Resistance to trimethoprim and 2,4-diamino-6,7-diisopropyl-pteridine (0/129) in Pasteurella haemolytica.

Thirteen strains of Pasteurella haemolytica resistant to moderate levels of trimethoprim (MICs from 8 to 64 micrograms/ml) and 0/129 (MICs from 16 to 64 micrograms/ml) were isolated from bovine specimens. Two strains, CNP330 and CNP334, were studied and found to harbour various plasmids but all attempts to cure trimethoprim resistance were unsuccessful. Resistance characters were not transferable to Escherichia coli or to Pasteurella multocida by conjugation and to E. coli by transformation. The resistance gene(s) was therefore tentatively assigned to a chromosomal location and cloned into E. coli where it conferred trimethoprim resistance. Trans-complementation analysis of a dihydrofolate reductase-deficient mutant of E. coli showed that trimethoprim resistance was secondary to synthesis of a dihydrofolate reductase. DNA/DNA hybridization of the hybrid plasmid and of strains CNP330 and CNP334 with probes specific for dihydrofolate reductase types I to V were negative, indicating that cross-resistance to trimethoprim and 0/129 in P. haemolytica was due to the acquisition by P. haemolytica of a new resistance determinant.