Structure of In31, a bla(IMP)-containing Pseudomonas aeruginosa integron phyletically related to In5, which carries an unusual array of gene cassettes

The location and environment of the acquired bla(IMP) gene, which encodes t he IMP-1 metallo-beta-lactamase, were investigated in a Japanese Pseudomona s aeruginosa clinical isolate (isolate 101/1477) that produced the enzyme. In this isolate, bla(IMP) was carried on a 36-Wb plasmid, and similar to th e identical alleles found in Serratia marcescens and Klebsiella pneumoniae clinical isolates, it was located on a mobile gene cassette inserted into a n integron. The entire structure of this integron, named In31, was determin ed. In31 is a class 1 element belonging to the same group of defective tran sposon derivatives that originated from Tn402-like ancestors such as In0, I n2, and In5. The general structure of In31 appeared to be most closely rela ted to that of In5 from pSCH884, suggesting a recent common phylogeny for t hese two elements. In In31, the bla(IMP) cassette is the first of an array of five gene cassettes that also includes an aacA4 cassette and three origi nal cassettes that have never been described in other integrons. The novel cassettes carry, respectively, (i) a new chloramphenicol acetyltransferase- encoding allele of the catB family, (ii) a qac allele encoding a new member of the small multidrug resistance family of proteins, and (iii) an open re ading frame encoding a protein of unknown function. All the resistance gene s carried on cassettes inserted in In31 were found to be functional in decr easing the in vitro susceptibilities of host strains to the corresponding a ntimicrobial agents.