MECHANISM OF RESISTANCE TO CARBAPENEM IN CLINICAL ISOLATES OF BACTEROIDES-FRAGILIS

Carbapenem (imipenem [IMI], meropenem [MER]) resistance in clinical isolates of Bacteroides fragilis is due to the production of a carbapenemase, a Zn++-dependent metallo beta-lactamase, encoded by the gene cfiA. This gene is carried by ca. 3 % of the B. fragilis strains isolated in France, but is expressed (MICs of IMI, MER greater-than-or-equal-to 64 mug/ml) in only one third of the cases, and silent (MICs of IMI/MER = 1/2 mug/ml) in the remaining two thirds. Activation of the silent gene occurs, in vitro and in vivo, after insertion of an IS element, most frequently IS1186, carrying a strong promoter, immediately upstream of the gene. The cfiA-positive strains differ from the cfiA-negative strains in that they harbor the IS elements known in B. fragilis (IS4351, IS1186, IS942) and they constitute, on the basis of molecular typing and sequencing of 16S RNA genes, a distinct taxonomical unit.