The role of small molecule signalling in biofilm migration of Pseudomonas aeruginosa

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Pseudomonas aeruginosa is a Gram-negative pathogen which exploits damaged epithelium to cause acute and chronic infections in a range of immunocompromised individuals. The chronic nature of infections caused by P. aeruginosa is often associated with the formation of biofilms. Extension and retraction of type IV pili (tfp) mediates a form of surface translocation, termed twitching motility, which is involved in active biofilm expansion and sessile biofilm formation. In P. aeruginosa the biogenesis, assembly and twitching motility function of tfp is controlled by a number of complex regulatory systems, however the signals that these systems respond to are not well characterised. The aim of this Thesis was to understand how intracellular and extracellular signals control P. aeruginosa twitching motility-mediated biofilm expansion. In this Thesis five independent fimL mutants, that had presumably acquired extragenic suppressor mutations which restored twitching motility ability, were characterised. All fimL revertants were found to have increased levels of intracellular cyclic AMP (icAMP). While an extragenic suppressor mutation in the cAMP phosphodiesterase CpdA was shown to be responsible for the increase in icAMP levels and restoration of twitching motility in one fimL revertant, the site of suppressor mutation(s) in the remaining four revertants was not identified. These results suggest that twitching motility reversion in fimL mutants occurs via at least two mechanisms and that an increase in icAMP levels is correlated with twitching motility. Extracellular ATP (eATP) is released by damaged epithelial cells which acts as a “danger” signal to recruit host immune system factors to repair the damage. As P. aeruginosa has a propensity for damaged epithelia the effect of eATP on P. aeruginosa biofilm expansion and formation was investigated. The results presented in this Thesis demonstrate that eATP inhibits P. aeruginosa twitching motility-mediated biofilm expansion and stimulates sessile biofilm formation, which may provide a potential advantage for P. aeruginosa within an infection setting. Additionally, our results suggest that high levels of endogenously-produced bacterial eATP acts to coordinate P. aeruginosa multicellular behaviours. This Thesis also reports the identification of a novel extracellular signal N-acetylglucosamine, which stimulates P. aeruginosa twitching motility. Additionally, the twitching motility response of P. aeruginosa to the host derived signals serum albumin, mucin and oligopeptides was characterised in detail. These analyses implicated the CheW-homolog, ChpC which is a component of the Chp chemosensory system, in this response. Overall the results presented in this Thesis provide insight into the regulation P. aeruginosa twitching motility by a number of intracellular and extracellular signals. Our results suggest that the adaptive response of P. aeruginosa to these signals is likely to have significant implications in the success of this pathogen within an infection setting.
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