PLASMID-MEDIATED DISSEMINATION OF THE METALLO-BETA-LACTAMASE GENE BLA(IMP) AMONG CLINICALLY ISOLATED STRAINS OF SERRATIA-MARCESCENS

The distribution of strains producing metallo-beta-lactamase among 105 strains of Serratia marcescens was investigated. All of these strains were isolated in seven general hospitals located in Aichi Prefecture, Japan, from April to May 1993. Southern hybridization analysis suggested that four S. marcescens strains, AK9373, AK9374, AK9385, and AK9391, had a metallo-beta-lactamase genes similar to the bla(IMP) gene found by our laboratory (E. Osano, Y. Arakawa, R. Wacharotayankun, M. Ohta, T. Horii, R. Ito, F. Yoshimura, and N. Kato, Antimicrob. Agents Chemother. 38:71-78, 1994), and these four strains showed resistance to carbapenems as well as to the other broad-spectrum beta-lactams. In particular, strains AK9373, AK9371, and AK9391 showed an extraordinarily high-level resistance to imipenem (MICs, greater than or equal to 64 mu g/ml), whereas strain AK9385 demonstrated moderate imipenem resistance (MIC, 8 mu g/ml). The imipenem resistance of AK9373 was transferred to Escherichia coli CSH2 by conjugation with a frequency of 10(-5). The DNA probe of the bla(IMP) gene hybridized to a large plasmid (approximately 120 kb) transferred into the E. coli transconjugant as well as to the large plasmids harbored by AK9373. On the other hand, although we failed in the conjugational transfer of imipenem resistance from strains AK9374, AK9385, and AK9391 to E. coli CSH2, imipenem resistance was transferred from these strains to E. coli HB101 by transformation. A plasmid (approximately 25 kb) was observed in each transformant which acquired imipenem resistance. The amino acid sequence at the N terminus of the enzyme purified from strain AK9373 was identical to that of the metallo-beta-lactamase IMP-1. In contrast, strains ES9348, AK9386, and AK93101, which were moderately resistant to imipenem (MICs, greater than or equal to 4 to less than or equal to 8 mu g/ml), had no detectable bla(IMP) gene. As a conclusion, 19% of clinically isolated S. marcescens strains in Aichi Prefecture, Japan, in 1993 were resistant to imipenem (MICs, greater than or equal to 2 mu g/ml), and strains which showed high-level imipenem resistance because of acquisition of a plasmid-mediated bla(IMP)-like metallo-beta-lactamase gene had already proliferated as nosocomial infections, at least in a general hospital.