Typhoidal Salmonella: Distinctive virulence factors and pathogenesis

Although nontyphoidal Salmonella (NTS; including Salmonella Typhimurium) mainly cause gastroenteritis, typhoidal serovars (Salmonella Typhi and Salmonella Paratyphi A) cause typhoid fever, the treatment of which is threatened by increasing drug resistance. Our understanding of S. Typhi infection in human remains poorly understood, likely due to the host restriction of typhoidal strains and the subsequent popularity of the S. Typhimurium mouse typhoid model. However, translating findings with S. Typhimurium across to S. Typhi has some limitations. Notably, S. Typhi has specific virulence factors, including typhoid toxin and Vi antigen, involved in symptom development and immune evasion, respectively. In addition to unique virulence factors, both typhoidal and NTS rely on two pathogenicity‐island encoded type III secretion systems (T3SS), the SPI‐1 and SPI‐2 T3SS, for invasion and intracellular replication. Marked differences have been observed in terms of T3SS regulation in response to bile, oxygen, and fever‐like temperatures. Moreover, approximately half of effectors found in S. Typhimurium are either absent or pseudogenes in S. Typhi, with most of the remaining exhibiting sequence variation. Typhoidal‐specific T3SS effectors have also been described. This review discusses what is known about the pathogenesis of typhoidal Salmonella with emphasis on unique behaviours and key differences when compared with S. Typhimurium.     

Local mobility in lipid domains of supported bilayers characterized by atomic force microscopy and fluorescence correlation spectroscopy

Fluorescence correlation spectroscopy (FCS) is used to examine mobility of labeled probes at specific sites in supported bilayers consisting of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) lipid domains in 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC). Those sites are mapped beforehand with simultaneous atomic force microscopy and submicron confocal fluorescence imaging, allowing characterization of probe partitioning between gel DPPC and disordered liquid DOPC domains with corresponding topography of domain structure. We thus examine the relative partitioning and mobility in gel and disordered liquid phases for headgroup- and tailgroup-labeled GM1 ganglioside probes and for headgroup- and tailgroup-labeled phospholipid probes. For the GM1 probes, large differences in mobility between fluid and gel domains are observed; whereas unexpected mobility is observed in submicron gel domains for the phospholipid probes. We attribute the latter to domain heterogeneities that could be induced by the probe. Furthermore, fits to the FCS data for the phospholipid probes in the DOPC fluid phase require two components (fast and slow). Although proximity to the glass substrate may be a factor, local distortion of the probe by the fluorophore could also be important. Overall, we observe nonideal aspects of phospholipid probe mobility and partitioning that may not be restricted to supported bilayers.     

Temporal Shedding Patterns and Virulence Factors of Escherichia coli Serogroups O26, O103, O111, O145, and O157 in a Cohort of Beef Calves and Their Dams

This study investigated the shedding of Escherichia coli O26, O103, O111, O145, and O157 in a cohort of beef calves from birth over a 5-month period and assessed the relationship between shedding in calves and shedding in their dams, the relationship between shedding and scouring in calves, and the effect of housing on shedding in calves. Fecal samples were tested by immunomagnetic separation and by PCR and DNA hybridization assays. E. coli O26 was shed by 94% of calves. Over 90% of E. coli O26 isolates carried the vtx₁, eae, and ehl genes, 6.5% carried vtx₁ and vtx₂, and one isolate carried vtx₂ only. Serogroup O26 isolates comprised seven pulsed-field gel electrophoresis (PFGE) patterns but were dominated by one pattern which represented 85.7% of isolates. E. coli O103 was shed by 51% of calves. Forty-eight percent of E. coli O103 isolates carried eae and ehl, 2% carried vtx₂, and none carried vtx₁. Serogroup O103 isolates comprised 10 PFGE patterns and were dominated by two patterns representing 62.5% of isolates. Shedding of E. coli O145 and O157 was rare. All serogroup O145 isolates carried eae, but none carried vtx₁ or vtx₂. All but one serogroup O157 isolate carried vtx₂, eae, and ehl. E. coli O111 was not detected. In most calves, the temporal pattern of E. coli O26 and O103 shedding was random. E. coli O26 was detected in three times as many samples as E. coli O103, and the rate at which calves began shedding E. coli O26 for the first time was five times greater than that for E. coli O103. For E. coli O26, O103, and O157, there was no association between shedding by calves and shedding by dams within 1 week of birth. For E. coli O26 and O103, there was no association between shedding and scouring, and there was no significant change in shedding following housing.     

Automated robotic harvesting of protein crystals—addressing a critical bottleneck or instrumentation overkill?

One of the critical steps in high throughput crystallography that so far has evaded automation is the actual harvesting of the delicate crystals from the mother liquor in which they are growing. The late-stage operation of harvesting is presently a most risky and loss-intensive procedure, compounded by its tight integration with the critical steps of cryo-protection and cryo-quenching. Recent advances in micromanipulation robotics and micro-fabrication have made it possible to seriously consider automation of protein crystal harvesting. Based on the experience gained during the development of an operator-assisted (and now operator-assisting) universal micromanipulation robot (UMR) prototype, we discuss the challenges ahead for the design of a fully autonomous, integrated system capable of the reliable harvesting of protein microcrystals. Experience from participation in NIH structural genomics projects and feedback from bottleneck workshops indicates that genuine demand exists in the high throughput community as well as in pharmaceutical production pipelines, justifying the effort and resources to develop autonomous harvesting robotics.     

Binding of intimin from enteropathogenic Escherichia coli to Tir and to host cells

Enteropathogenic Escherichia coli (EPEC) induce characteristic attaching and effacing (A/E) lesions on epithelial cells. This event is mediated, in part, by binding of the bacterial outer membrane protein, intimin, to a second EPEC protein, Tir (translocated intimin receptor), which is exported by the bacteria and integrated into the host cell plasma membrane. In this study, we have localized the intimin-binding domain of Tir to a central 107-amino-acid region, designated Tir-M. We provide evidence that both the amino- and carboxy-termini of Tir are located within the host cell. In addition, using immunogold labelling electron microscopy, we have confirmed that intimin can bind independently to host cells even in the absence of Tir. This Tir-independent interaction and the ability of EPEC to induce A/E lesions requires an intact lectin-like module residing at the carboxy-terminus of the intimin polypeptide. Using the yeast two-hybrid system and gel overlays, we show that intimin can bind both Tir and Tir-M even when the lectin-like domain is disrupted. These data provide strong evidence that intimin interacts not only with Tir but also in a lectin-like manner with a host cell intimin receptor.     

A rostrocaudal muscular dystrophy caused by a defect in choline kinase beta, the first enzyme in phosphatidylcholine biosynthesis

Muscular dystrophies include a diverse group of genetically heterogeneous disorders that together affect 1 in 2000 births worldwide. The diseases are characterized by progressive muscle weakness and wasting that lead to severe disability and often premature death. Rostrocaudal muscular dystrophy (rmd) is a new recessive mouse mutation that causes a rapidly progressive muscular dystrophy and a neonatal forelimb bone deformity. The rmd mutation is a 1.6-kb intragenic deletion within the choline kinase beta (Chkb) gene, resulting in a complete loss of CHKB protein and enzymatic activity. CHKB is one of two mammalian choline kinase (CHK) enzymes (alpha and beta) that catalyze the phosphorylation of choline to phosphocholine in the biosynthesis of the major membrane phospholipid phosphatidylcholine. While mutant rmd mice show a dramatic decrease of CHK activity in all tissues, the dystrophy is only evident in skeletal muscle tissues in an unusual rostral-to-caudal gradient. Minor membrane disruption similar to dysferlinopathies suggest that membrane fusion defects may underlie this dystrophy, because severe membrane disruptions are not evident as determined by creatine kinase levels, Evans Blue infiltration, and unaltered levels of proteins in the dystrophin-glycoprotein complex. The rmd mutant mouse offers the first demonstration of a defect in a phospholipid biosynthetic enzyme causing muscular dystrophy, representing a unique model for understanding mechanisms of muscle degeneration.     

Redox reactions associated with iron release from mammalian ferritin

The early redox events involved in iron reduction and mobilization in mammalian ferritin have been investigated by several techniques. Sedimentation velocity measurements of ferritin samples with altered core sizes, prepared by partial reduction and Fe2+ chelation, suggest two different events occur during iron loss from the ferritin core. Reductive optical titrations confirm this biphasic behavior by showing that the first 20-30% of core reduction has different optical properties than the latter 70-80%. Proton uptake during initial core reduction is near zero, but as the percent core reduction increases, the proton uptake (H+/e) values increase to 2 H+/e (2 H+/Fe3+ reduced) as core reduction approaches 1 e/Fe3+. Coulometric reduction of ferritin by mediators of different redox potential and different cross-sectional areas show a two-phase sigmoidal reaction pattern in which initial core reduction occurs at a slower rate than later core reduction. The above experiments were all conducted in the absence of iron chelators so that the observed results were all attributed to core reduction rather than the combined effects of core reduction accompanied by Fe2+ chelation. The coulometric reduction of ferritin by various mediators shows a correlation more with reduction potential than with molecular cross-sectional area. The role of the ferritin channels in core reduction is considered in terms of the reported results.     

Role of arginine 180 and glutamic acid 177 of ricin toxin A chain in enzymatic inactivation of ribosomes.

The gene for ricin toxin A chain was modified by site-specific mutagenesis to change arginine 180 to alanine, glutamine, methionine, lysine, or histidine. Separately, glutamic acid 177 was changed to alanine and glutamic acid 208 was changed to aspartic acid. Both the wild-type and mutant proteins were expressed in Escherichia coli and, when soluble, purified and tested quantitatively for enzyme activity. A positive charge at position 180 was found necessary for solubility of the protein and for enzyme activity. Similarly, a negative charge with a proper geometry in the vicinity of position 177 was critical for ricin toxin A chain catalysis. When glutamic acid 177 was converted to alanine, nearby glutamic acid 208 could largely substitute for it. This observation provided valuable structural information concerning the nature of second-site mutations.