Bacterial adaptation to host innate immunity responses

Late repression of NF-κB activity by invasive but not non-invasive meningococcal isolates is required to display apoptosis of epithelial cells

Meningococcal invasive isolates of the ST-11 clonal complex are most frequently associated with disease and rarely found in carriers. Unlike carriage isolates, invasive isolates induce apoptosis in epithelial cells through the TNF-α signaling pathway. While invasive and non-invasive isolates are both able to trigger the TLR4/MyD88 pathway in lipooligosaccharide (LOS)-dependant manner, we show that only non-invasive isolates were able to induce sustained NF-κB activity in infected epithelial cells. ST-11 invasive isolates initially triggered a strong NF-κB activity in infected epithelial cells that was abolished after 9 h of infection and was associated with sustained activation of JNK, increased levels of membrane TNFR1, and induction of apoptosis. In contrast, infection with carriage isolates lead to prolonged activation of NF-κB that was associated with a transient activation of JNK increased TACE/ADAM17-mediated shedding of TNFR1 and protection against apoptosis. Our data provide insights to understand the meningococcal duality between invasiveness and asymptomatic carriage.

Inflammation and oxidative stress in vertebrate host-parasite systems

Innate, inflammation-based immunity is the first line of vertebrate defence against micro-organisms. Inflammation relies on a number of cellular and molecular effectors that can strike invading pathogens very shortly after the encounter between inflammatory cells and the intruder, but in a non-specific way. Owing to this non-specific response, inflammation can generate substantial costs for the host if the inflammatory response, and the associated oxygen-based damage, get out of control. This imposes strong selection pressure that acts to optimize two key features of the inflammatory response: the timing of activation and resolution (the process of downregulation of the response). In this paper, we review the benefits and costs of inflammation-driven immunity. Our aim is to emphasize the importance of resolution of inflammation as a way of maintaining homeostasis against oxidative stress and to prevent the 'horror autotoxicus' of chronic inflammation. Nevertheless, host immune regulation also opens the way to pathogens to subvert host defences. Therefore, quantifying inflammatory costs requires assessing (i) short-term negative effects, (ii) delayed inflammation-driven diseases, and (iii) parasitic strategies to subvert inflammation.

Mechanisms of pathogenesis and the evolution of parasite virulence

When studying how much a parasite harms its host, evolutionary biologists turn to the evolutionary theory of virulence. That theory has been successful in predicting how parasite virulence evolves in response to changes in epidemiological conditions of parasite transmission or to perturbations induced by drug treatments. The evolutionary theory of virulence is, however, nearly silent about the expected differences in virulence between different species of parasite. Why, for example, is anthrax so virulent, whereas closely related bacterial species cause little harm? The evolutionary theory might address such comparisons by analysing differences in tradeoffs between parasite fitness components: transmission as a measure of parasite fecundity, clearance as a measure of parasite lifespan and virulence as another measure that delimits parasite survival within a host. However, even crude quantitative estimates of such tradeoffs remain beyond reach in all but the most controlled of experimental conditions. Here, we argue that the great recent advances in the molecular study of pathogenesis provide a way forward. In light of those mechanistic studies, we analyse the relative sensitivity of tradeoffs between components of parasite fitness. We argue that pathogenic mechanisms that manipulate host immunity or escape from host defences have particularly high sensitivity to parasite fitness and thus dominate as causes of parasite virulence. The high sensitivity of immunomodulation and immune escape arise because those mechanisms affect parasite survival within the host, the most sensitive of fitness components. In our view, relating the sensitivity of pathogenic mechanisms to fitness components will provide a way to build a much richer and more general theory of parasite virulence.

Infectious response to E. coli: molecular and genetic pathways

Urinary tract infections are most commonly caused by type 1-piliated Escherichia coli (UPEC) and result in urothelial apoptosis, local cytokine release and neutrophil infiltration. A human urothelial cell line was incubated with various E. coli isolates and was then characterized by flow cytometry. UPEC induced rapid urothelial apoptosis that was dependent upon interactions mediated by type 1 pili. Laboratory isolates expressing type 1 pili-induced approximately 50% less apoptosis. UPEC blocked activity of a NF-kappaB-dependent reporter in response to inflammatory stimuli by stabilizing IkappaBalpha and UPEC rapidly altered cellular signalling pathways. Finally, blocking NF-kappaB activity increased the level of the laboratory strain-induced apoptosis to the level of apoptosis induced by UPEC. These results suggest that UPEC blocks NF-kappaB and enhances type 1 pili-induced apoptosis as a component of the uropathogenic programme.

Towards proteome database of Francisella tularensis

The accessibility of the partial genome sequence of Francisella tularensis strain Schu 4 was the starting point for a comprehensive proteome analysis of the intracellular pathogen F. tularensis. The main goal of this study is identification of protein candidates of value for the development of diagnostics, therapeutics and vaccines. In this review, the current status of 2-DE F. tularensis database building, approaches used for identification of biologically important subsets of F. tularensis proteins, and functional and topological assignments of identified proteins using various prediction programs and database homology searches are presented.

Reduced activation of inflammatory responses in host cells by mouse-adapted Helicobacter pylory isolates

Helicobacter pylori strains that harbour the Cag pathogenicity island (Cag PAI) induce interleukin (IL)-8 secretion in gastric epithelial cells, via the activation of NF- kappa B, and are associated with severe inflammation in humans. To investigate the influence of Cag PAI-mediated inflammatory responses on H. pylori adaptation to mice, a selection of H. pylori clinical isolates (n = 12) was cag PAI genotyped and tested in co-culture assays with AGS gastric epithelial cells, and in mouse colonization studies. Six isolates were shown to harbour a complete cag PAI and to induce NF- kappa B activation and IL-8 secretion in AGS cells. Of the eight isolates that spontaneously colonized mice, six had a cag PAI(-) genotype and did not induce pro-inflammatory responses in these cells. Mouse-to-mouse passage of the two cag PAI(+) -colonizing strains yielded host-adapted variants that infected mice with bacterial loads 100-fold higher than those of the respective parental strains (P= 0.001). These mouse-adapted variants were affected in their capacity to induce pro-inflammatory responses in host cells, yet no changes in cag PAI gene content were detected between the strains by DNA microarray analysis. This work provides evidence for in vivo selection of H. pylori bacteria with a reduced capacity to induce inflammatory responses and suggests that such bacteria are better adapted to colonize mice.

Uropathogenic Escherichia coli potentiates type 1 pilus-induced apoptosis by suppressing NF-kappaB

Urinary tract infections (UTIs) are among the most common inflammatory diseases. Acute UTIs are typically caused by type 1-piliated Escherichia coli and result in urothelial apoptosis, local cytokine release, and neutrophil infiltration. To examine the urothelial apoptotic response, a human urothelial cell line was incubated with various E. coli isolates and was then characterized by flow cytometry. Uropathogenic E. coli (UPEC) induced rapid urothelial apoptosis that was strictly dependent upon interactions mediated by type 1 pili. Interestingly, nonpathogenic HB101 E. coli expressing type 1 pili induced apoptosis at approximately 50% of the level induced by UPEC, suggesting that pathogenic strains contribute to apoptosis by pilus-independent mechanisms. Consistent with this possibility, UPEC blocked activity of an NF-kappaB-dependent reporter in response to inflammatory stimuli, yet this effect was independent of functional type 1 pili and was not mediated by laboratory strains of E. coli. UPEC suppressed NF-kappaB by stabilizing IkappaBalpha, and UPEC rapidly altered cellular signaling pathways. Finally, blocking NF-kappaB activity increased the level of piliated HB101-induced apoptosis to the level of apoptosis induced by UPEC. These results suggest that UPEC blocks NF-kappaB and thereby enhances type 1 pili-induced apoptosis as a component of the uropathogenic program.

Molecular secrets of bacterial type III effector proteins

Most Gram-negative phytopathogenic bacteria are thought to inject effector proteins into the plant cell via a type III secretion system that is essential for pathogenicity. Plant targets and the mode of action of type III effector proteins, which include avirulence (Avr) proteins, are largely unknown. However, recent findings have shed light on the molecular mechanisms of Avr action. Here, we focus on two classes of Avr proteins (the AvrBs3 and AvrRxv/YopJ families) that have been suggested to act as transcription factors and proteases, respectively.

Intestinal microflora in human and experimental inflammatory bowel disease

Commensal luminal bacteria stimulate protective or tolerogenic mucosal immune responses in normal (ie, resistant) hosts and detrimental responses, which result in chronic intestinal inflammation, in genetically susceptible hosts. Enteric pathogens appear to be important in the initiation and reactivation of human inflammatory bowel disease, and may be responsible for chronic inflammation in at least a subset of patients with inflammatory bowel diseases. Individual components of the commensal flora have variable abilities to induce inflammatory and protective immune responses; these preferential immune responses to individual bacterial species may be unique to each host's genetic background. Analogous to the balance between pro- and antiinflammatory cytokines and T-cell subsets, the ratio between protective (ie, probiotic) and aggressive commensal bacteria may determine whether there is mucosal homeostasis or chronic, relapsing intestinal inflammation. Therapeutic alteration of the luminal microenvironment by probiotic, prebiotic, and molecular strategies offers great promise for nontoxic treatment of inflammatory bowel disease.