A cell-lysate extract based assay reagent for detecting quorum sensing signals is generally provided, along with methods of making and using the same. The assay reagent generally includes a cell-lysate extract formed from a biosensor bacterium (e.g., Agrobacterium tumefaciens NTL4 (pCF218)(pCF372)) and a detecting substrate (e.g., an absorbance-based or luminescence-based substrate). The cell-lysate extract can be prepared by (1) disrupting the cell membranes of the biosensor bacterium to release the cellular components into a solution, (2) centrifuging the resulting solution, and (3) removing the resulting supernatant solution.