Genetic diversity of the tet(M) gene in tetracycline-resistant clonal lineages of Streptococcus pneumoniae

The aim of the present study was to examine the stability and evolution of tet(M)-mediated resistance to tetracyclines among members of different clon al lineages of Streptococcus pneumoniae. Thirty-two tetracycline-resistant isolates representing three national (Spanish serotype 14, Spanish serotype 15, and Polish serotype 23F) and one international (Spanish serotype 23F) multidrug-resistant epidemic clones were all found to be ter(M) positive an d tet(O), tet(K), and tet(L) negative. These isolates all carried the integ rase gene, int, which is associated with the Tn1545-Tn916 family of conjuga tive transposons. High-resolution restriction analysis of tet(M) products i dentified six alleles, tet(M)1 to tet(M)6: tet(M)1 to tet(M)3 and tet(M)5 i n isolates of the Spanish serotype 14 clone, tet(M)4 in both the Spanish se rotype 15 and 23F clones, and tet(M)6, the most divergent allele, in the Po lish 23F clone. This indicates that tet(M) variation can occur at the inter - and intraclone levels in pneumococci. Two alleles of int were identified, with int1 being found in all isolates apart from members of the internatio nal Spanish 23F clone, which carried int2. Susceptibility to tetracycline, doxycycline, and minocycline was evaluated for all isolates with or without preincubation in the presence of subinhibitory concentrations of tetracycl ines, Resistance to tetracyclines was found to be inducible in isolates of all clones; however, the strongest induction was observed in the Spanish se rotype 15 and 23F clones carrying tet(M)4. Tetracycline was found to be the strongest inducer of resistance, and minocycline was found to be the weake st inducer of resistance.