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

For 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.

A 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.

Here 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).

Our 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.