Formation of biologically influenced palladium microstructures by Desulfovibrio desulfuricans and Desulfovibrio ferrophilus IS5.

A range of Desulfovibrio spp. can reduce metal ions to form metallic nanoparticles that remain attached to their surfaces. The bioreduction of palladium (Pd) has been given considerable attention due to its extensive use in areas of catalysis and electronics and other technological domains. In this study we report, for the first time, evidence for Pd(II) reduction by the highly corrosive Desulfovibrio ferrophilus IS5 strain to form surface attached Pd nanoparticles, as well as rapid formation of Pd(0) coated microbial nanowires. These filaments reached up to 8 µm in length and led to the formation of a tightly bound group of interconnected cells with enhanced ability to attach to a low carbon steel surface. Moreover, when supplied with high concentrations of Pd (≥ 100 mmol Pd(II) g-1 dry cells), both Desulfovibrio desulfuricans and D. ferrophilus IS5 formed bacteria/Pd hybrid porous microstructures comprising millions of cells. These three-dimensional structures reached up to 3 mm in diameter with a dose of 1200 mmol Pd(II) g-1 dry cells. Under suitable hydrodynamic conditions during reduction, two-dimensional nanosheets of Pd metal were formed that were up to several cm in length. Lower dosing of Pd(II) for promoting rapid synthesis of metal coated nanowires and enhanced attachment of cells onto metal surfaces could improve the efficiency of various biotechnological applications such as microbial fuel cells. Formation of biologically stimulated Pd microstructures could lead to a novel way to produce metal scaffolds or nanosheets for a wide variety of applications.

Inhibition of Sulfate Reduction and Cell Division by Desulfovibrio desulfuricans Coated in Palladium Metal.

The growth of sulfate-reducing bacteria (SRB) and associated hydrogen sulfide production can be problematic in a range of industries such that inhibition strategies are needed. A range of SRB can reduce metal ions, a strategy that has been utilized for bioremediation, metal recovery, and synthesis of precious metal catalysts. In some instances, the metal remains bound to the cell surface, and the impact of this coating on bacterial cell division and metabolism has not previously been reported. In this study, Desulfovibrio desulfuricans cells (1g dry weight) enabled the reduction of up to 1500 mmol (157.5 g) palladium (Pd) ions, resulting in cells being coated in approximately 1 µm of metal. Thickly coated cells were no longer able to metabolize or divide, ultimately leading to the death of the population. Increasing Pd coating led to prolonged inhibition of sulfate reduction, which ceased completely after cells had been coated with 1200 mmol Pd g-1 dry cells. Less Pd nanoparticle coating permitted cells to carry out sulfate reduction and divide, allowing the population to recover over time as surface-associated Pd diminished. Overcoming inhibition in this way was more rapid using lactate as the electron donor, compared to formate. When using formate as an electron donor, preferential Pd(II) reduction took place in the presence of 100 mM sulfate. The inhibition of important metabolic pathways using a biologically enabled casing in metal highlights a new mechanism for the development of microbial control strategies. IMPORTANCE Microbial reduction of sulfate to hydrogen sulfide is highly undesirable in several industrial settings. Some sulfate-reducing bacteria are also able to transform metal ions in their environment into metal phases that remain attached to their outer cell surface. This study demonstrates the remarkable extent to which Desulfovibrio desulfuricans can be coated with locally generated metal nanoparticles, with individual cells carrying more than 100 times their mass of palladium metal. Moreover, it reveals the effect of metal coating on metabolism and replication for a wide range of metal loadings, with bacteria unable to reduce sulfate to sulfide beyond a specific threshold. These findings present a foundation for a novel means of modulating the activity of sulfate-reducing bacteria.

The Critical Role of Environmental Synergies in the Creation of Bionanohybrid Microbes.

A wide range of bacteria can synthesize surface-associated nanoparticles (SANs) through exogenous metal ions reacting with sulfide produced via cysteine metabolism, resulting in the emergence of a biological-nanoparticle hybrid (bionanohybrid). The attached nanoparticles may couple to extracellular electron transfer, facilitating de novo photoelectrochemical processes. While SAN-cell coupling in hybrid organisms is opening a range of biotechnological possibilities, observation of bionanohybrids in nature is not commonly reported and their lab-based behavior remains difficult to control. We describe the critical role environmental synergy (microbial growth stage, cell densities, cysteine, and exogenous metal concentrations) plays in controlling the form and occurrence of Escherichia coli and Moorella thermoacetica bionanohybrids. SAN development depends on an appropriate cell density to metal ratio, with too few cells resulting in nanoparticle suppression through cytotoxicity or inhibition of cysteine conversion, and with too many cells diluting the number and size of particles produced. This cell number is governed by the concentration of cysteine present, which acts to protect the cells from metal ion toxicity. Exposing cells to metal and cysteine during the lag phase leads to SAN development, whereas cells in the exponential growth phase predominantly produce dispersed nanoparticles. Applying these principles more broadly, E. coli is shown to biosynthesize composite Bi/Cu sulfide SANs, and Clostridioides difficile can be coaxed into a bionanohybrid lifestyle by fine-tuning the cysteine dosage. Bionanohybrids maintain a remarkable ability for binary fission and sustained growth, opening doors to the production of SANs tailored to specific technological functions. IMPORTANCE Some bacteria can produce nanoscale-sized particles, which remain attached to the surface of the organism. The surface association of these nanoparticles creates a new mode of interaction between the microbe's environment and its internal cellular function, giving rise to a new hybrid lifeform, a biological nanoparticle hybrid (bionanohybrid). These hybrid organisms gain new or enhanced biological functions, and thus their creation opens a wide range of biotechnological possibilities. Despite this potential, the fundamental controls on bionanohybrid formation and occurrence remain poorly constrained. In this study, Escherichia coli K-12, Moorella thermoacetica, and Clostridioides difficile were used to test the combined influences of the growth phase, cell density, cysteine dose, and metal concentration in determining single and composite metal sulfide surface-associated nanoparticle production. The significance of this study is that it defined the critical synergies controlling nanoparticle formation on bacterial cell surfaces, unlocking the potential for bionanohybrid applications in a range of organisms.