Petra Muir: Bacterial toxin delivery systems: Molecular mechanisms and potential use in probiotic bacteria

Abstract

The emergence of multidrug-resistant pathogenic bacteria and the lack of novel antibiotics reaching the market have led to an increase in treatment failures and mortality worldwide. Consequently, there is an urgent need for innovative alternative approaches to combat bacterial infections. Probiotic bacteria have demonstrated potential in both treating and preventing such infections. Efforts are underway to enhance probiotics, aiming for improved efficacy in targeting and inhibiting the colonization of pathogenic bacterial strains while ensuring their safety for use.  The work presented in this thesis enhances our understanding of bacterial toxin delivery systems, explores their adaptability for clinical applications in bioengineered probiotic bacteria, and offers insights into biocontainment strategies crucial for the secure utilization of these probiotic strains. My research has primarily focused on contact-dependent growth inhibition (CDI) systems, which deliver toxic proteins to closely related bacteria and require direct cell-to-cell contact. In order to use CDI systems in probiotics, we first need to expand our knowledge of the toxin delivery mechanisms employed by these systems.  In paper I, we show that class II CDI systems allow for broad-range cross-species toxin delivery and growth inhibition. We found that the CDI systems tested were able to inhibit the growth of clinically relevant species, such as Enterobacter cloacae and Enterobacter aerogenes. In paper II, we found that two toxins from two different bacterial species utilize the SecYEG translocon for target cell entry, and hence that, for these toxins at least, this crucial step lacks species-specificity. In paper III, we investigated the prevalence of CDI systems in E. coli and the potential advantages these bacteria gain from hosting multiple systems. In paper IV, we wanted to further our understanding of the roles of toxin delivery systems in colonization of host. We found that toxin delivery systems aid in colonization. In paper V, we developed a CRISPR-Cas9 systems that efficiently prevents horizontal gene transfer of antibiotic resistance genes in E. coli.  In conclusion, the findings presented in this thesis collectively highlights the potential of equipping probiotic bacteria with effective weapons, such as CDI systems, to directly target and inhibit the growth of pathogenic bacteria to function as an alternative to conventional antibiotic treatment.