Mechanisms of resistance to tetracyline in Streptococcus pneumoniae R6 mutants

Background This study aimed at characterizing determinants of resistance to tetracycline intrinsic to the genome of Streptococcus pneumoniae. Results Whole-genome sequencing of two S. pneumoniae R6 mutants resistant to tetracycline and cross-resistant to the fluoroquinolone ciprofloxacin revealed mutations in the gene rpsJ coding for the ribosomal protein S10 and in the operon coding for the ABC proteins PatA and PatB. Resistance reconstruction in a S. pneumoniae R6 tetracycline sensitive background confirmed the role of the rpsJ mutations in tetracycline resistance. Mutations located in the promoter region of patA also contributed to resistance to tetracycline, in addition to ciprofloxacin and ethidium bromide, by enhancing the transcription of the patA/B operon. Both mutants revealed a decreased accumulation of H3-tetracycline that was attributed to the increased expression of the PatA/B transporter. Mutations in the coding region of patA were also observed in both mutants and these contributed to resistance to tetracycline and ciprofloxacin, albeit to a lesser extent than the mutations in the promoter region. Comparative transcriptome profiling by RNA-sequencing further revealed a noticeable increase in the expression of several genes of the thiamine biosynthesis and salvage pathway in the two mutants and a role for some of these in resistance to tetracycline have been confirmed by gene inactivation. Conclusions The combination of genomic, transcriptomic and functional studies has allowed the discovery of novel tetracycline resistance mechanisms in S. pneumoniae.