A Selective Fluorescent l-Lactate Biosensor Based on an l-Lactate-Specific Transcription Regulator and Förster Resonance Energy Transfer.

Selective detection of l-lactate levels in foods, clinical, and bacterial fermentation samples has drawn intensive attention. Many fluorescent biosensors based on non-stereoselective recognition elements have been developed for lactate detection. Herein, the allosteric transcription factor STLldR from LT2 was identified to be stereo-selectively respond to l-lactate. Then, LldR was combined with Förster resonance energy transfer (FRET) to construct a fluorescent l-lactate biosensor FILLac. FILLac was further optimized by truncating the N- and C-terminal amino acids of LldR between cyan and yellow fluorescent proteins. The optimized biosensor FILLac exhibited a maximum emission ratio change (Δ) of 33.47 ± 1.91%, an apparent dissociation constant () of 6.33 ± 0.79 μM, and a limit of detection of 0.68 μM. FILLac was applied in 96-well microplates to detect l-lactate in bacterial fermentation samples and commercial foods such as Jiaosu and yogurt. The quantitation results of FILLac exhibited good agreement with that of a commercial l-lactate biosensor SBA-40D bioanalyzer. Thus, the biosensor FILLac compatible with high-throughput detection may be a potential choice for quantitation of l-lactate in different biological samples.