Abstract
Bacteria are often found in polymicrobial environments and, to ensure a fitness advantage, have adopted diverse strategies to fight other microbes within their preferred niche. An important mechanism utilised by Gram-negative bacteria is protein secretion, with bacterial secretion systems efficiently translocating particular proteins to the external medium or directly into competitor target cells. Recently, much progress has been made towards understanding the role and mechanism of the Type VI secretion system (T6SS). The T6SS is widespread in Gram-negative bacteria and can deliver toxins (‘effectors’) into both eukaryotic host cells and rival bacterial cells. Effector proteins are believed to be delivered by either covalent association ('specialised' effectors) or non-covalent association ('cargo' effectors) with components of the T6SS machinery. The opportunistic human pathogen, Serratia marcescens, displays a highly effective and offensive anti-bacterial T6SS. Multiple secreted effectors delivered by the T6SS of S. marcescens are known to represent efficient weapons against susceptible target cells. However, prior to this study, only cargo effectors had been described in the T6SS arsenal of this bacterium.In the first part of this study, two new polymorphic toxins containing Rhs repeat domains were identified as effectors secreted by the T6SS of S. marcescens. These two specialised effectors present a strong anti-bacterial activity which is neutralised in the presence of the cognate immunity protein. Both Rhs-family proteins contain a toxic C-terminal domain which acts in the cytoplasm of the target cell. One shows DNase activity whilst the other is a bacteriostatic toxin with the mode of action still to be confirmed. Importantly, in this study it has been demonstrated for the first time that Rhs proteins, rather than other T6SS-secreted effectors, are the primary determinant of intra-species competition in S. marcescens.
This study also reports the discovery of a new family of accessory proteins associated with T6SS effectors. EagR proteins are specifically required for deployment of their associated Rhs effectors. Furthermore, these accessory proteins are well conserved and are involved in the interaction of Rhs proteins with the T6SS apparatus. Evidence is presented for specific interactions between EagR and cognate Rhs proteins and support is provided for a general chaperone role of this family of proteins.
Together, the findings of this study provide important new insight into how bacteria can use the T6SS to deploy different polymorphic toxins, such as Rhs effectors, and mediate different types of inter-bacterial interactions.
| Date of Award | 2016 |
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| Original language | English |
| Awarding Institution |
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| Sponsors | Coordenação de Aperfeiçoamento de Pessoal de Nível Superior, American Society for Microbiology, Society for General Microbiology & British Society for Antimicrobial Chemotherapy |
| Supervisor | Sarah Coulthurst (Supervisor) |