Characterisation of viridans group streptococci with different levels of Tet(M)-mediated tetracycline resistance.

Streptococcus oralis 264-3, Streptococcus mitis 254-1 and S. mitis 264-1, isolated from the oral cavities of two children were each found to carry the tet(M) gene but exhibited different degrees of reduced susceptibility to tetracycline (tetracycline MICs of 2, 8 and 64 mg/L, respectively). The aim of this study was to determine the molecular basis for the different levels of tetracycline resistance (Tc(R)) observed. Escherichia coli HB101 carrying the cloned tet(M) genes exhibited similar levels of tetracycline susceptibility to those observed in the parental streptococcal strains (MICs of 1, 16, and 64 mg/L for tet(M) genes from S. oralis 264-3, S. mitis 254-1 and S. mitis 264-1, respectively). DNA sequencing revealed that S. oralis 264-3 had a tet(M) gene highly homologous to tet(M) carried by Tn916 from Enterococcus faecalis (99.6% identity), while the intermediate- and high-level Tc(R) strains had tet(M) sequences that resembled the tet(M) gene of Tn5251 from Streptococcus pneumoniae (99.3% and 99.4% identity, respectively). No differences were observed in the upstream attenuator structure for each of the strains and differences in reduced tetracycline susceptibilities could be attributed to changes in the deduced amino acid sequences of the Tet(M) proteins.