The cfiA gene is clustered in a bicistronic operon encoding an N-acetyltransferase and an O-acetyltransferase related to resistance markers. This genetic context, exclusively found in strains of Bacteroides fragilis division II, has been highly rearranged by the successive integration of two new mobile sequences, a miniature element and ISBf9. Besides that, among the DNA polymorphisms detected in the cfiA locus, only the integration of IS942 at its promoter was a determinant for expression of carbapenemase activity.