Tetracycline resistance determinants in streptococcal species isolated from the bovine mammary gland

Abstract

Seventy-one streptococci isolated from dairy cows with clinical mastitis were tested for tetracycline resistance. Twenty-one (30%) isolates were tetracycline resistant (Tcr), and eight hybridized with the Tet O, one hybridized with the Tet L, and one hybridized with both the Tet L and Tet K determinants. The remaining Tcr isolates did not hybridize with any of the 5 Gram-positive Tet determinants tested. The Tet O determinants were plasmid-mediated, and four selected strains transferred the Tet O determinant at frequencies of 10(-6) to 10(-8). Strains which did not hybridize with known probes were tested for resistance to minocycline. All of the Streptococcus dysgalactiae had low minimum inhibitory concentrations (MICs) for minocycline, while the S. agalactiae and the one S. uberis showed high MICs to minocycline. This suggests that at least two different uncharacterized Tet determinants exist in these isolates, one conferring high resistance to both tetracycline and minocycline and one conferring only tetracycline resistance.