There is an immediate need to drastically reduce the emissions associated with global fossil fuel consumption in order to limit climate change. However, carbon-based materials, chemicals, and ...transportation fuels are predominantly made from fossil sources and currently there is no alternative source available to adequately displace them. Gas-fermenting microorganisms that fix carbon dioxide (CO2) and carbon monoxide (CO) can break this dependence as they are capable of converting gaseous carbon to fuels and chemicals. As such, the technology can utilize a wide range of feedstocks including gasified organic matter of any sort (e.g., municipal solid waste, industrial waste, biomass, and agricultural waste residues) or industrial off-gases (e.g., from steel mills or processing plants). Gas fermentation has matured to the point that large-scale production of ethanol from gas has been demonstrated by two companies. This review gives an overview of the gas fermentation process, focusing specifically on anaerobic acetogens. Applications of synthetic biology and coupling gas fermentation to additional processes are discussed in detail. Both of these strategies, demonstrated at bench-scale, have abundant potential to rapidly expand the commercial product spectrum of gas fermentation and further improve efficiencies and yields.
Low carbon fuels and chemicals can be sourced from renewable materials such as biomass or from industrial and municipal waste streams. Gasification of these materials allows all of the carbon to ...become available for product generation, a clear advantage over partial biomass conversion into fermentable sugars. Gasification results into a synthesis stream (syngas) containing carbon monoxide (CO), carbon dioxide (CO
), hydrogen (H
) and nitrogen (N
). Autotrophy-the ability to fix carbon such as CO
is present in all domains of life but photosynthesis alone is not keeping up with anthropogenic CO
output. One strategy is to curtail the gaseous atmospheric release by developing waste and syngas conversion technologies. Historically microorganisms have contributed to major, albeit slow, atmospheric composition changes. The current status and future potential of anaerobic gas-fermenting bacteria with special focus on acetogens are the focus of this review.
Toxin-antitoxin (TA) systems in Escherichia coli may play a role in biofilm formation, but the mechanism involved remains debatable. It is not known whether the TA systems are responsible for ...extracellular DNA (eDNA) in biofilms. In this study, we investigated the function of the hipBA TA system in biofilm formation by Escherichia coli strain BW25113. First, the deletion of the HipBA TA system in E. coli BW25113 significantly reduced the biofilm biomass without antibiotic stress. Second, treatment of the BW25113 biofilm with DNase I caused a major reduction in biofilm formation, whereas similar treatment of the hipA mutant biofilm had only a minor effect. Third, the inactivation of HipA reduced the level of eDNA present in biofilm formation, and addition of BW25113 genomic DNA stimulated biofilm formation for both the wild-type and hipA mutant. Fourth, the wild-type cells underwent significantly more cell lysis than the hipA mutant. These results suggest that hipA plays a significant role during biofilm development and that eDNA is an important structural component of E. coli BW25113 biofilms. Thus, the TA system may enhance biofilm formation through DNA release.
Extracellular DNA can play a structural role in the microbial environment. Here evidence is presented that an environmental isolate of Acidovorax temperans utilises extracellular DNA for ...intercellular and cell-surface attachment and that Type IV pili and electrostatic interactions play a role in this interaction. Preliminary attempts to isolate and purify extracellular polysaccharides from A. temperans strain CB2 yielded significant amounts of DNA raising the question of whether this molecule was present as a structural component in the extracellular matrix. The role of DNA in attachment was indicated by experiments in which the addition of DNase to liquid medium inhibited the attachment of Acidovorax to glass wool. A Tn5 insertional mutant, lacking Type IV pili, was unable to initiate attachment. Addition of DNase caused rapid detachment of bound cells, but no detachment occurred when proteinase, RNase or inactivated DNase were used. Addition of MgCl₂ also caused significant detachment, supporting the possible mechanistic role of electrostatic interactions in the attachment process. Although attachment was apparent in early to mid-log phase growth, surprisingly DNA was not detected in the culture supernatant until late stationary phase and coincided with an appreciable loss of cell viability. This suggests that during log-phase growth attachment is mediated by eDNA that is released in low quantities and/or is highly localised within the extracellular matrix and also that stationary phase DNA release through widespread cell lysis may be a separate and unrelated event.
Extracellular DNA can play a structural role in the microbial environment. Here evidence is presented that an environmental isolate of
Acidovorax
temperans
utilises extracellular DNA for ...intercellular and cell-surface attachment and that Type IV pili and electrostatic interactions play a role in this interaction. Preliminary attempts to isolate and purify extracellular polysaccharides from
A. temperans
strain CB2 yielded significant amounts of DNA raising the question of whether this molecule was present as a structural component in the extracellular matrix. The role of DNA in attachment was indicated by experiments in which the addition of DNase to liquid medium inhibited the attachment of
Acidovorax
to glass wool. A Tn
5
insertional mutant, lacking Type IV pili, was unable to initiate attachment. Addition of DNase caused rapid detachment of bound cells, but no detachment occurred when proteinase, RNase or inactivated DNase were used. Addition of MgCl
2
also caused significant detachment, supporting the possible mechanistic role of electrostatic interactions in the attachment process. Although attachment was apparent in early to mid-log phase growth, surprisingly DNA was not detected in the culture supernatant until late stationary phase and coincided with an appreciable loss of cell viability. This suggests that during log-phase growth attachment is mediated by eDNA that is released in low quantities and/or is highly localised within the extracellular matrix and also that stationary phase DNA release through widespread cell lysis may be a separate and unrelated event.
All modern wastewater treatment systems rely on microbial processes for water clarification, with some 70% of systems utilising some form of the activated sludge process. This thesis describes an ...investigation into the extracellular polymers of a consistent member of the activated sludge microbial community: Acidovorax temperans. Several types of polymers are present on the cellular surface of A. temperans which were hypothesised to play a role in cellular and cell-surface interactions. The two morphotypes of A. temperans (Hpos and Hneg) were proposed to enable the study of these extracellular polymers in wastewater treatment related processes, such as biofilm formation and flocculation. The initial aim of this research was to chemically characterise and compare the exopolysaccharides produced by the two morphotypes. The results are described in Chapter Three; it was found the Hpos strain consistently produced exopolysaccharides in greater abundance, although exopolysaccharides from both strains shared a similar monosaccharide composition, which in turn, was nutrient dependent. Due to subsequent findings of large amounts of extracellular DNA in EPS fractions, it was further hypothesised that extracellular DNA is utilised as a structural element in biofilm formation by A. temperans. This is described in Chapter Four; enzymatic breakdown of extracellular DNA resulted in impaired biofilm formation. In addition, the presence of Type IV pili was also essential for cellular attachment to surfaces, while the underlying binding mechanism was concluded to be electrostatic. These results add further to the recently emerging notion that extracellular DNA could function as a structural molecule outside the cell. To date, this is the first reported study on an extracellular DNA and Type IV pili combined attachment mechanism. In addition, this thesis describes the most detailed study to date on exopolysaccharides from a member of the Comamonadaceae. An obvious function for Acidovorax in the activated sludge environment was not indicated by these studies on the presence, function and composition of extracellular polymers. However the discovery that eDNA and Type IV pili mediate cell-cell and cell-surface attachment in this organism provides a fertile starting point for investigating the contribution of these interactions to initiation and promotion of flocculation in activated sludge systems.
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