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Biofuel Cells Select for Microbial Consortia That Self-Mediate Electron Transfer. Korneel Rabaey, 2004.Microbial fuel cells hold great promise as a sustainable biotechnological solution to future energy needs . Current efforts to improve the efficiency of such fuel cells are limited by the lack of knowledge about the microbial ecology of these systems . The purposes of this study were (i) to elucidate whether a bacterial community, either suspended or attached to an electrode, can evolve in a microbial fuel cell to bring about higher power output, and (ii) to identify species responsible for the electricity generation . Enrichment by repeated transfer of a bacterial consortium harvested from the anode compartment of a biofuel cell in which glucose was used increased the output from an initial level of 0.6 W m–2 of electrode surface to a maximal level of 4.31 W m–2 (664 mV, 30.9 mA) when plain graphite electrodes were used . This result was obtained with an average loading rate of 1 g of glucose liter–1 day–1 and corresponded to 81% efficiency for electron transfer from glucose to electricity . Cyclic voltammetry indicated that the enhanced microbial consortium had either membrane-bound or excreted redox components that were not initially detected in the community . Dominant species of the enhanced culture were identified by denaturing gradient gel electrophoresis and culturing . The community consisted mainly of facultative anaerobic bacteria, such as Alcaligenes faecalis and Enterococcus gallinarum, which are capable of hydrogen production . Pseudomonas aeruginosa and other Pseudomonas species were also isolated . For several isolates, electrochemical activity was mainly due to excreted redox mediators, and one of these mediators, pyocyanin produced by P . aeruginosa, could be characterized . Overall, the enrichment procedure, irrespective of whether only attached or suspended bacteria were examined, selected for organisms capable of mediating the electron transfer either by direct bacterial transfer or by excretion of redox components . Two Novel Type III-Secreted Proteins of Xanthomonas campestris pv . vesicatoria Are Encoded within the hrp Pathogenicity Island. Laurent Noël, 2002.The Hrp type III protein secretion system (TTSS) is essential for pathogenicity of gram-negative plant pathogen Xanthomonas campestris pv . vesicatoria . cDNA-amplified fragment length polymorphism and reverse transcription-PCR analyses identified new genes, regulated by key hrp regulator HrpG, in the regions flanking the hrp gene cluster . Sequence analysis revealed genes encoding HpaG, a predicted leucine-rich repeat-containing protein, the lysozyme-like HpaH protein, and XopA and XopD, which are similar in sequence to Hpa1 from Xanthomonas oryzae pv . oryzae and PsvA from Pseudomonas syringae, respectively . XopA and XopD (Xanthomonas outer proteins) are secreted by the Xanthomonas Hrp TTSS and thus represent putative effector proteins . Mutations in xopA, but not in xopD, resulted in reduced bacterial growth in planta and delayed plant reactions in susceptible and resistant host plants . Since the xopD promoter contains a putative hrp box, which is characteristic of hrpL-regulated genes in P . syringae and Erwinia spp., the gene was probably acquired by horizontal gene transfer . Interestingly, the regions flanking the hrp gene cluster also contain insertion sequences and genes for a putative transposase and a tRNAArg . These features suggest that the hrp gene cluster of X . campestris pv . vesicatoria is part of a pathogenicity island . Chimeric Analysis of AcrA Function Reveals the Importance of Its C-Terminal Domain in Its Interaction with the AcrB Multidrug Efflux Pump. Christopher A. Elkins, 2003.AcrAB-TolC is the major, constitutively expressed efflux protein complex that provides resistance to a variety of antimicrobial agents in Escherichia coli . Previous studies showed that AcrA, a periplasmic protein of the membrane fusion protein family, could function with at least two other resistance-nodulation-division family pumps, AcrD and AcrF, in addition to its cognate partner, AcrB . We found that, among other E . coli resistance-nodulation-division pumps, YhiV, but not MdtB or MdtC, could also function with AcrA . When AcrB was assessed for the capacity to function with AcrA homologs, only AcrE, but not YhiU or MdtA, could complement an AcrA deficiency . Since AcrA could, but YhiU could not, function with AcrB, we engineered a series of chimeric mutants of these proteins in order to determine the domain(s) of AcrA that is required for its support of AcrB function . The 290-residue N-terminal segment of the 398-residue protein AcrA could be replaced with a sequence coding for the corresponding region of YhiU, but replacement of the region between residues 290 and 357 produced a protein incapable of functioning with AcrB . In contrast, the replacement of residues 357 through 397 of AcrA still produced a functional protein . We conclude that a small region of AcrA close to, but not at, its C terminus is involved in the interaction with its cognate pump protein, AcrB .
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