title: Biofilm formation by Pseudomonas aeruginosa: role of the C4-HSL cell-to-cell signal and inhibition by azithromycin.
abstract: OBJECTIVE
  In Pseudomonas aeruginosa, biofilm formation is controlled by a cell-to-cell signalling circuit relying on the secretion of 3-oxo-C12-HSL and C4-HSL. Previous studies suggested that C4-HSL plays no significant role in biofilm formation. However the wild-type PAO1 strain PAO-BI, used as a control in these studies is itself impaired in the production of C4-HSL. We wondered therefore whether the role of C4-HSL in biofilm formation might have been underestimated, and whether azithromycin inhibits biofilm formation by interfering with cell-to-cell signalling.
  
  
  METHODS
  We used isogenic mutants of wild-type PAO1 strains PAO-BI and PT5 in a static biofilm model. Biofilm formation was quantified using Crystal Violet staining and exopolysaccharide measurements.
  
  
  RESULTS
  Wild-type strain PAO-BI, as a result of its reduced C4-HSL secretion, produced 40% less biofilm compared with the wild-type PAO1 strain PT5. Using isogenic mutants of strain PT5 we have shown that whereas a lasI mutant (deficient in 3-oxo-C12-HSL) produced similar amounts of biofilm to the wild-type, a rhlI mutant (deficient in C4-HSL) produced 70% less biofilm. In the latter strain, biofilm formation could be restored by addition of exogenous C4-HSL. Azithromycin, known to reduce the production of both 3-oxo-C12-HSL and C4-HSL, inhibited biofilm formation of wild-type PT5 by 45%. This inhibition could be reversed by the addition of both cell-to-cell signals.
  
  
  CONCLUSIONS
  Our results indicate that C4-HSL also plays a significant role in biofilm formation. Furthermore, we demonstrate the potential of using cell-to-cell signalling blocking agents such as azithromycin to interfere with biofilm formation.
authors.affiliation: Department of Genetics and Microbiology, University of Geneva, CMU, 1, rue Michel-Servet, CH-1211 Geneva 4, Switzerland.
authors.forename: Sabine
authors.initials: S
authors.lastname: Favre-Bonté
authors.forename: Thilo
authors.initials: T
authors.lastname: Köhler
authors.forename: Christian
authors.initials: C
authors.lastname: Van Delden
mesh_terms: D015107:4-Butyrolactone / Q000031:analogs & derivatives* / Q000037:antagonists & inhibitors* / Q000378:metabolism; D017963:Azithromycin / Q000494:pharmacology*; D018441:Biofilms / Q000187:drug effects / Q000254:growth & development*; D002450:Cell Communication / Q000187:drug effects* / Q000502:physiology; D006801:Humans; D011550:Pseudomonas aeruginosa; D015398:Signal Transduction / Q000187:drug effects
chemical_list: C092312:N-butyrylhomoserine lactone; D017963:Azithromycin; D015107:4-Butyrolactone