Palenzuela Bretones, Diana
(2017)
Linking fatty acid derivatives with signalling and cross-talk in Pseudomonas aeruginosa.
PhD thesis, University of Nottingham.
Abstract
Xanthomonas campestris is a bacterial plant pathogen that produces diffusible signal factor (DSF), a signalling fatty acid mediating the regulation of important pathogenicity traits, such as protease production and biofilm formation. The production of DSF is dependent on the activity of an enoyl CoA hydratase (ECH) enzyme. Chemically related long chain fatty acids, the DSF type (DSF T) molecules, have since been found to be produced by a large range of bacterial species including Stenotrophomonas maltophilia and Burkholderia cenocepacia, in which they have similar effects and are subject to a similar regulation. Interestingly, these molecules participate in inter species signalling as well, as they can affect the behaviour of species that do not produce them.
Pseudomonas aeruginosa produces the DSF T cis-2-decenoic acid (CDA), to disperse its biofilms and inhibit biofilm formation, but the extent of its effects, the regulatory network and the conditions leading to the production of this molecule are not yet understood. Whole transcriptome shotgun sequencing (RNA seq) previously performed at The University of Nottingham on P. aeruginosa grown in the presence and absence of CDA was further analysed in Chapter 3 to better understand the effects of this signalling molecule. The analysis of this transcriptomic approach and of other transcriptomic profiles performed with DSF T molecules indicate that the effects of these signals are highly dependent on the conditions of growth. This chapter also investigates how non cognate DSF T affect the colonisation capacities of P. aeruginosa. It is of special interest that X. campestris DSF and B. cenocepacia BDSF limit the colonisation capacities of P. aeruginosa in in vitro experiments, which constitutes a starting point for future work on these communication networks in ecological settings.
Previous studies suggested that PA0744 and PA3426 encode enzymes which are involved in the metabolism of CDA, though further work was required in this direction. This line of work was continued in Chapter 4 where it is shown that the regulation of CDA is especially complex and seems to involve other regulatory elements such as the intracellular second messenger cyclic diguanylate (c di GMP) and inter-cellular signalling by quorum sensing (QS). Although P. aeruginosa presents homologues of the sensor proteins and synthases of DSF T, bioinformatic studies have not identified clear lead proteins that might perform these functions.
To get a broader view of small signalling molecules that affect the regulation of community behaviours, cultures of P. aeruginosa were treated with 2 tridecanone (2TDC) in Chapter 5. This molecule has been shown to affect biofilm formation and motility in Sinorhizobium meliloti, Pseudomonas syringae and Salmonella enterica, so the aim was to investigate what effects it might have in P. aeruginosa. Interestingly, this molecule inhibited the establishment of biofilms and swarming motility in P. aeruginosa. These two phenotypes are generally subjected to opposite regulations, thus 2TDC affects the behaviour of P. aeruginosa in a novel way.
In summary, various small molecules and their role in the regulation of colonisation mechanisms in P. aeruginosa were investigated in this study. Results give new perspectives to understand cell to cell communication in this bacterium and show that small molecules present overlapping yet distinct effects in the biology of P. aeruginosa, the knowledge of which could one day pave the way to new treatments against bacterial infections.
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