Adaptive synthesis of a rough lipopolysaccharide in Geobacter sulfurreducens for metal reduction and detoxification

The ability of some metal-reducing bacteria to produce a rough (no O-antigen) lipopolysaccharide (LPS) could facilitate surface interactions with minerals and metal reduction. Consistent with this, the laboratory model metal reducer Geobacter sulfurreducens PCA produced two rough LPS isoforms (with or without a terminal methyl-quinovosamine sugar) when growing with the soluble electron acceptor, fumarate, but only expressed the shorter and more hydrophilic variant when reducing iron oxides. We reconstructed from genomic data conserved pathways for the synthesis of the rough LPS and generated heptosyltransferase mutants with partial (Δ rfaQ ) and complete (Δ rfaC ) truncations in the core oligosaccharide. The stepwise removal of the LPS core sugars reduced the hydrophilicity of the cell and increased outer membrane vesiculation. These changes in outer membrane charge and remodeling did not substantially impact planktonic growth but disrupted the developmental stages and structure of electroactive biofilms. Furthermore, the mutants assembled conductive pili for the extracellular mineralization of the toxic uranyl cation, yet were unable to prevent the permeation and mineralization of the radionuclide in the cell envelope. Hence, not only does the rough LPS promote cell-cell and cell-mineral interactions critical to biofilm formation and metal respiration, but it also functions as a permeability barrier to toxic metal cations. In doing so, the rough LPS maximizes the extracellular reduction of soluble and insoluble metals and preserves cell envelope functions critical to the environmental survival of Geobacter bacteria in metal rich environments and their performance in bioremediation and bioenergy applications. Importance Some metal-reducing bacteria produce a lipopolysaccharide (LPS) without the repeating sugars (O-antigen) that decorate the surface of most Gram-negative bacteria, but the biological significance of this adaptive feature has never been investigated. Using the model representative Geobacter sulfurreducens strain PCA and mutants carrying stepwise truncations in the LPS core sugars, we demonstrate the importance of the rough LPS in the control of cell surface chemistry during the respiration of iron minerals and the formation of electroactive biofilms. Importantly, we describe hitherto overlooked roles for the rough LPS in metal sequestration and outer membrane vesiculation that are critical for the extracellular reduction and detoxification of toxic metals and radionuclides. These results are of interest for the optimization of bioremediation schemes and electricity-harvesting platforms using these bacteria.

Chromium Reduction via a Semi-Conducting Hematite Electrode: Implications for Microbial Cycling of Metals in Natural Soils

Semi-conducting Fe oxide minerals, such as hematite, are well known to influence the fate of contaminants and nutrients in many environmental settings and influence microbial growth under suboxic to anoxic conditions through a myriad of different processes. Recent studies of Fe oxide reduction by Fe(II) have demonstrated that reduction of Fe at one surface can result in the release of Fe(II) different one. Termed Fe(II) catalyzed recrystallization, this phenomena is attributed to conduction of additional electrons through the mineral structure from the point of contact to another which occurs because of the minerals’ semi-conductivity. While it is well understood that Fe(II) plays a central role in redox cycling of elements, the environmental implications of Fe(II) catalyzed recrystallization need to be further explored. Here, we provide evidence that the Fe mineral conductivity underpinning Fe(II) catalyzed recrystallization can couple the reduction of Cr, a priority metal contaminant, with an electron source that is cannot directly affect Cr. This is shown for both an abiotic electron source, a potentiostat, as well as the metal reducing bacteria Shewanella Putrefaciens. The implications of this work show that semiconductive minerals may be links in subsurface electrical networks that physically distribute redox chemistry and suggests novel methods for remediating Cr contamination in groundwater.

Chromium Reduction via a Semi-Conducting Hematite Electrode: Implications for Microbial Cycling of Metals in Natural Soils

Semi-conducting Fe oxide minerals, such as hematite, are well known to influence the fate of contaminants and nutrients in many environmental settings and influence microbial growth under suboxic to anoxic conditions through a myriad of different processes. Recent studies of Fe oxide reduction by Fe(II) have demonstrated that reduction of Fe at one surface can result in the release of Fe(II) different one. Termed Fe(II) catalyzed recrystallization, this phenomena is attributed to conduction of additional electrons through the mineral structure from the point of contact to another which occurs because of the minerals’ semi-conductivity. While it is well understood that Fe(II) plays a central role in redox cycling of elements, the environmental implications of Fe(II) catalyzed recrystallization need to be further explored. Here, we provide evidence that the Fe mineral conductivity underpinning Fe(II) catalyzed recrystallization can couple the reduction of Cr, a priority metal contaminant, with an electron source that is cannot directly affect Cr. This is shown for both an abiotic electron source, a potentiostat, as well as the metal reducing bacteria Shewanella Putrefaciens. The implications of this work show that semiconductive minerals may be links in subsurface electrical networks that physically distribute redox chemistry and suggests novel methods for remediating Cr contamination in groundwater.

Influence of pore geometry on motility and trapping of metal reducing bacteria

<p>Diverse processes such as bioremediation, biofertilization, and microbial drug delivery<br>rely on bacterial migration in porous media. However, how pore-scale confinement alters<br>bacterial motility is unknown due to the inherent heterogeneity in porous media. As a<br>result, models of migration are limited and often employ ad hoc assumptions.<br>We aim to determine the impact of pore confinement in the spreading dynamics of two<br>populations of motile metal reducing bacteria by directly visualizing individual <em>Acidovorax</em><br>and <em>Pelosinus</em> in an unconfined liquid medium and in a microfluidic chip containing regular<br>placed pillars. We observe that the length of runs of the two species differs from the<br>unconfined and confined medium. Results show that bacteria in the confined medium<br>display a systematic shorter jumps due to grain obstacles when compared to the open<br>porous medium. Close inspection of the trajectories reveals that cells are intermittently<br>and transiently trapped, which produces superdiffusive motion at early and subdiffusion<br>behavior at late times, as they navigate through the confined pore space. While in the open<br>medium, we observe a linearly increasing variance with respect to time for <em>Acidovorax</em>, and<br>for <em>Pelosinus</em> the variance increases at a much faster rate showing super diffusive behavior<br>at early times. At late times, the rate of growth in spreading increases for <em>Acidovorax</em> while<br>it reduces for <em>Pelosinus</em>. We finally discuss that the paradigm of run-and-tumble motility<br>is dramatically altered in the confined porous medium and its practical applications of<br>these effects on large-scale transport.</p>

On Modeling Ensemble Transport of Metal Reducing Motile Bacteria

Many metal reducing bacteria are motile with their run-and-tumble behavior exhibiting series of flights and waiting-time spanning multiple orders of magnitude. While several models of bacterial processes do not consider their ensemble motion, some models treat motility using an advection diffusion equation (ADE). In this study, Geobacter and Pelosinus, two metal reducing species, are used in micromodel experiments for study of their motility characteristics. Trajectories of individual cells on the order of several seconds to few minutes in duration are analyzed to provide information on (1) the length of runs, and (2) time needed to complete a run (waiting or residence time). A Continuous Time Random Walk (CTRW) model to predict ensemble breakthrough plots is developed based on the motility statistics. The results of the CTRW model and an ADE model are compared with the real breakthrough plots obtained directly from the trajectories. The ADE model is shown to be insufficient, whereas a coupled CTRW model is found to be good at predicting breakthroughs at short distances and at early times, but not at late time and long distances. The inadequacies of the simple CTRW model can possibly be improved by accounting for correlation in run length and waiting time.