Optimisation of a Whole Cell Bacterial Biosensor for Propionate

Propionate is a short chain fatty acid produced by microbial fermentation of dietary fibre in the gastrointestinal tract and plays a role in several metabolic and inflammatory conditions including inflammatory bowel disease. In Escherichia coli, prpR encodes the positive regulator of the prpBCDE promoter (PprpB) which is induced in the presence of propionate and is responsible for expression of the propionate catabolic genes (prpBCDE). PrpR specifically senses 2-methylcitrate (2-MC), an intermediate of the methylcitrate cycle used by this bacterium to convert propionate to pyruvate. The CIDAR MoClo assembly method was used to construct a whole cell propionate biosensor based on the pPro expression vectors that contain a gene encoding green fluorescent protein (GFP) under the control of the PprpB promoter and the prpR transcriptional gene, which is activated by 2-MC signalling the presence of propionate in the environment. The biosensor can only function in the presence of the prpBCDE operon on the chromosome of the host strain as it requires the production of enzymes needed to breakdown propionate to 2-MC. Our propionate biosensor exhibited a concentration dependent response to propionate. The biosensor was optimised by changing the strength of the promoter controlling the expression level of prpR followed by a directed evolution approach to simultaneously select for increased sensitivity, lower background expression and high dynamic range. This biosensor could play a valuable role in further research of the influence of propionate on the gut microbiome and the diagnosis of gut metabolic and inflammatory conditions.