title: An unusual class of anthracyclines potentiate Gram-positive antibiotics in intrinsically resistant Gram-negative bacteria.
abstract: OBJECTIVE
  An orthogonal approach taken towards novel antibacterial drug discovery involves the identification of small molecules that potentiate or enhance the activity of existing antibacterial agents. This study aimed to identify natural-product rifampicin adjuvants in the intrinsically resistant organism Escherichia coli.
  
  
  METHODS
  E. coli BW25113 was screened against 1120 actinomycete fermentation extracts in the presence of subinhibitory (2 mg/L) concentrations of rifampicin. The active molecule exhibiting the greatest rifampicin potentiation was isolated using activity-guided methods and identified using mass and NMR spectroscopy. Susceptibility testing and biochemical assays were used to determine the mechanism of antibiotic potentiation.
  
  
  RESULTS
  The anthracycline Antibiotic 301A(1) was isolated from the fermentation broth of a strain of Streptomyces (WAC450); the molecule was shown to be highly synergistic with rifampicin (fractional inhibitory concentration index = 0.156) and moderately synergistic with linezolid (FIC index = 0.25) in both E. coli and Acinetobacter baumannii. Activity was associated with inhibition of efflux and the synergistic phenotype was lost when tested against E. coli harbouring mutations within the rpoB gene. Structure-activity relationship studies revealed that other anthracyclines do not synergize with rifampicin and removal of the sugar moiety of Antibiotic 301A(1) abolishes activity.
  
  
  CONCLUSIONS
  Screening only a subsection of our natural product library identified a small-molecule antibiotic adjuvant capable of sensitizing Gram-negative bacteria to antibiotics to which they are ordinarily intrinsically resistant. This result demonstrates the great potential of this approach in expanding antibiotic effectiveness in the face of the growing challenge of resistance in Gram-negatives.
authors.affiliation: Department of Biochemistry and Biomedical Sciences, M. G. DeGroote Institute for Infectious Disease Research, McMaster University, 1280 Main St W, Hamilton, Ontario L8S 4K1, Canada.
authors.forename: Georgina
authors.initials: G
authors.lastname: Cox
authors.affiliation: Department of Biochemistry and Biomedical Sciences, M. G. DeGroote Institute for Infectious Disease Research, McMaster University, 1280 Main St W, Hamilton, Ontario L8S 4K1, Canada.
authors.forename: Kalinka
authors.initials: K
authors.lastname: Koteva
authors.affiliation: Department of Biochemistry and Biomedical Sciences, M. G. DeGroote Institute for Infectious Disease Research, McMaster University, 1280 Main St W, Hamilton, Ontario L8S 4K1, Canada wrightge@mcmaster.ca.
authors.forename: Gerard D
authors.initials: GD
authors.lastname: Wright
mesh_terms: D000081:Acetamides / Q000494:pharmacology; D040981:Acinetobacter baumannii / Q000187:drug effects; D018943:Anthracyclines / Q000737:chemistry / Q000302:isolation & purification / Q000378:metabolism / Q000494:pharmacology*; D000900:Anti-Bacterial Agents / Q000737:chemistry / Q000302:isolation & purification / Q000494:pharmacology*; D012321:DNA-Directed RNA Polymerases / Q000235:genetics / Q000378:metabolism; D004357:Drug Synergism; D004926:Escherichia coli / Q000187:drug effects*; D000069349:Linezolid; D009682:Magnetic Resonance Spectroscopy; D013058:Mass Spectrometry; D023303:Oxazolidinones / Q000494:pharmacology; D012293:Rifampin / Q000494:pharmacology*; D013302:Streptomyces / Q000378:metabolism; D013329:Structure-Activity Relationship
chemical_list: D000081:Acetamides; D018943:Anthracyclines; D000900:Anti-Bacterial Agents; D023303:Oxazolidinones; D012321:DNA-Directed RNA Polymerases; C105078:RNA polymerase beta subunit; D000069349:Linezolid; D012293:Rifampin
keywords: antibiotic adjuvants; intrinsic antibiotic resistance; pathogens; resistome