Shewanella oneidensis MR-1 as an avenue for green nanoparticle biosynthesis and next-generation biosensing

Abstract

As global resource consumption grows and the pressures of industrialization and climate change place stress on Earth’s natural sources of freshwater, the preservation of these water systems has become a matter of increasing urgency. Accomplishing this task will require many strategies and technologies, such as engineering non-polluting renewable energy systems, remediation techniques for polluted bodies of water, and most recently, arrays of smart sensors, which can collect detailed data of lake dynamics and these complex systems to be modeled.

In addition to performing controlled nanoparticle synthesis with Shewanella, I investigated the interactions of the bacteria with a range of solid substrates in an aqueous growth medium. I was able to successfully observe the interactions of Shewanella biofilms with substrates of silicon, microporous alumina, and reduced graphene oxide. Furthermore, I performed cyclic voltammetry on a three-electrode system containing Shewanella culture, a steel working electrode, a titanium counter electrode and a Gamry Ag/AgCl reference electrode. The cyclic voltammetric measurements revealed reduction-oxidation activity that corresponded to visible changes in the culture, such as the evolution of a biofilm coating and the corrosion of iron.

Three types of semiconductor nanoparticles - lead sulfide, cadmium sulfide and molybdenum disulfide - were synthesized in the first known controlled studies using the metal and sulfur-reducing capabilities of Shewanella oneidensis MR-1 to produce these materials. Batches of Shewanella were cultivated anaerobically with sodium thiosulfate supplied as an electron acceptor. Metal ions were added to the media through the addition of dissolved salts, and the batches were incubated for several days. In all trials, precipitates formed in inoculated batches that did not exist in sterile batches of the same liquid medium. Scanning electron microscopy and energy dispersive X-ray spectroscopy revealed substantial biofilm growth that was associated with concentrated levels of metals from the supplied metal salts along with sulfur, suggesting the formation of metal sulfides at the biofilm sites. Using a combination of X-ray diffraction and Raman spectroscopy, we were then able to confirm the presence of the targeted nanoparticulate materials.

The results of this work offer new insights into the potential for biologically-assisted creation of electronic devices. Semiconductor nanoparticles have a wide range of industry applications ranging from photonic sensors to transistors to photovoltaic arrays. Compared to existing chemical methods, biological synthesis of semiconductor nanoparticles requires substantially less heat input as well as no added chemical surfactants or stabilizers. Meanwhile, my studies of electron transfer between bacterial biofilms and solid electrodes contribute to existing knowledge of electro-biochemical sensing, which may offer substantial cost advantages over existing aqueous chemical sensors by allowing direct sensing of microbial metabolic activity and its by-products.

In this work I describe the cultivation of a dissimilatory metal-reducing bacterium, Shewanella oneidensis MR-1, in controlled environments, demonstrating novel methods of synthesis for several types of metal sulfide nanoparticles with semiconductor properties and applications in semiconductor device and sensor engineering. Shewanella is an obligate anaerobe that can grow either aerobically or anaerobically but performs metal or sulfur reduction in anaerobic environments.

As microbiologists and engineers have recently discovered, organisms living in or near these water systems may help provide a solution to these engineering problems. A class of bacteria known as dissimilatory metal-reducing bacteria, or DMRB, have adapted to metal-rich waters by reducing ions of metals such as manganese, arsenic, and iron as part of their metabolic processes. In the process, DMRB often trigger the precipitation of various insoluble or minimally-soluble minerals consisting of reduced metals. DMRB can adapt well to healthy freshwater environments as well as contaminated or extreme environments such as hydrothermal vents or acid mine drainage sites.