It is becoming increasingly apparent that the microbiota has a profound effect on human health and disease. Modern synthetic biology provides tools that can be used to engineer new diagnostic and therapeutic circuits- facilitating microbiome engineering and the creation of engineered biotherapeutics. These engineered biotherapeutics have the potential to expand our knowledge of microbial communities, host-microbe interactions and human health. However, to achieve these ambitious goals several challenges remain to be solved. These involve the creation of novel model systems and design strategies that can be used to characterise and improve these engineered strains. The primary focus of this thesis are whole-cell biosensors, that can be used to monitor molecules relevant to human health. Within this work I develop a novel model system, based on the Caenorhabditis elegans nematode that can be used to characterise biosensor strains in vivo. Through the developed protocols I use the nematode model to show that ratiometric biosensors can detect and report on changes within the C. elegans digestive tract. This model could be used to improve engineered biosensor strains, while also expanding our understanding of nematode biology and host-microbe interactions. In addition, I engineer a range of new ratiometric plasmids that can be used in conjunction with the C. elegans model system in future. Finally, I develop a range of acetoacetate-inducible biosensors; while also exploring methods of rationally improving two-component system biosensors. Two component systems are a common sensing mechanism that can be used to create a range of biosensors, therefore methods of rationally improving these biosensors would be an invaluable tool. Overall, it is hoped that the tools developed within this thesis can be used to further engineer whole-cell biosensors, which may help expand our knowledge of host-microbe interactions and human health
