Objectives: In this study, 94 tetracycline-resistant coagulase-positive and coagulase-variable staphylococci were investigated for their genetic basis of tetracycline resistance. The isolates had been collected in Germany during 2004-2006 and tested for their MICs to tetracycline in the BfT-GermVet study. The staphylococci were obtained from various disease conditions of farm and companion animals, including dogs/cats suffering from respiratory tract infections (n=20) or skin/ear/mouth infections (n=37) as well as from pigs suffering from skin infection (n=23) or urinary-genital tract infections (n=14).
Methods: Tetracycline-resistant staphylococci were tested by PCR for the resistance genes tet(K) and tet(L) coding for efflux pumps and for tet(M) and tet(O) coding for ribosome protective proteins. A plasmid localization was investigated for the resistance genes tet(K) and tet(L) by Southern blot hybridization and/or by protoplast transformation.
Results: The tet(M) gene was detected in 72 isolates: in 52 isolates alone and in 13 isolates together with tet(L) and in seven isolates in combination with tet(K). From the remaining isolates ten harboured tet(L) and 12 tet(K). The gene tet(O) was not detected in any of the isolates. Seven tet(L)- and three tet(K)-carrying isolates were plasmid-free. Nine isolates harboured the gene tet(L) on a plasmid and 14 isolates had a plasmid-located tet(K). The plasmids harbouring tet(K) were about 4 kb in size and structurally similar to the prototype plasmid pT181. Two of the nine plasmids carrying tet(L) were approximately 6 kb whereas the remaining seven tet(L)-carrying plasmids had a size of about 40 kb. Three of the transformants with a 40-kb plasmid were also resistant to erythromycin (n=2) or to trimethoprim (n=1).
Conclusions: These data show that tetracycline resistance in staphylococci was mainly due to the gene tet(M) with 76.6% of the resistant isolates harbouring this gene. Plasmid location was confirmed for 23 of 42 isolates carrying genes encoding efflux pumps, thus underlining the potential for horizontal transfer of tetracycline resistance via plasmids among staphylococci from animals.