The biosynthesis of the exopolysaccharide gellan results in the decrease of Sphingomonas paucimobilis tolerance to copper

Although it is generally accepted that extracellular anionic polysaccharide-producing bacteria are more tolerant to toxic cations than the nonproducing strains, the gellan-producing Sphingomonas paucimobilis R40 was definitively less tolerant to sublethal concentrations of copper (Cu 2+) than the nonproducing variant RP10. This nonmucoid strain was spontaneously obtained during the cultivation of mucoid R40. The two variants exhibited indistinguishable genome fingerprints with the rare-cutting restriction enzymes, AseI and BfrI, which indicates a close genetic relationship. The lower tolerance to Cu 2+ of the gellan-producing strain was revealed by the range of CuCl 2 concentrations that allowed growth (up to 0.35 or 0.50 mmol l −1 for R40 or RP10, respectively) and for identical Cu 2+ concentrations by the more drastic inhibition of the maximum specific growth rate and longer duration of the growth lag phase observed for R40 as compared with RP10. The possible basis for the higher susceptibility of the mucoid strain to Cu 2+ is discussed. The presence of increasing concentrations of Cu 2+ in the growth medium affected cell morphology and association and led to decreased concentrations of final biomass and gellan produced by R40; however, gellan specific production (gellan produced cell −1) appeared to be stimulated in Cu 2+-stressed cells although the molecular mass was reduced as judged by viscosimetric measurements.