Plasmid-encoded metallo-beta-lactamase (IMP-6) conferring resistance to carbapenems, especially meropenem

In 1996, Serratia marcescens KU3838 was isolated from the urine of a patien t with a urinary tract infection at a hospital in northern Japan and was fo und to contain the plasmid pKU501, Previously, we determined that pKU501 ca rries bla,,, and the genes for TEM-l-type p-lactamases as well as producing both types of p-lactamases (H, Yano, A. Kuga, K, Irinoda, R, Okamoto, T, K obayashi, and M, Inoue, J. Antibiot. 52:1135-1139, 1999), pKU502 is a recom binant plasmid that contains a 1.5-kb DNA fragment, including the metallo-b eta -lactamase gene, and is obtained by PCR amplification of pKU501, The se quence of the metallo-P-lactamase gene in pKU502 was determined and reveale d that this metallo-P-lactamase gene differed from the gene encoding IMP-I 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 replace d by Gly in the mature enzyme. This enzyme was designated IMP-6, The strain s that produced IMP-6 were resistant to carbapenems, The MICs of panipenem and especially meropenem were higher than the MIC of imipenem for these str ains. The k(cat)/K-m value of IMP-6 was about sevenfold higher against mero penem than against imipenem, although the MIC of meropenem for KU1917, whic h 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 I MP-6 has extended substrate profiles against carbapenems, However, the acti vity of IMP-6 was very low against penicillin G and piperacillin, These res ults suggest that IMP-6 acquired high activity against carbapenems, especia lly meropenem, via the point mutation but in the process lost activity agai nst penicillins. Although IMP-6 has reduced activity against penicillins du e to this point mutation, pKU501 confers resistance to a variety of antimic robial agents because it also produces TEM-1-type enzyme.