Cardoso Ramalhete, Rita (2025) Investigation of Pseudomonas aeruginosa Species-Specific Resistance to Type 6 Secretion System Attack. PhD thesis, Birkbeck, University of London.
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Abstract
The Type 6 Secretion System (T6SS) is a contact-dependent mechanism employed by many Gram-negative bacteria to deliver toxic proteins into the extracellular milieu or neighbouring cells. Pseudomonas aeruginosa is an opportunistic pathogen that encodes three T6SS apparatuses, one of which (H1-T6SS) specifically assembles and fires in response to exogenous T6SS activity. This response is lethal to competitors and provides P. aeruginosa a competitive advantage within microbial communities. Here, we demonstrated that inactivating H1-T6SS increases the susceptibility of P. aeruginosa to Acinetobacter baylyi T6SS attacks of than to those of Vibrio cholerae. This observation greatly motivated our exploration of species-specific behaviour mediated by the T6SS. Next, we investigated whether P. aeruginosa was selectively more resistant to V. cholerae T6SS due to effector toxicity by endogenously expressing V. cholerae effectors into P. aeruginosa. Unlike cytosolic expression, periplasmic expression of V. cholerae effectors results in significant toxicity, suggesting that proper cellular effector localisation is critical for toxicity and that P. aeruginosa is not inherently resistant to V. cholerae effectors. We then investigated whether V. cholerae T6SS could deliver effectors directly into the cytosol of P. aeruginosa by using fluorescence- and antibiotic-based reporters for cytosolic delivery. Despite our efforts, these reporter systems were ineffective in determining T6SS-mediated cytosolic delivery. We thus asked instead, whether P. aeruginosa cell wall could specifically prevent V. cholerae T6SS attacks. We revealed that P. aeruginosa mutants lacking one or more genes encoding for exopolysaccharides or lipid transport pathway proteins were not more susceptible to V. cholerae T6SS attacks. This suggests that P. aeruginosa cell wall might not play a role in species-specific resistance against V. cholerae T6SS. Lastly, we investigated the role of P. aeruginosa retaliatory H1-T6SS in a multispecies community consisting of T6SS-aggressor and T6SS-sensitive species. Our results revealed that P. aeruginosa is able to protect T6SS-sensitive E. coli from T6SS-aggressor V. cholerae in a H1-T6SS-dependent manner and by creating a physical barrier between aggressors and their victims. Collectively, our study underscores the pivotal role of P. aeruginosa retaliatory H1-T6SS in modulating interbacterial competition and shaping population dynamics by influencing species-specificity and the spatial organisation of microbial communities.
Metadata
| Item Type: | Thesis |
|---|---|
| Copyright Holders: | The copyright of this thesis rests with the author, who asserts his/her right to be known as such according to the Copyright Designs and Patents Act 1988. No dealing with the thesis contrary to the copyright or moral rights of the author is permitted. |
| Depositing User: | Acquisitions And Metadata |
| Date Deposited: | 13 Aug 2025 15:07 |
| Last Modified: | 05 Sep 2025 03:36 |
| URI: | https://eprints.bbk.ac.uk/id/eprint/56052 |
| DOI: | https://doi.org/10.18743/PUB.00056052 |
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