Effects of divalent cations on the synthesis of alginic acid-like exopolysaccharide from mucoid Pseudomonas aeruginosa.

The effects of modulated culture calcium (Ca2+) and magnesium (Mg2+) concentrations on the growth of two mucoid Pseudomonas aeruginosa strains of cystic fibrosis origin and on the synthesis of their extracellular polyuronic acids (EPA) were examined. Both strains required a minimum Mg2+ concentration for growth but differed in their Mg2+ requirements to achieve maximum growth potential. The chemical composition of the EPA produced by either strain was not effected by the Mg2+ or Ca2+ concentration of the medium. The exopolysaccharide of strain PM1 was polymannuronic acid and acetylated polymannuronic acid for strain PM9. However, the molecular size of the exopolysaccharide produced by strain PM1 alone was markedly influenced by culture Ca2+ and Mg2+ levels. When the Mg2+ concentration was 3.0 mM or above, the exopolysaccharide was monodispersed and of high molecular weight. At lower Mg2+ concentrations the polymer was degraded. The evidence suggests that exopolysaccharide degradation resulted from the release or activation of an enzyme in response to reduced Mg2+.