title: In vitro evaluation of the antibacterial activity of extracts from 34 species of North American lichens.
abstract: BACKGROUND
  The emergence of antibiotic resistant pathogens is a serious global health threat. Hence, the search for new antibiotic drugs from various natural sources should be given high priority. Lichens produce a variety of low molecular weight metabolic compounds and many cultures have utilized these compounds in traditional medicine for centuries.
  
  
  OBJECTIVE
  Report the antibiotic properties of extracts from 34 North American lichens screened against four pathogenic bacteria.
  
  
  METHODS
  The micro-well dilution method was used to determine the minimum inhibitory concentration (MIC) of acetone and methanol extracts of 34 lichen species against four bacterial strains. Major chemical compounds in each species were identified using thin layer chromatography (TLC).
  
  
  RESULTS
  Most of the lichen extracts demonstrated inhibitory effects against Staphylococcus aureus, Pseudomonas aeruginosa, and methicillin-resistant S. aureus (MRSA) with MIC values ranging from 3.9 to 500 µg/ml. In addition, extracts from three species, Letharia columbiana (Nutt.) J. W. Thomson (Parmeliaceae), Letharia vulpina (L.) Hue (Parmeliaceae), and Vulpicida canadensis (Räsänen) J.-E. Mattsson & M. J. Lai (Parmeliaceae) (MIC = 125-500 µg/ml) were also effective against Escherichia coli. Generally, acetone extractions were found to be more effective than methanol extractions.
  
  
  CONCLUSIONS
  Results of this study show that lichen extracts provide significant antimicrobial activity against both Gram-positive and Gram-negative bacteria. These results suggest that lichens may be an important potential source of antibacterial drugs.
authors.affiliation: Department of Biology .
authors.forename: Gajendra
authors.initials: G
authors.lastname: Shrestha
authors.forename: Jocelyn
authors.initials: J
authors.lastname: Raphael
authors.forename: Steven D
authors.initials: SD
authors.lastname: Leavitt
authors.forename: Larry L
authors.initials: LL
authors.lastname: St Clair
mesh_terms: D000900:Anti-Bacterial Agents / Q000302:isolation & purification* / Q000494:pharmacology; D004353:Drug Evaluation, Preclinical / Q000379:methods; D008011:Lichens / Q000235:genetics; D055624:Methicillin-Resistant Staphylococcus aureus / Q000187:drug effects / Q000502:physiology; D008826:Microbial Sensitivity Tests / Q000379:methods; D009656:North America; D010936:Plant Extracts / Q000302:isolation & purification* / Q000494:pharmacology; D011550:Pseudomonas aeruginosa / Q000187:drug effects* / Q000502:physiology; D013045:Species Specificity; D013211:Staphylococcus aureus / Q000187:drug effects* / Q000502:physiology
chemical_list: D000900:Anti-Bacterial Agents; D010936:Plant Extracts
keywords: Antibiotic; MIC; TLC; micro-broth dilution; natural products