Inability of purified Pseudomonas aeruginosa exopolysaccharide to bind selected antibiotics.

It has been proposed that the exopolysaccharide (alginate) of mucoid Pseudomonas aeruginosa strains which infect cystic fibrosis patients might bind and hence protect this pathogen from antibiotics. To test this hypothesis, we employed equilibrium dialysis to measure the binding between several antibiotics and purified Pseudomonas alginate. Binding was calculated from the residual concentrations of antibiotics in free solution by a biological assay. The detectable binding of antibiotics to alginate was consistent with expectations; the positively charged antibiotics steptomycin and tobramycin, bound to the polyanion (0.047 and 0.024 mumol/mg of alginate, respectively), whereas the neutral species, clindamycin and penicillin, bound negligibly or not at all (0.0011 and 0 mumol/mg of alginate, respectively). When these experiments were performed in the presence of physiological concentrations of saline, none of the antibiotics bound to the polysaccharide. Since the binding observed was abrogated by salt concentrations typical of the tracheobronchial secretions of cystic fibrosis patients, the data suggest that tight binding of antibiotics to the exopolysaccharide of a mucoid P. aeruginosa strain does not provide increased antibiotic resistance.