Induced tigecycline resistance inStreptococcus pneumoniaemutants reveals mutations in ribosomal proteins and rRNA

Objectives Tigecycline is a broad-spectrum antibiotic acting at the level of the 30S ribosomal subunit to inhibit translation. While Streptococcus pneumoniae remains susceptible to tigecycline, resistance is beginning to emerge in some species and mainly involves efflux or mutations in ribosome constituents. We describe here the characterization of S. pneumoniae mutants selected for resistance to tigecycline. Methods Molecular determinants of resistance to tigecycline in S. pneumoniae were studied through WGS of two series of mutants made resistant to tigecycline in vitro in a stepwise fashion and by reconstructing tigecycline resistance using DNA transformation. Results The tigecycline-resistant S. pneumoniae M1TGC-6 and M2TGC-6 mutants were cross-resistant to tetracycline and minocycline. A role in tigecycline resistance could be attributed to 4 of the 12 genes that were mutated in both mutants. Mutations in ribosomal proteins S10 and S3, acquired early and late during selection, respectively, were implicated in resistance in both mutants. Similarly, mutations were detected in the four alleles of the 16S ribosomal RNA at sites involved in tigecycline binding and the number of mutated alleles correlated with the level of resistance. Finally, the gene spr1784 encodes an RsmD-like 16S rRNA methyltransferase for which inactivating mutations selected in the S. pneumoniae tigecycline-resistant mutants were found to decrease susceptibility to tigecycline. Conclusions This first report about tigecycline resistance mechanisms in S. pneumoniae revealed that, in contrast to Gram-negative species, for which efflux appears central for tigecycline resistance, resistance in the pneumococcus occurs through mutations related to ribosomes.