Caenorhabditis elegans as a model to elucidate host-pathogen interactions for human bacterial pathogens

The soil nematode Caenorhabditis elegans is a popular host utilized to model bacterial virulence and microbial pathogenesis in vivo. This thesis explores the use of C. elegans for the study of host-pathogen interactions for two Gram-negative bacteria, Burkholderia thailandensis and Salmonella enterica.

We conducted a RNA interference screen to identify host genes capable of modulating the infection outcomes of C. elegans infected with B. thailandensis. We discovered that during infection, the cell junction protein LIN-7 appeared to modulate the evolutionarily conserved DAF-2 insulin/IGF-1 signalling pathway, culminating on both the FOXO transcription factor DAF-16 and the heat-shock factor 1. Moreover, LIN-7 regulated nematode survival during infection with other Gram-negative bacteria. Tissue-specific experiments also revealed that this interaction between LIN-7 and the DAF-2 signalling pathway operated mainly in nematode tissues outside the intestine (Paper I).

Through a forward genetics screen using ultraviolet light, we identified pt1 as a novel allele of the unc-7 innexin gene. We found that the pt1 mutant exhibited enhanced survival only when infected with Burkholderia spp. We further defined a specific subclass of unc-7 interacting genes, unc-9 and goa-1, in a unique pathway which probably involves calcium ion fluxes (Paper II).

Next we characterized a new aspect of S. enterica virulence. We observed that S. enterica provoked oxidative stress in the hypodermal tissues of infected C. elegans even though there was no apparent invasion beyond the intestinal epithelium. Via chemical and mutational interference, we found this phenomenon to be deleterious to the host. Genetic inactivation of the bacterial thioredoxin 1 strongly abrogated pathogenicity of S. enterica as well as the emergence of oxidative stress, thereby suggesting a novel role for this virulence factor (Paper III).

Finally, we investigated the combinatorial effects of the proton pump inhibitor omeprazole and the salicylidene acylhydrazide INP0010 during S. enterica infection. We observed disparate effects when they were used in combination and applied to different infection models including the epithelial and macrophage-like cell lines and C. elegans. The nematode can thus provide a platform for testing virulence inhibitors, allowing the elucidation of their mechanisms in the context of a whole organism (Paper IV).

List of scientific papers

I. XIAOHUI SEM, Jason F. Kreisberg, Trupti Kawli, Man-Wah Tan, Mikael Rhen and Patrick Tan. Modulation of Caenorhabditis elegans infection sensitivity by the LIN-7 cell junction protein. Cellular Microbiology. 2012 Oct; 14(10):1584-99.
https://doi.org/10.1111/j.1462-5822.2012.01824.x

II. Kelvin Wong, XIAOHUI SEM, Siew Hoon Sim, Bernice Sim, Mikael Rhen and Patrick Tan. Gap junction proteins in Caenorhabditis elegans affect host susceptibility to Burkholderia infection. [Submitted]

III. XIAOHUI SEM and Mikael Rhen. Pathogenicity of Salmonella enterica in Caenorhabditis elegans relies on disseminated oxidative stress in the infected host. PLoS One. 2012 Sep; 7(9):e45417.
https://doi.org/10.1371/journal.pone.0045417

IV. Speranta Puiac, XIAOHUI SEM, Aurel Negrea and Mikael Rhen. Small-molecular virulence inhibitors show divergent and immunomodulatory effects in infection models of Salmonella enterica serovar Typhimurium. International Journal of Antimicrobial Agents. 2011 Nov; 38(5):409-16.
https://doi.org/10.1016/j.ijantimicag.2011.06.009