title: Multiple mechanisms to ameliorate the fitness burden of mupirocin resistance in Salmonella typhimurium.
abstract: We examined how the fitness costs of mupirocin resistance caused by mutations in the chromosomal isoleucyl-tRNA synthetase gene (ileS) can be ameliorated. Mupirocin-resistant mutants were isolated and four different, resistance-conferring point mutations in the chromosomal ileS gene were identified. Fifty independent lineages of the low-fitness, resistant mutants were serially passaged to evolve compensated mutants with increased fitness. In 34/50 of the evolved lineages, the increase in fitness resulted from additional point mutations in isoleucine tRNA synthetase (IleRS). Measurements in vitro of the kinetics of aminoacylation of wild-type and mutant enzymes showed that resistant IleRS had a reduced rate of aminoacylation due to altered interactions with both tRNAIle and ATP. The intragenic compensatory mutations improved IleRS kinetics towards the wild-type enzyme, thereby restoring bacterial fitness. Seven of the 16 lineages that lacked second-site compensatory mutations in ileS, showed an increase in ileS gene dosage, suggesting that an increased level of defective IleRS compensate for the decrease in aminoacylation activity. Our findings show that the fitness costs of ileS mutations conferring mupirocin resistance can be reduced by several types of mechanisms that may contribute to the stability of mupirocin resistance in clinical settings.
authors.affiliation: Department of Bacteriology, Swedish Institute for Infectious Disease Control and Microbiology, Tumor and Cell Biology Center, Karolinska Institute, S-171 82 Solna, Sweden.
authors.forename: Wilhelm
authors.initials: W
authors.lastname: Paulander
authors.forename: Sophie
authors.initials: S
authors.lastname: Maisnier-Patin
authors.forename: Dan I
authors.initials: DI
authors.lastname: Andersson
mesh_terms: D000255:Adenosine Triphosphate / Q000378:metabolism; D019943:Amino Acid Substitution; D049148:Aminoacylation; D000900:Anti-Bacterial Agents / Q000494:pharmacology*; D004252:DNA Mutational Analysis; D024881:Drug Resistance, Bacterial; D018628:Gene Dosage; D007533:Isoleucine-tRNA Ligase / Q000737:chemistry / Q000235:genetics* / Q000378:metabolism*; D007700:Kinetics; D008958:Models, Molecular; D016712:Mupirocin / Q000494:pharmacology*; D017354:Point Mutation; D011485:Protein Binding; D012355:RNA, Transfer, Ile / Q000378:metabolism; D012486:Salmonella typhimurium / Q000187:drug effects* / Q000201:enzymology
chemical_list: D000900:Anti-Bacterial Agents; D012355:RNA, Transfer, Ile; D000255:Adenosine Triphosphate; D016712:Mupirocin; D007533:Isoleucine-tRNA Ligase