibiot. 52:1135-1139, 1999). pKU502 is a recombinant plasmid that contains a 1.5-kb DNA fragment, including the metallo--lactamase gene, and is obtained by PCR amplification of pKU501. The sequence of the metallo--lactamase gene in pKU502 was determined and revealed that this metallo--lactamase gene differed from the gene encoding IMP-1 by one point mutation, leading to one amino acid substitution: 640-A in the base sequence of the IMP-1 gene was replaced by G, and Ser-196 was replaced by Gly in the mature enzyme. This enzyme was designated IMP-6. The strains that produced IMP-6 were resistant to carbapenems. The MICs of panipenem and especially meropenem were higher than the MIC of imipenem for these strains. The k cat /K m value of IMP-6 was about sevenfold higher against meropenem than against imipenem, although the MIC of meropenem for KU1917, which produced IMP-1, was lower than that of imipenem, and the MIC of panipenem was equal to that of imipenem. These results support the hypothesis that IMP-6 has extended substrate profiles against carbapenems. However, the activity of IMP-6 was very low against penicillin G and piperacillin. These results suggest that IMP-6 acquired high activity against carbapenems, especially meropenem, via the point mutation but in the process lost activity against penicillins. Although IMP-6 has reduced activity against penicillins due to this point mutation, pKU501 confers resistance to a variety of antimicrobial agents because it also produces TEM-1-type enzyme.Carbapenems such as imipenem, panipenem, and meropenem are the most potent agents for treatment of gramnegative infections due to the stability of these agents against the majority of -lactamases and their high rate of permeation through bacterial outer membranes. In addition, imipenem, panipenem, and meropenem have a high affinity for penicillinbinding protein (PBP) 2 of gram-negative bacteria except for Pseudomonas aeruginosa. In P. aeruginosa, imipenem and panipenem have a high affinity for PBP 2 and meropenem has an affinity for PBP 3 (6, 19, 40).However, some clinical isolates have been reported to be resistant to carbapenems due to production of metallo--lactamases. There have been increasing reports, especially from Japan (7,14,21,27,35), of gram-negative bacteria that carry the transferable carbapenem resistance gene bla IMP , including isolates of P. aeruginosa and Serratia marcescens. The metallo--lactamases are class B enzymes in Ambler's molecular classification (1) and belong to Bush group 3 (3,31). Most of these metallo--lactamases confer resistance not only to carbapenems but also to other -lactams an...