INSERTION OF A NOVEL DNA-SEQUENCE, IS-1186, UPSTREAM OF THE SILENT CARBAPENEMASE GENE CFIA, PROMOTES EXPRESSION OF CARBAPENEM RESISTANCE INCLINICAL ISOLATES OF BACTEROIDES-FRAGILIS

A small number of isolates of Bacteroides fragilis, an anaerobic patho gen of the human intestinal flora, carries a copy (or copies) of the c arbapenem-resistance gene, cfiA, which may be silent or expressed. We have studied the mechanism of activation of the frequently silent gene in in vitro-selected mutants and in clinical isolates. In both types of strains, activation was observed as the consequence of the insertio n, at several possible sites, of a novel 1.3 kb insertion sequence, IS 1186, immediately upstream of the carbapenemase gene. IS1186 has two o pen reading frames, on opposite strands, with coding capacities for a 41.2 kDa (ORF1) and a 22.5 kDa (ORF2) protein. The 41.2 kDa protein ha s homology with some proteins predicted from open reading frames of IS elements or DNA direct repeats of aerobic, but not anaerobic, Gram-ne gative bacteria. Upon insertion, transcription of cfiA was found to be driven from a promoter identified on the right end of IS1186. In one instance, insertion occurred into the putative ribosome-binding site o f cfiA, leaving intact the tetranucleotide AGAA which is concluded to be a fully functional ribosome-binding site. Between 3 and 14 copies o f IS 1186 were detected per genome and the element was found, within t he species B. fragilis, almost exclusively in the subgroup carrying th e cfiA gene.