Tetracycline resistance genes in isolates of Pasteurella multocida, Mannheimia haemolytica, Mannheimia glucosida and Mannheimia varigena from bovine and swine respiratory disease: intergeneric spread of the tet(H) plasmid pMHT1

Tetracycline-resistant isolates of Pasteurella multocida and Mannheimia spp . from respiratory diseases in cattle and swine were investigated for the c lasses of tet gene and their chromosomal or plasmid location. The 34 isolat es comprised eight P. multocida, 23 Mannheimia haemolytica, two Mannheimia varigena and a single Mannheimia glucosida isolate. Identification of the t et genes was achieved by PCR analysis and hybridization with specific probe s. Transformation and hybridization experiments served to confirm the plasm id location of tet genes. Selected tet genes and their adjacent regions wer e sequenced. The tet genes tet(B), tet(G) and tet(H) were detected. The gen e tef(H) was present in 26 isolates. The 4.4 kb tet(H)-carrying plasmid pMH T1 was detected in six isolates representing all four species. In the remai ning 28 isolates, copies of tet(B), tet(G) and tet(H) were identified as ch romosomal. No correlation between the tet gene type and the MIC of tetracyc line, or between the number of tet gene copies and the MIC of tetracycline was observed. Tetracycline resistance in P. multocida and Mannheimia spp. i s mediated by at least three different tet genes. A new type of tet(H)-carr ying plasmid, pMHT1, was identified. The detection of pMHT1 in M. glucosida and M. varigena is the first report of resistance plasmids in isolates of these two species. For the first time, tet(G) genes were detected in member s of the family Pasteurellaceae.