Assessment by electron-microscopy of recombinant Vibrio cholerae and Salmonella vaccine strains expressing enterotoxigenic Escherichia coli-specific surface antigens

Salmonella Typhimurium, the major causative agent of foodborne illness inactivated by a phage lysis system provides effective protection against lethal challenge by induction of robust cell-mediated immune responses and activation of dendritic cells

Salmonella Typhimurium infection via foodborne transmission remains a major public health threat even in developed countries. Vaccines have been developed to reduce the disease burden at the pre-harvest stage, but the cell-mediated immune response against intracellular invasion of the pathogen is not sufficiently elicited by conventional killed Salmonella vaccines, which are safer than live vaccines. In this study, we developed a genetically inactivated vaccine candidate by introducing lysis plasmid pJHL454 harboring the λ phage holin-endolysin system into S. Typhimurium; we designated this vaccine JOL1950. In vitro expression of endolysin was validated by immunoblotting, and complete inactivation of JOL1950 cells was observed following 36 h of the lysis. Electron microscopic examinations by scanning electron microscopy and immunogold labeling transmission EM revealed conserved surface antigenic traits of the JOL1950 cells after lysis. An in vivo immunogenicity study in mice immunized with lysed cells showed significantly increased serum IgG, IgG1, and IgG2a levels. Further, we observed markedly increased in vitro cell proliferation and upregulation of Th1, Th2, and Th17 cytokines in the repulsed splenic T-cells of immunized mice. In dendritic cells (DCs) treated with lysed JOL1950, we observed a significant increase in dendritic cell activation, co-stimulatory molecule production, and levels of immunomodulatory cytokines. In addition, Th1 and Th17 cytokines were also released by naïve CD4⁺ T-cells pulsed with primed DCs. Lysed JOL1950 also protected against lethal challenge in immunized mice. Together, these results indicate that our vaccine candidate has great potential to induce cell-mediated immunity against S. Typhimurium by facilitating the activation of DCs.

Functional expression of the entire adhesiome of Salmonella enterica serotype Typhimurium

Adhesins are crucial virulence factors of pathogenic bacteria involved in colonization, transmission and pathogenesis. Many bacterial genomes contain the information for a surprisingly large number of diverse adhesive structures. One prominent example is the invasive and facultative intracellular pathogen Salmonella enterica with an adhesiome of up to 20 adhesins. Such large repertoire of adhesins contributes to colonization of a broad range of host species and may allow adaptation to various environments within the host, as well as in non-host environments. For S. enterica, only few members of the adhesiome are functionally expressed under laboratory conditions, and accordingly the structural and functional understanding of the majority of adhesins is sparse. We have devised a simple and versatile approach to functionally express all adhesins of S. enterica serotype Typhimurium, either within Salmonella or within heterologous hosts such as Escherichia coli. We demonstrate the surface expression of various so far cryptic adhesins and show ultrastructural features using atomic force microscopy and transmission electron microscopy. In summary, we report for the first time the expression of the entire adhesiome of S. enterica serotype Typhimurium.

Expression and morphology of enterotoxigenic Escherichia coli surface antigen CS31A in E. coli K12 and Vibrio cholerae

Enterotoxigenic Escherichia coli (ETEC) is known as a worldwide cause of diarrheal disease. The pathogenesis involves the attachment of the microorganisms to the mucosa and the production of enterotoxins. Surface expression of CS31A fimbriae was assessed by Western blots, dot blots, immunofluorescence, and electron microscopy using negative staining and immunogold labeling. These investigations revealed significant differences in both the morphology of the wild-type and recombinant strains and the antigen exposure of CS31A in the wild-type and recombinant strains. In the wild-type ETEC strain, expression of CS31A was subject to phase variation. The recombinant E. coli strain produced CS31A but was prone to epitope shedding. In Vibrio cholerae vaccine strain CVD 103-HgR, the recombinant CS31A antigen was expressed but was only found intracellularly. Thus, E. coli strains seem to lend themselves better to the development of recombinant vaccines expressing ETEC-specific antigens at the cell's surface than strains from other orders or genera such as V. cholerae.