Structure and function of the Salmonella Typhi chimaeric A(2)B(5) typhoid toxin

Functional mapping of community-acquired respiratory distress syndrome (CARDS) toxin of Mycoplasma pneumoniae defines regions with ADP-ribosyltransferase, vacuolating and receptor-binding activities

Community-acquired respiratory distress syndrome (CARDS) toxin from Mycoplasma pneumoniae is a 591-amino-acid virulence factor with ADP-ribosyltransferase (ADPRT) and vacuolating activities. It is expressed at low levels during in vitro growth and at high levels during colonization of the lung. Exposure of experimental animals to purified recombinant CARDS toxin alone is sufficient to recapitulate the cytopathology and inflammatory responses associated with M. pneumoniae infection in humans and animals. Here, by molecular modelling, serial truncations and site-directed mutagenesis, we show that the N-terminal region is essential for ADP-ribosylating activity. Also, by systematic truncation and limited proteolysis experiments we identified a portion of the C-terminal region that mediates toxin binding to mammalian cell surfaces and subsequent internalization. In addition, the C-terminal region alone induces vacuolization in a manner similar to full-length toxin. Together, these data suggest that CARDS toxin has a unique architecture with functionally separable N-terminal and C-terminal domains.

The cytolethal distending toxin effects on Mammalian cells: a DNA damage perspective

The cytolethal distending toxin (CDT) is produced by many pathogenic Gram-negative bacteria and is considered as a virulence factor. In human cells, CDT exposure leads to a unique cytotoxicity associated with a characteristic cell distension and induces a cell cycle arrest dependent on the DNA damage response (DDR) triggered by DNA double-strand breaks (DSBs). CDT has thus been classified as a cyclomodulin and a genotoxin. Whereas unrepaired damage can lead to cell death, effective, but improper repair may be detrimental. Indeed, improper repair of DNA damage may allow cells to resume the cell cycle and induce genetic instability, a hallmark in cancer. In vivo, CDT has been shown to induce the development of dysplastic nodules and to lead to genetic instability, defining CDT as a potential carcinogen. It is therefore important to characterize the outcome of the CDT-induced DNA damage and the consequences for intoxicated cells and organisms. Here, we review the latest results regarding the host cell response to CDT intoxication and focus on DNA damage characteristics, cell cycle modulation and cell outcomes.

Structural and enzymatic characterization of a host-specificity determinant from Salmonella

GtgE is an effector protein from Salmonella Typhimurium that modulates trafficking of the Salmonella-containing vacuole. It exerts its function by cleaving the Rab-family GTPases Rab29, Rab32 and Rab38, thereby preventing the delivery of antimicrobial factors to the bacteria-containing vacuole. Here, the crystal structure of GtgE at 1.65 Å resolution is presented, and structure-based mutagenesis and in vivo infection assays are used to identify its catalytic triad. A panel of cysteine protease inhibitors were examined and it was determined that N-ethylmaleimide, antipain and chymostatin inhibit GtgE activity in vitro. These findings provide the basis for the development of novel therapeutic strategies to combat Salmonella infections.

Salmonella vaccines: lessons from the mouse model or bad teaching?

Salmonella enterica subsp. enterica includes several very important human serovars including Typhi, Paratyphi, Typhimurium and Enteritidis. These bacteria cause a significant global burden of disease, typically classified into enteric fever, gastroenteritis and, more recently, invasive non-typhoidal salmonellosis (iNTS). Vaccines have been developed for one of these serovars, S. Typhi and the recent increase in iNTS cases has resulted in a push to develop new vaccines that will inhibit disease by S. Typhimurium and S. Enteritidis, the most common iNTS S. enterica serovars. The development of new human vaccines has been informed by studies in the murine model of typhoid fever based on S. Typhimurium infections of very 'sensitive' (Nramp-1(S)) mice, which has some obvious deficiencies, not the least that antibodies protect humans against S. Typhi infection but are only weakly protective in 'sensitive' mice infected with S. Typhimurium. S. Typhimurium also lacks Vi, the target of protective antibodies in typhoid fever. Notwithstanding these deficiencies, the murine model has identified a very complex series of innate and adaptive immune responses to infection that might be exploited to develop new vaccines. Equally, advances in understanding the pathogenesis of infection, through pathogenomics and more sophisticated animal models will likely contribute to the development of novel immunogens.

Typhoid epidemiology, diagnostics and the human challenge model

PURPOSE OF REVIEW

Infection caused by ingestion of human-restricted Salmonella enterica serovars Typhi and Paratyphi predominantly affects the most impoverished sections of society. In this review, we describe recent advances made in estimating the burden of illness and the important role improved diagnostic tests may have in controlling infection and report the development of a new human challenge model of typhoid infection.

RECENT FINDINGS

Typhoid continues to be a major cause of morbidity, particularly in children and young adults in south east Asia, although accurate assessments are still hindered by the lack of reliable surveillance data. Recent reports of high rates of infection in Africa and the dominance of paratyphoid in several geographic areas are of particular concern. Diagnosis of enteric fever remains frustrated by the nonspecific clinical presentation of cases and the lack of test sensitivity. Methods to improve diagnostic accuracy are hindered by the incomplete understanding of immunobiological mechanisms of infection and lack of a suitable animal infection model.

SUMMARY

Enteric fever is a major global problem, the burden of which has only partially been recognized. Control strategies utilizing cheap accurate diagnostics and effective vaccines are urgently required, and their development should be accelerated by the use of a human challenge model.

Immunoproteomic analysis of antibody in lymphocyte supernatant in patients with typhoid fever in Bangladesh

We have previously shown that an assay based on detection of anti-Salmonella enterica serotype Typhi antibodies in supernatant of lymphocytes harvested from patients presenting with typhoid fever (antibody in lymphocyte supernatant [ALS] assay) can identify 100% of patients with blood culture-confirmed typhoid fever in Bangladesh. In order to define immunodominant proteins within the S. Typhi membrane preparation used as antigen in these prior studies and to identify potential biomarkers unique to S. Typhi bacteremic patients, we probed microarrays containing 2,724 S. Typhi proteins with ALS collected at the time of clinical presentation from 10 Bangladeshis with acute typhoid fever. We identified 62 immunoreactive antigens when evaluating both the IgG and IgA responses. Immune responses to 10 of these antigens discriminated between individuals with acute typhoid infection and healthy control individuals from areas where typhoid infection is endemic, as well as Bangladeshi patients presenting with fever who were subsequently confirmed to have a nontyphoid illness. Using an ALS enzyme-linked immunosorbent assay (ELISA) format and purified antigen, we then confirmed that immune responses against the antigen with the highest immunoreactivity (hemolysin E [HlyE]) correctly identified individuals with acute typhoid or paratyphoid fever in Dhaka, Bangladesh. These observations suggest that purified antigens could be used with ALS and corresponding acute-phase activated B lymphocytes in diagnostic platforms to identify acutely infected patients, even in areas where enteric fever is endemic.

Helicobacter and salmonella persistent infection strategies

Some host-adapted bacterial pathogens are capable of causing persistent infections in humans. For example, Helicobacter pylori inhabits the human gastric mucosa and persistence can be lifelong. Salmonella enterica serovar Typhi causes systemic infections that involve colonization of the reticuloendothelial system and some individuals become lifelong carriers. In this review, I compare and contrast the different lifestyles of Helicobacter and Salmonella within the host and the strategies they have evolved to persist in mammalian hosts. Persistently infected carriers serve as the reservoirs for these pathogens, and the carrier state is an essential feature that is required for survival of the bacteria within a restricted host population. Therefore, investigating the chronic carrier state should provide insight into bacterial survival strategies, as well as new therapeutic approaches for treatments.

EcxAB is a founding member of a new family of metalloprotease AB5 toxins with a hybrid cholera-like B subunit

AB5 toxins are composed of an enzymatic A subunit that disrupts cellular function associated with a pentameric B subunit required for host cell invasion. EcxAB is an AB5 toxin isolated from clinical strains of Escherichia coli classified as part of the cholera family due to B subunit homology. Cholera-group toxins have catalytic ADP-ribosyltransferases as their A subunits, so it was surprising that EcxA did not. We confirmed that EcxAB self-associates as a functional toxin and obtained its structure. EcxAB is a prototypical member of a hybrid AB5 toxin family containing metzincin-type metalloproteases as their active A subunit paired to a cholera-like B subunit. Furthermore, EcxA is distinct from previously characterized proteases and thus founds an AB5-associated metzincin family that we term the toxilysins. EcxAB provides the first observation of conserved B subunit usage across different AB5 toxin families and provides evidence that the intersubunit interface of these toxins is far more permissive than previously supposed.