Identification of quorum-sensing signal molecules and the LuxRI homologs in fish pathogen Edwardsiella tarda

Edwardsiella tarda is a gram-negative bacterium that causes septicaemia in fish and serious damage to the aquaculture industry. The virulence factors of this pathogen and control mechanisms of the expression of virulence genes have not yet been clearly elucidated. A number of gram-negative pathogenic bacteria have a quorum-sensing system. These bacteria produce N-acyl-L-homoserine lactone (AHL) that they use them as a quorum-sensing signal molecule. In this study, we found that E. tarda isolated from deceased flounder produces AHLs. Thin layer chromatography analysis indicated that the two kinds of AHL produced by E. tarda seemed to be N-hexanoyl-L-homoserine lactone (C6-HSL) and N-heptanoyl-L-homoserine lactone (C7-HSL). We have cloned and sequenced the quorum-sensing genes, luxI homolog (edwI) and luxR homolog (edwR). EdwI and EdwR showed high identity with CarI/CarR and ExpI/ExpR from Erwinia carotovora, respectively. SDS-PAGE analysis of extracellular proteins revealed that the expression of the 55-kDa protein, which was reported as a virulent-strain-specific protein, is controlled by AHLs. These results suggest that some virulence factors are regulated by the quorum-sensing system in E. tarda.     

Development-inhibiting activity of some tropical plants against Sitophilus zeamais Motschulsky (Coleoptera: Curculionidae)

Thirteen tropical plants were evaluated for development-inhibiting activity against Sitophilus zeamais. The bioassays were carried out by incorporating seeds or leaves at various dose levels into an artificial diet for the test insect. It was found that seeds of Basella alba and leaves of Operculina turpethum and Calotropis gigantea were potent in delaying development and in reducing adult emergence, and hence the capacity for population increase. At 0.5% concentration, adult emergence in tests with B. alba, O. turpethum and C. gigantea was reduced by 62, 95 and 70%, respectively. In B. alba and C. gigantea, the development periods were 2.2 and 1.8 times those in the control and the capacities for increase/day were only 0.0324 and 0.0328 compared with 0.1004 in the control. B. alba, O. turpethum and C. gigantea were active at concentrations as low as 0.01, 0.05 and 0.1%. The potential of these materials in insect pest management is discussed.     

Oxygen reduction behavior of rutile-type iridium oxide in sulfuric acid solution

Two different forms of rutile-type iridium oxide catalysts were prepared: IrO₂-coated titanium plate electrocatalysts prepared by a dip-coating method (IrO₂/Ti) and iridium oxide nanoparticles (IrO₂) prepared by a wet method, the Adams fusion method. The catalytic behavior of the oxygen reduction reaction (ORR) was evaluated by cyclic voltammetry in 0.5 M H₂SO₄ at 60 °C. Both catalysts were found to exhibit considerable activity for the ORR; however, the former oxide electrodes showed higher activity than the latter ones. All the IrO₂/Ti catalyst electrodes heat-treated at a temperature between 400 °C and 550 °C showed ca. 0.84 V (vs. RHE) of the onset potential for the ORR, E_ORR, where the reduction current of oxygen had begun to be observed during the cathodic potential sweep of the test electrodes. It has been confirmed clearly that IrO₂, but neither metallic Ir nor the hydrated IrO₂, behaves as an active catalyst for the ORR in an acidic solution. It was also demonstrated that the enlargement of the surface area of the IrO₂/Ti with the help of lanthanum is effective for the enhancement of the catalytic activity in the reaction.     

Enhancement of electron field emission from carbon nanofiber bundles separately grown on Ni catalyst in Ni-Cr alloy

The enhancement of electron field emission was successfully obtained from carbon nanofiber (CNF) bundles separately grown on Ni grains in Ni-Cr alloy compared with that from CNF bundles on pure nickel. Ni grains less than 1 μm in size were separated by Cr grains in the alloy prepared by cosputtering with Ni pellets on a Cr plate without a lithographic patterning process. CNFs grew on Ni grains as bundles of 100 nm diameter and 1 μm length by helicon wave plasma-enhanced chemical vapor deposition, controlling the distance between adjacent CNF bundles by 500-1000 nm. The field emission characteristics of electrons from CNF bundles were evaluated and compared using current-voltage characteristic curves and a Fowler-Nordheim (F-N) plot converted using a work function value of 4.7 eV for CNFs. The field enhancement factor of FE characteristics obtained from the slopes in the F-N plot increased with the average distance defined as the space between adjacent CNF bundles. CNFs separately grown on a Ni-Cr catalyst are a candidate for effective two-dimensional field emitter devices.