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Fundamentals of Photosynthetic Microbial Fuel Cell
Published in Lakhveer Singh, Durga Madhab Mahapatra, Waste to Sustainable Energy, 2019
Nanowires are extensions of the outer membrane containing the c-type cytochrome MtrC, which is involved in Fe(III) and Mn(IV) and the outer membrane cytochrome (OmcA) to make physical connections with neighboring cells (Shi et al. 2016). Such nanostructures are composed by the outer membrane Mtr proteins MtrC and OmcA in the entire surface. This observation agrees with the anteriorly proposed multi-step redox hopping mechanism of electron transport along the nanostructure extension (McCormick et al. 2015). Although many microorganisms can donate electrons to the anode, only a few iron reducers, such as Geobacter sulfurreducens and Shewanella oneidensis MR-1 have been reported to generate nanowires than can be used as mediators for direct electron transport the electrode (Eaktasang et al. 2016). The conductive pili of G. sulfurreducens PCA are assemblies of the pilin protein PilA (Cologgi et al. 2011). The conductivity of Geobacter spp. nanowires increases when temperature or pH decreases. This type of temperature-dependent and pH-dependent conductivity is a property that is shared with some conductive polymers where Geobacter spp. nanowires are proposed to transfer electrons by a metallic-like electron transfer mechanism (Shi et al. 2016). Despite outer membrane c-type cytochromes not being critical for conductivity along the pilus, they are probably intricated in some electron transport steps. For instance, the hexa-heme OmcS seem to be involved in the electron transport from nanowires to Fe(III) oxides, and could allow electron flow between nanofilaments (McCormick et al. 2015).
Application of AFM in Microbial Energy Systems
Published in Cai Shen, Atomic Force Microscopy for Energy Research, 2022
First, the conducting probe AFM played a critical role in the extracellular electron transfer mechanism of microorganisms. A breakthrough in the mechanism of microbial direct electron transfer was reported by Reguera et al. in 2005; the pili of G. sulfurreducens were imaged in the contact mode of AFM, and its correspondence between the current and applied voltage was measured by conducting-probe AFM, indicating that pili of G. sulfurreducens might serve as nanowires to transfer electrons extracellularly [19]. Geobacter nanowires are presumed to be conductive as a result of the amino acid sequence of the type IV pilin subunit PilA and, possibly, the tertiary structure of the assembled pilus. Since then, the conductive mechanisms of nanowires have attracted much interest and debate. However, electrical measurements of conducting probe AFM gave current-voltage (I-V) curves as important evidence. For example, Figure 10.8 shows the electrical transport along a bacterial nanowire of S. oneidensis MR-1, electron transport rates up to 109/s at 100 mV of applied bias, and a measured resistivity on the order of 1 Ω·cm [20]. In addition, the ΔmtrC/omcA mutants produced appendages morphologically consistent with wild-type nanowires but that were found to be nonconductive. Otherwise, conducting probe AFM was also used to investigate the electronic transport characteristics of S. oneidensis MR-1 nanowires, and the nanowires exhibit p-type, tunable electronic behavior with a field-effect mobility on the order of 10−1 cm2/(V s), comparable to devices based on synthetic organic semiconductors [21].
Cell Aggregation and Sedimentation
Published in Martin A. Hjortso, Joseph W. Roos, Cell Adhesion, 2018
To get around the problem of small settling velocities of recombinant bacterial cells, Henry et al. (102) took advantage of the genetic control of flocculation. They used a host strain which was Pil− (lacking the gene for pilin protein) and inserted a plasmid, pORN108, which contains the pil operon (10). As a result, the plasmid-bearing cells synthesize type 1 pili, which leads to flocculation, whereas the plasmid-free segregant cells do not synthesize pili and are nonflocculent. They then used this strain in the reactor/settler system shown in Figure 19. When no recycle was used, the plasmid-bearing (+) cells were overtaken by plasmid-free (–) cells in only two days (see Fig. 21). This was due to the relatively high segregation probability (p = 0.03), which followed from the low average copy number of 3.8 plasmids per cell, and to the large growth rate differential (μm+ = 0.72/h vs. μm− = 1.14/h), which followed from the significant metabolic burden associated with the overproduction of pili. In contrast, the plasmid-bearing strain was maintained as dominant when the inclined settler was used (see Fig. 21). The inclined settler was designed so that most of the flocculent cells had time to settle out of suspension and be recycled to the bioreactor (γ+ = 0.45), while the nonflocculent cells did not settle and were removed through the settler effluent (γ− = 1.0).
Effect of photocatalysis (TiO2/UVA) on the inactivation and inhibition of Pseudomonas aeruginosa virulence factors expression
Published in Environmental Technology, 2021
Faouzi Achouri, Myriam Ben Said, Mohamed Ali Wahab, Latifa Bousselmi, Serge Corbel, Raphaël Schneider, Ahmed Ghrabi
Mobility linked to environmental variations is a major phenomenon in the colonization and infection processes of P. aeruginosa. Accordingly, we have investigated the effect of photocatalytic treatment on the three mobility types of P. aeruginosa known as swimming, twitching and swarming. As shown in Figure 4, the bacterium lost their mobility for both types twitching and swarming after 60 min of photocatalytic treatment with a progressive decrease in the swimming motility up to 60 min of treatment. Therefore, the photocatalytic treatment affected the cellular appendages, flagella and pili of the bacterial cells reducing their abilities to ‘probe’ the surfaces and to move in the middle. In fact, the polar flagellum gives the bacterium the ability to move not only in an aqueous medium, but also on semi-solid surfaces (swarming) and an affection of the activity of the flagellum under photocatalytic treatment led to a reduction of bacteria ability to colonize and spread on surfaces [41]. In addition, mobility also involves type IV pili. Consisting mainly of pilin monomers (PilA), their retraction capacity allows twitching and swarming mobility, permitting dispersion of bacteria on wet surfaces [42]. They are also known to play a crucial role in mucosal adhesion and colonization [43]. Thus any alteration of these pili would lead to a decrease in the adhesion to epithelial cells in vitro and as consequence reduce their ability to infect cells [44]. Furthermore, the retention of motility cells even after 60 min of photocatalytic treatment was due to cells that have lost their cultivability but still viable. Indeed, during severe environmental conditions, microorganisms develop various resistance strategies to maximize the use of resources while maintaining structural and genetic integrity and increasing tolerance to harmful conditions [45]. However, a particular survival strategy in bacteria is the ability to enter a viable but non-cultivable state (VBNC) which allows the resistance to adverse environmental conditions [46,47]. In this regard, the VBNC state is similar to dormancy when cells retain an intact membrane and an undamaged genetic material. However, in contrast to dormant cells, VBNC cells are by definition not culturable on routine laboratory media [48].