Typhoid fever: pathogenesis and disease

Flagellin is the major proinflammatory determinant of enteropathogenic Salmonella

The gastroenteritis-causing pathogen Salmonella typhimurium induces profound transcriptional changes in intestinal epithelia resulting in the recruitment of neutrophils whose presence is the histopathologic hallmark of salmonellosis. Here we used cDNA microarray expression profiling to define the molecular determinants that mediate such changes in model intestinal epithelia. Enteropathogenic Salmonella induced a classical proinflammatory gene expression program similar to that activated by the canonical proinflammatory agonist TNF-alpha. Nonproinflammatory bacteria, both commensals (Escherichia coli) and systemic pathogens (S. typhi), did not activate this expression profile. While S. typhimurium strains lacking the SPI-1-encoded type III system were fully proinflammatory, strains lacking the genes for the flagellar structural component flagellin were nearly devoid of proinflammatory signaling. Lastly, the epithelial proinflammatory response could be largely recapitulated by basolateral addition of purified flagellin. Thus, S. typhimurium flagellin is the major molecular trigger by which this pathogen activates gut epithelial proinflammatory gene expression.

Typhoid fever as cellular microbiological model

The knowledge about typhoid fever pathogenesis is growing in the last years, mainly about the cellular and molecular phenomena that are responsible by clinical manifestations of this disease. In this article are discussed several recent discoveries, as follows: a) Bacterial type III protein secretion system; b) The five virulence genes of Salmonella spp. that encoding Sips (Salmonella invasion protein) A, B, C, D and E, which are capable of induce apoptosis in macrophages; c) The function of Toll R2 and Toll R4 receptors present in the macrophage surface (discovered in the Drosophila). The Toll family receptors are critical in the signalizing mediated by LPS in macrophages in association with LBP and CD14; d) The lines of immune defense between intestinal lumen and internal organs; e) The fundamental role of the endothelial cells in the inflammatory deviation from bloodstream into infected tissues by bacteria. In addition to above subjects, the authors comment the correlation between the clinical features of typhoid fever and the cellular and molecular phenomena of this disease, as well as the therapeutic consequences of this knowledge.

Role of Toll-like receptor 4 in macrophage activation and tolerance during Salmonella enterica serovar Typhimurium infection

Toll-like receptors (TLRs) play an important role in the innate immune response, particularly in the initial interaction between the infecting microorganism and phagocytic cells, such as macrophages. We investigated the role of TLR4 during infection of primary murine peritoneal macrophages with Salmonella enterica serovar Typhimurium. We found that macrophages from the C3H/HeJ mouse strain, which carries a functionally inactive Tlr4 gene, exhibit marked impairment of tumor necrosis factor alpha (TNF-alpha) secretion in response to S. enterica serovar Typhimurium infection. However, activation of extracellular growth factor-regulated kinase and NF-kappa B signaling pathways was relatively unaffected, as was increased expression of TNF-alpha mRNA. Furthermore, macrophage tolerance, which is associated with increased expression of the NF-kappa B p50 and p52 subunits, was induced by S. enterica serovar Typhimurium even in the absence of functional TLR4. These results indicate that during infection of macrophages by S. enterica serovar Typhimurium, TLR4 signals are required at a posttranscriptional step to maximize secretion of TNF-alpha. Signals delivered by pattern recognition receptors other than TLR4 are sufficient for the increased expression of the TNF-alpha transcript and at least some genes associated with macrophage tolerance.

Current trends in typhoid Fever

Typhoid fever, a systemic infection caused by Salmonella enterica serotype typhi, remains an important worldwide cause of morbidity and mortality. Endemic cases in the United States are unusual, with most following foreign travel to the Indian subcontinent, Africa, Asia, or Latin America. The classic findings of typhoid fever include rose spots, relative bradycardia, and stepwise fevers, but unfortunately these signs are frequently absent. Gastrointestinal manifestations may include diffuse abdominal pain, bleeding, perforation, cholecystitis, and cholangitis. The diagnosis should be suspected after collection of the appropriate clinical and travel history with confirmation by blood or bone marrow culture. Novel methods are in development to establish the diagnosis when cultures are negative or unavailable. Multidrug resistance has increased worldwide, and decisions on antimicrobial therapy must take such resistance into account. The empiric treatment of choice is a fluoroquinolone drug; ceftriaxone and azithromycin are alternatives. Preventive strategies include good sanitation and food handling practices along with vaccination of selected groups.

The basis of persistent bacterial infections

Selected bacterial pathogens, such as Helicobacter pylori and Mycobacterium tuberculosis, establish persistent infections in mammalian hosts despite activating inflammatory and antimicrobial responses. The strategies used to overcome host defense responses vary with the anatomical location of the infection but often rely on deliberate manipulations of the host cell responses. Phylogenetically unrelated bacteria can share similar strategies for the establishment of persistence and, in selected examples, one even can define homologous "persistence" genes. Such observations suggest that persistent infection is a specific phase in infection pathogenesis rather than a fortuitous imbalance in the host-pathogen interaction.

Animal models paving the way for clinical trials of attenuated Salmonella enterica serovar Typhi live oral vaccines and live vectors

Attenuated Salmonella enterica serovar Typhi (S. Typhi) strains can serve as safe and effective oral vaccines to prevent typhoid fever and as live vectors to deliver foreign antigens to the immune system, either by the bacteria expressing antigens through prokaryotic expression plasmids or by delivering foreign genes carried on eukaryotic expression systems (DNA vaccines). The practical utility of such live vector vaccines relies on achieving a proper balance between minimizing the vaccine's reactogenicity and maximizing its immunogenicity. To advance to clinical trials, vaccine candidates need to be pre-clinically evaluated in relevant animal models that attempt to predict what their safety and immunogenicity profile will be when administered to humans. Since S. Typhi is a human-restricted pathogen, a major obstacle that has impeded the progress of vaccine development has been the shortcomings of the animal models available to assess vaccine candidates. In this review, we summarize the usefulness of animal models in the assessment of the degree of attenuation and immunogenicity of novel attenuated S. Typhi strains as vaccine candidates for the prevention of typhoid fever and as live vectors in humans.

Cytopathic effects observed upon expression of a repressed collagenase gene present in Salmonella and related pathogens: mimicry of a cytotoxin from multiple antibiotic-resistant Salmonella enterica serotype Typhimurium phagetype DT104

Recently, we reported that certain strains of Salmonella enterica serovar Typhimurium phagetype DT104 (DT104) secrete a putative cytotoxin. While searching for the gene that encodes this toxin, we noted a previously reported but uncharacterized DNA fragment (clg) in Salmonella that could be potentially relevant to cytotoxin-like activity. Therefore, we cloned and expressed clg in cytotoxin-negative Escherichia coli and Salmonella and subsequently assessed the bioactivity of Clg in vitro and in vivo. Lysates containing Clg from both expression hosts exerted cytopathic effects on murine enterocytes while semi-purified Clg was determined to be cytopathic to HEp-2 cells. Sequence and RT-PCR analyses of the clg gene indicated that a homologue of clg exists in different Gram-negative bacteria although the gene is not expressed in vitro. Although Clg-mediated lesions are similar to those mediated by the DT104 cytotoxin, further investigations are necessary to examine the relationship between these two proteins in DT104. Nonetheless, we report here a defined Salmonella protein that is capable of inflicting damage on tissue culture cells and murine enterocytes.