PhD, La Trobe University
Investigating the interaction dynamics between two bacteria capable of electric current production
Electrogenic bacteria are unique in their ability to respire through metallic surfaces. They are called electrogenic because they literally generate electrical current by breathing through these metal surfaces such as a graphite electrode found in microbial electrochemical systems (MESs). These systems are a newly developing technology that can be utilized for renewable and sustainable energy production, bioremediation of natural ecosystems and monitoring environmental health. Electric bacteria which interact with insoluble electron acceptors and donors like metals, live in complex communities of microbial biofilms where both syntrophic and competitive interactions dictate their ability to thrive and in turn drive MES function. However, the interactions that occur within these communities are poorly understood. It is important to dissect the interaction dynamics between each of the members in the electric biofilm community in order to fully understand how to optimize their functionality in the context of MESs. Two well characterized electrogenic bacteria, Geobacter sulfurreducens and Pseudomonas aeruginosa co-habit many of the same environments such as ocean sediments and anaerobic soils, however their ability to interact has not yet been determined. By placement of these two bacteria in cocultures in a strict environment, we have found a syntrophic interaction exists, where one microbe, Pseudomonas, essentially feeds the other, Geobacter, via electron transfer. This interaction underwent adaptive evolution over many generations and the resulting cocultures were sequenced to determine adaptations on the genetic level. By observing the protein and genetic changes throughout their evolution, we see an initial dependency that is then replaced with a competitive interaction as they adapt over time. We will be testing for potential antimicrobial activity being elicited by either organism to confirm whether this initially harmonious interaction has evolved into a competitive one. Gaining deeper understanding of the ecological interactions that take place in electrogenic biofilms will contribute to our ability to harness their interactions in the environment and aid in the effort to make MES technologies a powerful eco-friendly technology of the future.
Abstract: The possibility that metatranscriptomic analysis could distinguish between direct interspecies electron transfer (DIET) and H2 interspecies transfer (HIT) in anaerobic communities was investigated by comparing gene transcript abundance in cocultures in which Geobacter sulfurreducens was the electron-accepting partner for either Geobacter metallireducens, which performs DIET, or Pelobacter carbinolicus, which relies on HIT. Transcript abundance for G. sulfurreducens uptake hydrogenase genes was 7-fold lower in cocultures with G. metallireducens than in cocultures with P. carbinolicus, consistent with DIET and HIT, respectively, in the two cocultures. Transcript abundance for the pilus-associated cytochrome OmcS, which is essential for DIET but not for HIT, was 240-fold higher in the cocultures with G. metallireducens than in cocultures with P. carbinolicus. The pilin gene pilA was moderately expressed despite a mutation that might be expected to repress pilA expression. Lower transcript abundance for G. sulfurreducens genes associated with acetate metabolism in the cocultures with P. carbinolicus was consistent with the repression of these genes by H2 during HIT. Genes for the biogenesis of pili and flagella and several c-type cytochrome genes were among the most highly expressed in G. metallireducens. Mutant strains that lacked the ability to produce pili, flagella, or the outer surface c-type cytochrome encoded by Gmet_2896 were not able to form cocultures with G. sulfurreducens. These results demonstrate that there are unique gene expression patterns that distinguish DIET from HIT and suggest that metatranscriptomics may be a promising route to investigate interspecies electron transfer pathways in more-complex environments.
Pub.: 05 Feb '13, Pinned: 30 Aug '17
Abstract: Electrode-associated microbial biofilms are essential to the function of bioelectrochemical systems (BESs). These systems exist in a number of different configurations but all rely on electroactive microorganisms utilizing an electrode as either an electron acceptor or an electron donor to catalyze biological processes. Investigations of the structure and function of electrode-associated biofilms are critical to further the understanding of how microbial communities are able to reduce and oxidize electrodes. The community structure of electrode-reducing biofilms is diverse and often dominated by Geobacter spp. whereas electrode-oxidizing biofilms are often dominated by other microorganisms. The application of a wide range of tools, such as high-throughput sequencing and metagenomic data analyses, provide insight into the structure and possible function of microbial communities on electrode surfaces. However, the development and application of techniques that monitor gene expression profiles in real-time are required for a more definite spatial and temporal understanding of the diversity and biological activities of these dynamic communities. This mini review summarizes the key gene expression techniques used in BESs research, which have led to a better understanding of population dynamics, cell-cell communication and molecule-surface interactions in mixed and pure BES communities.
Pub.: 17 Dec '14, Pinned: 30 Aug '17
Abstract: Electrodes are unnatural electron acceptors, and it is yet unknown how some Geobacter species evolved to use electrodes as terminal electron acceptors. Analysis of different Geobacter species revealed that they varied in their capacity for current production. Geobacter metallireducens and G. hydrogenophilus generated high current densities (ca. 0.2 mA/cm(2)), comparable to G. sulfurreducens. G. bremensis, G. chapellei, G. humireducens, and G. uraniireducens, produced much lower currents (ca. 0.05 mA/cm(2)) and G. bemidjiensis was previously found to not produce current. There was no correspondence between the effectiveness of current generation and Fe(III) oxide reduction rates. Some high-current-density strains (G. metallireducens and G. hydrogenophilus) reduced Fe(III)-oxides as fast as some low-current-density strains (G. bremensis, G. humireducens, and G. uraniireducens) whereas other low-current-density strains (G. bemidjiensis and G. chapellei) reduced Fe(III) oxide as slowly as G. sulfurreducens, a high-current-density strain. However, there was a correspondence between the ability to produce higher currents and the ability to grow syntrophically. G. hydrogenophilus was found to grow in co-culture with Methanosarcina barkeri, which is capable of direct interspecies electron transfer (DIET), but not with Methanospirillum hungatei capable only of H2 or formate transfer. Conductive granular activated carbon (GAC) stimulated metabolism of the G. hydrogenophilus - M. barkeri co-culture, consistent with electron exchange via DIET. These findings, coupled with the previous finding that G. metallireducens and G. sulfurreducens are also capable of DIET, suggest that evolution to optimize DIET has fortuitously conferred the capability for high-density current production to some Geobacter species.
Pub.: 19 Aug '15, Pinned: 30 Aug '17
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