Bacteria use N-acyl homoserine lactone (AHL) molecules to regulate the expression of genes in a density-dependent manner. Several biosensors have been developed and engineered to detect the presence of all types of AHLs.
Bernier, S. P., Beeston, A. L., & Sokol, P. A. (2008). Detection of N-acyl homoserine lactones using a traI-luxCDABE-based biosensor as a high-throughput screening tool. BMC Biotechnology, 8. https://doi.org/10.1186/1472-6750-8-59