Embodiments of this invention are directed towards the sensitive, fast, and accurate identification and/or characterization of a single cell or bacterium, particularly phenotypic characterization. Certain aspects of the invention include assays that include functional nucleic acid probes (FNAPs). FNAPs can be used to generate deoxyribozyme cleavage cascades (DRCC) initiated by activation of a FNAP resulting in a detectable signal from a single cell.