Characterization of the cytolethal distending toxin (typhoid toxin) in non-typhoidal Salmonella serovars.

Lorraine D Rodriguez-Rivera,Henk C Den Bakker,Barbara M Bowen,Gerald E Duhamel,Martin Wiedmann

doi:10.1186/s13099-015-0065-1

Abstract

BackgroundFor many putative Salmonella enterica subsp. enterica virulence genes, functional characterization across serovars has been limited. Cytolethal distending toxin B (CdtB) is an incompletely characterized virulence factor that is found not only in Salmonella enterica subsp. enterica serovar Typhi (Salmonella Typhi) and dozens of Gram negative bacterial pathogens, but also in non-typhoidal Salmonella (NTS) serovars.MethodsA comparative genomics approach was performed to characterize sequence conservation of the typhoid toxin (TT), encoded in the CdtB-islet, between Salmonella Typhi and NTS serovars. The cytotoxic activity of representative Salmonella enterica subsp. enterica serovars Javiana, Montevideo and Schwarzengrund strains and their respective isogenic cdtB mutants was determined in human intestinal epithelial Henle-407 cells by assessment of cell cycle progression of infected cells using fluorescence-activated cell sorting (FACS). Two-way analysis of variance (ANOVA) was used to determine whether cdtB deletion had a significant (p < 0.05) effect on the percentage of Henle-407 cells at each stage of the cell cycle.ResultsHere we show that a CdtB-islet encoding the cytolethal distending toxin B (CdtB), pertussis-like toxin A (PltA), and pertussis-like toxin B (PltB) is present in a dozen NTS serovars and that these proteins have a high level of sequence conservation and each form monophyletic clades with corresponding Salmonella Typhi genes. Human epithelial Henle-407 cells infected with three representative CdtB-encoding NTS serovars displayed G2/M phase cell cycle arrest that was absent in cells infected with corresponding isogenic cdtB null mutants (p < 0.0001 for the factor ∆cdtB deletion).ConclusionOur results show that CdtB encoded by NTS serovars has a genomic organization, amino acid sequence conservation and biological activity similar to the TT, and thus, may contribute to disease pathogenesis.Electronic supplementary materialThe online version of this article (doi:10.1186/s13099-015-0065-1) contains supplementary material, which is available to authorized users.

Highlights

For many putative Salmonella enterica subsp. enterica virulence genes, functional characterization across serovars has been limited
Typhi’s cytolethal distending toxin B (CdtB) A maximum likelihood (ML) based phylogeny of CdtB amino acid sequences showed that homologs of this gene are widely distributed among Gram-negative bacteria (Fig. 1), including 11 serovars classified into S. enterica subsp. enterica clade B (13 isolates, Fig. 1), S. enterica subsp. enterica serovar Inverness (FSL R8-3668), S. enterica subsp. arizonae (RSK2980), and S. bongori (NCTC 12419)
Consistent with our results, a recent study confirmed the presence of a functional CdtB in a strain of Salmonella Javiana [21]; this study showed that the cell cycle arrest pattern observed in cells infected with the Salmonella Typhi control strain was similar to that observed for the Salmonella Javiana test strain

Summary

Introduction

For many putative Salmonella enterica subsp. enterica virulence genes, functional characterization across serovars has been limited. The study by den Bakker et al [3] identified an islet encoding the cytolethal distending toxin B (CdtB-islet) in the genomes of 56 non-typhoidal Salmonella isolates, including (i) 37/38 clade B isolates, 14/115 non-typhoidal clade A isolates, and five isolates that did not clearly group into clade A or B [3]. Despite these observations and the important role of non-typhoidal Salmonella as foodborne and zoonotic pathogens, the role of CdtB in non-typhoidal serovars has remained understudied; studies that explore the functionality of CdtB in non-typhoidal serovars are essential

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Discussion

Conclusion