Virulence phenotypes of Shigella flexneri 2a avirulent mutant 24570 can be complemented by the plasmid-coded positive regulator virF gene

Avirulent mutation of an opaque colony variant of Shigella flexneri 2a designated 24570 has been believed to be linked with the glpK locus of the chromosome. However, avirulent phenotypes of the 24570 strain could be complemented by the invasion plasmid-coded virF gene, a positive regulator for invasion genes. The 24570 strain had a DNA structural alteration upstream of the virF gene.     

Complete genome sequence of Shigella flexneri 5b and comparison with Shigella flexneri 2a

Background

On most common microarray platforms many genes are represented by multiple probes. Although this is quite common no one has systematically explored the concordance between probes mapped to the same gene.

Results

Here we present an analysis of all the cases of multiple probe sets measuring the same gene on the Affymetrix U133a GeneChip and found that although in the majority of cases both measurements tend to agree there are a significant number of cases in which the two measurements differ from each other. In these cases the measurements can not be simply averaged but rather should be handled individually.

Conclusion

Our analysis allows us to provide a comprehensive list of the correlation between all pairs of probe sets that are mapped to the same gene and thus allows microarray users to sort out the cases that deserve further analysis. Comparison between the set of highly correlated pairs and the set of pairs that tend to differ from each other reveals potential factors that may affect it.     

Structure and composition of the Shigella flexneri "needle complex", a part of its type III secreton

Type III secretion systems (TTSSs or secretons), essential virulence determinants of many Gram-negative bacteria, serve to translocate proteins directly from the bacteria into the host cytoplasm. Electron microscopy (EM) indicates that the TTSSs of Shigella flexneri are composed of: (1) an external needle; (2) a transmembrane domain; and (3) a cytoplasmic bulb. EM analysis of purified and negatively stained parts 1, 2 and a portion of 3 of the TTSS, together termed the "needle complex" (NC), produced an average image at 17 A resolution in which a base, an outer ring and a needle, inserted through the ring into the base, could be discerned. This analysis and cryoEM images of NCs indicated that the needle and base contain a central 2-3 nm canal. Five major NC components, MxiD, MxiG, MxiJ, MxiH and MxiI, were identified by N-terminal sequencing. MxiG and MxiJ are predicted to be inner membrane proteins and presumably form the base. MxiD is predicted to be an outer membrane protein and to form the outer ring. MxiH and MxiI are small hydrophilic proteins. Mutants lacking either of these proteins formed needleless secretons and were unable to secrete Ipa proteins. As MxiH was present in NCs in large molar excess, we propose that it is the major needle component. MxiI may cap at the external needle tip.     

外环境压力对福氏志贺菌荚膜异多糖酸合成调节子转录的影响

荚膜异多糖酸合成调节子(regulator of colanic acid capsule synthesis,Rcs)对大肠埃希菌适应外环境压力具有重要调控功能,但其在志贺菌中的功能尚未见报道。为探索外环境压力对福氏志贺菌Rcs编码基因rcs转录水平的影响,本研究采用核苷酸序列比对及蛋白结构域预测等生物信息学方法分析福氏志贺菌的Rcs编码基因簇rcsBDC,利用实时定量聚合酶链反应(quantitative real-time polymerase chain reaction,qRT-PCR),对该菌不同生长时期的rcsB、rcsD、rcsC基因转录水平进行分析,并检测在不同pH值培养基、渗透压条件下的基因转录水平。结果显示,福氏志贺菌rcsBDC在培养5～6 h(对数中期)时转录水平较高,8～10 h(稳定期)时转录水平较低(P<0.001);10 h时,rcsB和rcsD在酸性、渗透压条件下的转录水平均显著高于正常条件下的转录水平(P<0.05)。结果提示,外环境刺激可提高福氏志贺菌在稳定期的rcsB和rcsD转录水平,为志贺菌适应胃肠道酸性、渗透压环境的机制研究提供了一定理论基础。     

Structural characterization of the type-III pilot-secretin complex from Shigella flexneri

Assembly of the type-III secretion apparatus, which translocates proteins through both membranes of Gram-negative bacterial pathogens into host cells, requires the formation of an integral outer-membrane secretin ring. Typically, a small lipidated pilot protein is necessary for the stabilization and localization of this ring. Using NMR spectroscopy, we demonstrate that the C-terminal residues 553-570 of the Shigella flexneri secretin MxiD encompass the minimal binding domain for its cognate pilot MxiM. Although unstructured in isolation, upon complex formation with MxiM, these residues fold into an amphipathic turn-helix motif that caps the elongated hydrophobic cavity of the cracked beta-barrel pilot. Along with a rearrangement of core aromatic residues, this prevents the binding of lipids within the cavity. The mutually exclusive association of lipids and MxiD with MxiM establishes a framework for understanding the role of a pilot in the outer-membrane insertion and multimerization of the secretin ring.     

Acid tolerance of Shigella sonnei and Shigella flexneri

AIMS: Determination of the behaviour of Shigella sonnei and Sh. flexneri under acid conditions. METHODS AND RESULTS: The growth and survival of Shigella spp. (9 isolates) in acidified Brain Heart Infusion (BHI) (pH 5.0-3.25 with pH intervals of 0.25) was determined after 6, 24 and 30 h incubation at 37 degrees C. Subsequently, survival of shigellae was studied in apple juice and tomato juice stored at 7 degrees C and 22 degrees C for up to 14 days and in strawberries and a fresh fruit salad, kept at 4 degrees C for 4 and 48 h. CONCLUSIONS: The minimum pH for growth in acidified BHI for Sh. flexneri and Sh. sonnei was, respectively, pH 4.75 and pH 4.50. Survival in fruit juices and fresh fruits depended upon their pH, the type of strain and the incubation temperature. Shigella spp. Survived for up to 14 days in tomato juice and apple juice stored at 7 degrees C. The shortest survival time (2-8 d) was observed in apple juice at 22 degrees C. Sh. sonnei but not Sh. flexneri was recovered after 48 h from strawberries and fruit salad kept at 4 degrees C. SIGNIFICANCE AND IMPACT OF THE STUDY: Acid foods, especially if kept at refrigeration temperatures, support survival of Shigella spp. and may cause Shigella food poisoning.