title: Activation of cryptic aminoglycoside resistance in Salmonella enterica.
abstract: Aminoglycoside resistance in bacteria can be acquired by several mechanisms, including drug modification, target alteration, reduced uptake and increased efflux. Here we demonstrate that increased resistance to the aminoglycosides streptomycin and spectinomycin in Salmonella enterica can be conferred by increased expression of an aminoglycoside adenyl transferase encoded by the cryptic, chromosomally located aadA gene. During growth in rich medium the wild-type strain was susceptible but mutations that impaired electron transport and conferred a small colony variant (SCV) phenotype or growth in glucose/glycerol minimal media resulted in activation of the aadA gene and aminoglycoside resistance. Expression of the aadA gene was positively regulated by the stringent response regulator guanosine penta/tetraphosphate ((p)ppGpp). SCV mutants carrying stop codon mutations in the hemA and ubiA genes showed a streptomycin pseudo-dependent phenotype, where growth was stimulated by streptomycin. Our data suggest that this phenotype is due to streptomycin-induced readthrough of the stop codons, a resulting increase in HemA/UbiA levels and improved electron transport and growth. Our results demonstrate that environmental and mutational activation of a cryptic resistance gene can confer clinically significant resistance and that a streptomycin-pseudo-dependent phenotype can be generated via a novel mechanism that does not involve the classical rpsL mutations.
authors.affiliation: Department of Medical Biochemistry and Microbiology, Uppsala University, Box 582, SE-751 23 Uppsala, Sweden.
authors.forename: Sanna
authors.initials: S
authors.lastname: Koskiniemi
authors.forename: Maria
authors.initials: M
authors.lastname: Pränting
authors.forename: Erik
authors.initials: E
authors.lastname: Gullberg
authors.forename: Joakim
authors.initials: J
authors.lastname: Näsvall
authors.forename: Dan I
authors.initials: DI
authors.lastname: Andersson
mesh_terms: D000617:Aminoglycosides / Q000494:pharmacology*; D000900:Anti-Bacterial Agents / Q000494:pharmacology*; D001426:Bacterial Proteins / Q000235:genetics / Q000378:metabolism; D024881:Drug Resistance, Bacterial; D015964:Gene Expression Regulation, Bacterial / Q000187:drug effects; D006801:Humans; D009154:Mutation; D012480:Salmonella Infections / Q000382:microbiology; D019779:Salmonella enterica / Q000187:drug effects* / Q000201:enzymology / Q000235:genetics* / Q000378:metabolism; D013307:Streptomycin / Q000494:pharmacology; D015533:Transcriptional Activation / Q000187:drug effects*; D014166:Transferases / Q000235:genetics / Q000378:metabolism
chemical_list: D000617:Aminoglycosides; D000900:Anti-Bacterial Agents; D001426:Bacterial Proteins; D014166:Transferases; D013307:Streptomycin