Disparate Efficacy of Tobramycin on Ca(2+)-, Mg(2+)-, and HEPES-treated Pseudomonas Aeruginosa Biofilms

Can J Microbiol. 1992 Nov;38(11):1214-8. doi: 10.1139/m92-201.


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.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Binding Sites
  • Calcium / pharmacology
  • Diffusion
  • Drug Resistance, Microbial
  • HEPES / pharmacology
  • Magnesium / pharmacology
  • Pseudomonas aeruginosa / drug effects*
  • Pseudomonas aeruginosa / growth & development
  • Pseudomonas aeruginosa / metabolism
  • Tobramycin / metabolism
  • Tobramycin / pharmacology*


  • Magnesium
  • Calcium
  • Tobramycin