Genetic Optimisation of Bacteria-Induced Calcite Precipitation

Abstract

Bacteria in our environment contribute to Earth’s landscapes through mineral deposits via a process known as bacteria-induced calcite precipitation (BICP). Over recent years, such bacteria have been at the basis of innovative biotechnologies arising within civil engineering sectors, finding application for example in self-healing concrete where encapsulated bacteria (such as those of the genus Bacillus) facilitate the repair of cracks that appear during aging of built structures. BICP occurs as a product of bacterial metabolism, which creates a microenvironment that favours the precipitation of calcium cations and carbonate anions in the form of mineral calcite. This process is dependent on changes in pH, availability of cell surface nucleation sites, and ion concentrations. Current approaches using this technology in industrial applications require bacteria that are both capable of BICP, as well as possessing specific growth characteristics required for the respective application (e.g. pH/salt tolerance). This project explored the genetic optimisation of BICP using Bacillus subtilis as a model Gram-positive and industrially relevant organism. Genetic engineering was coupled with functional characterisation of the resulting strains, quantitative and qualitative assessment of BICP under laboratory conditions and electron microscopy imaging of resulting crystals. This work identified key molecular components needed for BICP to occur and a way to mobilise these into better-suited chassis organisms for application predominantly in the context of improving self-healing concrete. Results showed that heterologous expression of the ureolytic pathway and modulation of biofilm production offer mechanisms whereby BICP can be engineered into a non-precipitating strain. In contrast modulation of bacterial surface charge was found to be ineffective in improving BICP. A plasmid suitable for mobilisation to a broad host range of Gram-positive bacteria was also developed to facilitate the future shuttling of BICP-promoting genes to application relevant bacteria. The results presented here provide a systematic exploration of the genetic components that drive BICP and will help pave the way for the rational design or selection of better precipitators for application. The ultimate goal is for these fundamental findings to contribute to the formulation of bio-concrete that increases the lifespan of cementitious structures and consequently decreases the maintenance costs and carbon dioxide release associated with concrete production and building.

Date of Award	17 Feb 2021
Original language	English
Awarding Institution	University of Bath
Supervisor	Susanne Gebhard (Supervisor) & Kevin Paine (Supervisor)