title: Disparate efficacy of tobramycin on Ca(2+)-, Mg(2+)-, and HEPES-treated Pseudomonas aeruginosa biofilms.
abstract: Mucoid exopolysaccharide (MEP) obtained from Pseudomonas aeruginosa 579 was suspended in 10 mM N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid (HEPES) pH 7.2 containing 0.1-10.0 mM of CaCl2.2H2O or MgCl2.4H2O. MEP treated with HEPES or < 5.0 mM of the Ca2+ or Mg2+ salts remained soluble and bound tobramycin in an equilibrium dialysis bioassay. MEP treated with 5.0 or 10.0 mM of the Ca2+ or Mg2+ salts did not bind tobramycin. Five and 10 mM Ca(2+)-treated MEP precipitated but Mg(2+)-treated MEP did not. Pseudomonas aeruginosa 579 biofilms formed using a defined growth medium having < 1 mM Ca2+ or Mg2+ were treated for 1 h with 10 mM HEPES +/- 5.0 mM CaCl2.2H2O or MgCl2.4H2O, prior to an 8-h exposure to HEPES, or the defined growth medium, +/- 125 micrograms/mL of tobramycin. The tobramycin kill kinetics for the HEPES-, Mg(2+)-, and Ca(2+)-treated biofilms were similar and gradual from T = 0-6 h. The viability of the HEPES- and Mg(2+)-treated populations declined sharply (from 6 to 8 h). Bacteria dispersed from the MEP in control biofilms at 0 and 8 h did not grow in the presence of 7.81 micrograms/mL of tobramycin. Thus, binding of tobramycin of P. aeruginosa 579 MEP may not be as influential to the impediment of tobramycin diffusion as is the steric hindrance imposed by the Ca2+ condensation of the polymer.
authors: Department of Biological Sciences, University of Calgary, Alta., Canada.
authors: B D
authors: BD
authors: Hoyle
authors: C K
authors: CK
authors: Wong
authors: J W
authors: JW
authors: Costerton
mesh_terms: D001665:Binding Sites; D002118:Calcium / Q000494:pharmacology; D004058:Diffusion; D004352:Drug Resistance, Microbial; D006531:HEPES / Q000494:pharmacology; D008274:Magnesium / Q000494:pharmacology; D011550:Pseudomonas aeruginosa / Q000187:drug effects* / Q000254:growth & development / Q000378:metabolism; D014031:Tobramycin / Q000378:metabolism / Q000494:pharmacology*
chemical_list: D008274:Magnesium; D006531:HEPES; D002118:Calcium; D014031:Tobramycin