The combined effect of the Cfr methyltransferase and ribosomal protein L3 mutations on resistance to ribosome targeting antibiotics [PublishAheadOfPrint]

Several groups of antibiotics inhibit bacterial growth by binding to bacterial ribosomes. Mutations in the ribosomal protein L3 have been associated with resistance to linezolid and tiamulin, which both bind at the peptidyl transferase center in the ribosome. Resistance to these and other antibiotics also occurs through methylation of 23S ribosomal RNA at position A2503 by the methyltransferase Cfr. The mutations in L3 and the cfr gene have been found together in clinical isolates, raising the question whether they provide a combined effect on antibiotic resistance or growth. We transformed a plasmid borne cfr gene into an uL3 depleted E. coli strain containing either wild type L3 or one of the seven L3 mutations: G147R, Q148F, N149S, N149D, N149R, Q150L, or T151P expressed from plasmid coded rplC genes. The L3 mutations are well tolerated with small to moderate decreases in growth rate. The presence of Cfr has a very minor influence on growth rate. The resistance of the transformants against linezolid, tiamulin, florfenicol, and synercid was measured by minimum inhibitory concentration assays. The resistance from Cfr is in all cases stronger than the effects from the L3 mutations but various effects were obtained from the combinations of Cfr and L3 mutations ranging from a synergistic to an antagonistic effect. The susceptibility to linezolid and tiamulin varies highly between the L3 mutations while no significant effects are seen for florfenicol and synercid. This study underscores the complex interplay between various resistance mechanisms and cross-resistance, even from antibiotics with overlapping binding sites.

http://ift.tt/2tHIOCB