Plasmid-encoded AmpC beta-lactamases: How far have we gone 10 years after the discovery?

The dogma that ampC genes are located exclusively on the chromosome was dom inant until about 10 years ago. Since 1989 over 15 different plasmid-encode d AmpC beta-lactamases have been reported from several countries. Most of t hese enzymes evolved in two clusters. The major cluster includes several en zymes with a high similarity to CMY-2 which is the closest related chromoso mal AmpC enzyme of Citrobacter Freundii: A second cluster centers around CM Y-1. It is less homogeneous and not closely related chromosomal AmpC enzyme s. Molecular diversification by amino acid substitutions does not usually t ranslate into a change in the resistance phenotype. At this time, CMY-2 app ears to be the most prevalent and widely distributed. Further global increa se of prevalence and diversity of plasmidic AmpC beta-lactamases have to be anticipated in the next millenium.