Biotechnological exploitation of bacterial communication processes: from quorum sensing inhibition to the generation of synthetic cells interfacing with natural cells

Abstract

The asocial existence of the bacterial cell has been a major paradigm in microbiology for centuries. In the 300 years since van Leeuwenhoek’s descriptions of the microbial world, bacteria have been regarded as deaf mute individual cells designed to proliferate but unable to communicate and interact with each other. The discovery of quorum sensing (QS) communication systems in recent times dramatically changed our way of thinking at bacterial cells. Indeed, it is now acknowledged that via QS bacteria can monitor their cell density and consequently coordinate gene expression at the population level, thus displaying collective behaviours that resemble capacities typical of multicellular organisms. Bacterial social phenotypes controlled by QS systems include bioluminescence, motility, biofilm formation, synthesis of secondary metabolites, competence, and production of virulence factors in plant, animal and human pathogens. In the last decades many scientists aimed at untangling complex QS networks governing bacterial social behaviours, others at exploiting QS circuits as useful tools in synthetic biology applications or as targets for new therapeutic approaches. In particular, bacterial QS systems have been employed in synthetic biology to generate engineered bacterial cells able to synchronize gene expression at the population level. Moreover, since QS controls the expression of virulence traits in different human pathogens, QS systems are considered promising targets for the development of antivirulence drugs that reduce bacterial pathogenic potential with limited selective pressure for the emergence of resistance. One of the model organisms for QS studies is the Gram-negative bacterium Pseudomonas aeruginosa, one of the most dreaded opportunistic human pathogens, in which a complex network of four interconnected QS circuits controls the expression of multiple virulence factors. P. aeruginosa infections are difficult to eradicate since this pathogen is particularly resistant to currently available antibiotics. The necessity for new therapeutic options to treat P. aeruginosa infections is testified by a recent release from the World Health Organization ranking this bacterium in the Priority 1 group of antibiotic-resistant pathogens for which new drugs are urgently needed. In this context, QS in P. aeruginosa represents both a suitable system to untangle the complex regulatory networks leading to the emergence of social traits in unicellular organisms, and a potential target for the development of antivirulence drugs decreasing P. aeruginosa pathogenicity. This PhD thesis builds up on previous knowledge on QS and virulence in P. aeruginosa, on regulatory properties emerging from complex regulatory circuits, on synthetic biology applications and on antivirulence strategies, to exploit the QS communication systems of P. aeruginosa for future biomedical applications. In details, experimental and methodological works, as well as review articles, will be presented aimed at i) understanding new regulatory properties emerging from the peculiar topological architecture of the P. aeruginosa las QS circuit, ii) exploiting the P. aeruginosa rhl QS system for the generation of synthetic cells able to interface with natural cells, and iii) identifying new antivirulence drugs targeting the P. aeruginosa pqs QS system. Overall, the results produced in this PhD thesis increase our knowledge of the P. aeruginosa QS circuits and pave the way for future therapeutic applications based on synthetic cells and on repurposed drugs with antivirulence activity.