Trimethoprim Resistance In Multiple Genera of Enterobacteriaceae at a U.S. Hospital: Spread of the Type II Dihydrofolate Reductase Gene by a Single Plasmid

Abstract

The percentage of clinical isolates of several species of Enterobacteriaceae, particularly Escherichia coli and Klebsiella pneumoniae, resistant to trimethoprim (TMP^R) has increased gradually at the Brigham and Women's Hospital (Boston) in recent years. Thirtyseven of 42 TMP^R isolates from six species of gram-negative bacilli conjugally transferred TMP resistance to K12 E. coli. β-Lactam resistance cotransferred from 21 of the 37 donors, and sulfamethoxazole (SMZ) resistance cotransferred from five of the 37 donors. Plasmids that encoded TMP resistance either alone or with SMZ resistance had a molecular size of ∼52.5 kilobases, with identical restriction endonuclease-generated “fingerprints.” Plasmids encoding β-lactam-mediated resistance (β^R) were approximately four kilobases larger and had fragment patterns that were identical for all of the TMP^R/β^R plasmids tested and had many restriction endonuclease-generated bands in common with TMP^R plasmids. Radiolabeled dihydrofolate reductase (DHFR) probes identified the type II DHFR as the determinant of TMP resistance. In contrast with reports from Europe, TMP resistance in multiple species of Enterobacteriaceae was found to be spread in one hospital by a single, stable conjugative plasmid that has a wide host range and encodes the type II DHFR gene.