Investigation into the transcriptional regulation and role in virulence of the Type VI Secretion System of Serratia marcescens Db10

Abstract

The Type 6 Secretion system (T6SS) is the most recently identified secretion system in Gram-negative bacteria. It is widely distributed and has been found to target both eukaryotic and prokaryotic cells. The T6SS has been described as resembling an inverted bacteriophage cell puncturing device which can deliver toxic effectors directly into target in cells in one step. It has become increasingly apparent that the activity of T6SSs in different bacteria is specifically adapted to enable the organism to survive in its particular environmental niche. Serratia marcescens (Sma) is an opportunistic enteric pathogen which is responsible for causing severe hospital acquired infections in vulnerable patients. The insect pathogen Sma Db10 possesses a functional T6SS which targets antibacterial effector molecules to kill competitor bacteria. The availability of a sequenced genome for Sma Db10 makes it a tractable model for the study of T6SS function and regulation in this important opportunistic pathogen.

This study has established that the Sma Db10 T6SS is encoded in a large cluster of 38 genes which are all co-transcribed. The system is constitutively active but has been shown not to be required for virulence in non-mammalian infection models. The regulation of T6SSs can be complex and is often integrated into existing regulatory networks. This study has revealed that expression of the Sma Db10 T6SS is under the control of Hfq, a global post-transcriptional regulator. Hfq plays a critical role in small (s) RNA mediated- regulation, acting as a chaperone to facilitate the interaction of sRNAs with their target mRNAs. Deletion of Hfq in Sma Db10 results in a severe reduction in transcript levels of genes within the T6SS gene cluster, and results in a total loss of T6SS activity. Hfq was also shown to regulate 35% of the Sma Db10 genome, including genes responsible for production of virulence factors such as haemolysin, chitinase, protease and lipase. Deletion of Hfq resulted in attenuated virulence in the non-mammalian infection models, Caenorhabditis elegans and Galleria mellonella.

The findings of this study support the idea that the Sma Db10 T6SS represents an “offensive” T6SS which is always primed and ready to attack competitor bacteria, a strategy which is likely to be of significant value in the niches occupied by this opportunistic pathogen.

Date of Award	2013
Original language	English
Awarding Institution	University of Dundee
Supervisor	Sarah Coulthurst (Supervisor)