Bulgarian Academy of Sciences

About: Bulgarian Academy of Sciences is a government organization based out in Sofia, Bulgaria. It is known for research contribution in the topics: Catalysis & Coupling constant. The organization has 17989 authors who have published 36276 publications receiving 642820 citations. The organization is also known as: Bulgarian Academy of Sciences，簡稱：BAS & Balgarska Akademiya na Naukite.

Lipopolysaccharide O antigen status of Yersinia enterocolitica O:8 is essential for virulence and absence of O antigen affects the expression of other Yersinia virulence factors

Abstract: Summary Lipopolysaccharide (LPS) is the major component of the outer membrane of Gram-negative bacteria. Although much attention has been given to the biological effects of its lipid A portion, a great body of evidence indicates that its O chain polysaccharide (O antigen) portion plays an important role in the bacterium‐host interplay. In this work we have studied in-depth the role of the O antigen in Yersinia enterocolitica serotype O:8 pathogenesis. We made a detailed virulence analysis of three mutants having different O antigen phenotypes: (i) LPS with no O antigen (rough mutant); (ii) LPS with one O unit (semirough mutant) and (iii) LPS with random distribution of O antigen chain lengths. We demonstrated that these LPS O antigen mutants were attenuated in virulence regardless of the infection route used. Coinfection experiments revealed that the rough and semirough mutants were severely impaired in their ability to colonize the Peyer’s patches and in contrast to the wild-type strain they did not colonize spleen and liver. The mutant with random distribution of O antigen chain lengths, however, survived better but started to be cleared from mouse organs after 8 days. As an explanation to this attenuation we present here evidence that other Yersinia virulence factors depend on the presence of O antigen for their proper function and/or expression. We demonstrated that in the rough mutant: (i) the YadA function but not its expression was altered; (ii) Ail was not expressed and (iii) inv expression was downregulated. On the other hand, expression of flhDC , the flagellar master regulatory operon, was upregulated in this mutant with a concomitant increase in the production of flagellins. Finally, expression of yplA , encoding for the Yersinia phospholipase A, was also upregulated accompanied by an increased flagellar type III secretion system mediated secretion of YplA to culture medium. Together these findings suggest that the absence of O antigen in the outer membrane of Yersinia either directly or indirectly, for example through a cellular or membrane stress, could act as a regulatory signal.

A comparative study of nanosized IB/ceria catalysts for low-temperature water-gas shift reaction

Abstract: The water-gas shift (WGS) activity of different M/ceria catalysts, where M is a metal from IB group (i.e. Au, Ag, Cu) has been studied. Two different techniques were used for the preparation of the catalysts—deposition–precipitation and a modified version of deposition–precipitation. HRTEM combined with EDS, X-ray diffraction, FT-IR of the molecules involved in the WGS reaction at 90 and 300 K as well as of forward and reverse WGS reaction up to 573 K, XPS and H 2 -TPR were employed to obtain information about the effect of different synthesis procedures on the surface and on the bulk structure of the catalysts and to explain the differences observed in the catalytic activity. The influence of each IB metal on the WGS activity has been investigated, too. The experimental results revealed that deposition–precipitation is the preferred method for preparation of active ceria-based catalysts because it allowed most part of IB metal promoters to be exposed at the catalyst surface. WGS reaction proceeds at the boundary between nanosized metallic particles and ceria, where CO adsorption on IB metals and H 2 O dissociation on oxygen-vacancy defects of ceria take place. The role of formate species as intermediates in the WGS reaction is confirmed by FT-IR spectroscopy. The correlation between the stability of formates and the WGS activity suggests that the decomposition of formates is a rate-limiting step.

Effect of synthesis procedure on the low-temperature WGS activity of Au/ceria catalysts

Abstract: Gold/ceria catalysts were prepared by two different methods (deposition-precipitation and modified deposition-precipitation). Considerable differences in the low-temperature water-gas shift (WGS) activity of both catalysts were observed. The preparation technique strongly influenced catalytic activity due to the large differences in gold particles size and to the availability of active gold sites in close contact with ceria defects on the surface. HRTEM and EDS have shown presence of very highly dispersed gold clusters ( d about 1 nm) on the surface of the catalyst prepared by deposition-precipitation. Large gold particles (average d ≈15 nm) have been found in gold/ceria catalyst prepared by modified deposition-precipitation. FTIR spectroscopy of adsorbed CO and X-ray photoelectron spectroscopy were employed to obtain information about the surface structure of both catalysts and to explain the differences in catalytic activity.