Terminal electron acceptors influence the quantity and chemical composition of capsular exopolymers produced by anaerobically growing Shewanella spp.

Bacterial exopolymers perform various roles, including acting as a carbon sink, a protective layer against desiccation or antimicrobial agents, or a structural matrix in biofilms. Despite such varied roles, little is known about the heterogeneity of bacterial exopolymer production under varying growth conditions. Here we describe experiments designed to characterize the quantity and quality of exopolymers produced by two commonly studied members of the widely distributed genus Shewanella. Electrokinetic, spectroscopic, and electron microscopic techniques were employed to demonstrate that cell surfaces of Shewanella oneidensis MR-1 (electrophoretic softness, lambda(-1), range from 0.4 to 2.6 nm) are associated with less extracellular polymeric material than surfaces of Shewanella putrefaciens 200R (lambda(-1) range from 1.6 to 3.0 nm). Both species exhibit similar responses to changes in electron acceptor with nitrate- and fumarate-grown cells producing relatively little exopolymer compared to trimethylamine N-oxide (TMAO)-grown cells. In S. oneidensis, the increase in exopolymers has no apparent effect upon cell-surface fixed charge density (-7.7 to -8.7 mM), but for S. putrefaciens a significant drop in fixed charge density is observed between fumarate/nitrate-grown cells (-43 mM) and TMAO-grown cells (-20.8 mM). For both species, exopolymers produced during growth on TMAO have significant amide functionality, increasing from approximately 20-25% of C-containing moieties in nitrate-grown cells to over 30% for TMAO-grown cells (determined from X-ray photoelectron spectroscopy). The increased exopolymer layer associated with TMAO-grown cells appears as a continuous, convoluted layer covering the entire cell surface when viewed by low-temperature, high-resolution scanning electron microscopy. Such significant changes in cell-surface architecture, dependent upon the electron acceptor used for growth, are likely to influence a variety of cell interactions, including aggregation and attachment to surfaces, and the binding of aqueous metal species.