Examining the response of Escherichia coli and Pseudomonas aeruginosa to organic acid stress.

Bushell, Francesca Maria Lucy (2019). Examining the response of Escherichia coli and Pseudomonas aeruginosa to organic acid stress. University of Birmingham. Ph.D.

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Abstract

Weak organic acids have been used for centuries to treat infections and preserve food. However, the detailed mechanisms by which they exert their effect on the cell are not fully understood. Two aspects of organic acid action are presented in this thesis. In the first study, a combination of transcriptomics, transposon-insertion sequencing (TraDIS) and evolution were used to probe the molecular mechanism by which acetic, propionic and butyric acid at both neutral and mildly acidic pH affect cellular process in the UPEC strain E. coli EO499. The effects were numerous and complex, with all three organic acids having a large impact on metabolic processes. Six populations of EO 499 evolved independently at pH 5.5 with 4 mM acetic acid showed increased fitness in that environment, but that fitness was not replicated in an environment of pH 5.5 alone. A cross-comparison of the data derived from these different approaches reveals a highly complex network of genes and responses which are not easily interpreted. Comparison of the methods revealed some overlap between the TraDIS and RNAseq data, with a large proportion of the genes which were considered significant in TraDIS also showing differential regulation. This indicates that with more sophisticated bioinformatics techniques, it should be possible to build up intricate networks of the response to organic acids. The second study examined the inhibition of growth and biofilm formation in P. aeruginosa PA01 and a clinical isolate PA1054 by a range of organic acids. This study was devised to help inform the use of organic acids as a topical treatment for burn wounds. A laboratory plate reader was used to generate growth data in eight different organic acids at three different concentrations and five different pH. This data was then analysed (in collaboration with colleagues) using three mathematical approaches, two parametric and one Bayesian. The plates used for growth were also stained to quantify biofilm formation. This study found that the most effective acid at inhibiting both growth and biofilm formation was propionic acid and the least effective was benzoic acid. The effects of all acids were significantly enhanced by reduction in pH. These data indicate that the use of organic acids as a topical treatment would likely inhibit wound infections. However, the currently used concentrations of ~800 mM could be reduced if combined with mildly acidic solution.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):