Engineered E. coli biosensor tracks resveratrol production with HPLC-level accuracy

Researchers have shown that a genetically engineered Escherichia coli biosensor can quantify resveratrol made inside a microbial production strain with results closely matching conventional chromatography. The work extends a sensing platform that had previously been characterized only with purified, authentic flavonoid standards into the messier territory of real biosynthetic samples, where culture extracts contain a mixture of metabolites rather than a single dissolved compound.

The biosensor is built on the ttg operon system of Pseudomonas putida. That soil bacterium carries the ttgABC operon, which encodes a multidrug efflux pump that helps the cell tolerate a wide range of aromatic compounds. Expression of the pump is controlled by TtgR, a ligand-responsive transcriptional repressor that binds the operator region of the ttgABC promoter. When no inducing compound is present, TtgR shuts transcription down. When a suitable ligand binds, the repressor loosens its grip on the operator, transcription switches on, and in the biosensor configuration this activation drives expression of an enhanced green fluorescent protein (eGFP) reporter. The result is a genetically encoded signal that scales with the amount of ligand inside the sensing cells, converting molecular recognition into fluorescence that a standard plate reader can measure.

Engineered E. coli strains typically carry a simplified pathway in which tyrosine is converted to resveratrol through three heterologous enzymes: tyrosine ammonia lyase (TAL), 4-coumaroyl-CoA ligase (4CL) and stilbene synthase (STS). Despite steady gains in production titers, the authors identify accurate and rapid quantification of resveratrol as a persistent bottleneck for pathway optimization and process monitoring. HPLC and mass spectrometry deliver accurate numbers, but they are time-consuming and demand specialized instrumentation, which limits their usefulness for screening large numbers of engineered strains or following production dynamics in real time.

In the new study, the production strain carried two plasmids, PC-Sc4CL-VvSTS and PA-aroG-SeTAL-tyrA, encoding the heterologous biosynthetic pathway. Cells were grown in lysogeny broth to an optical density of roughly 0.4, harvested and resuspended in M9 minimal medium, and production was triggered with 0.1 mM isopropyl β-D-1-thiogalactopyranoside. Cultures were incubated with shaking at 30 degrees Celsius for 24 hours, and samples were collected at 0, 2, 4, 8, 12 and 24 hours. Resveratrol was extracted from one-milliliter culture samples with ethyl acetate, evaporated to dryness and reconstituted either in DMSO for the biosensor assay or in methanol for HPLC analysis, so that both methods measured material drawn from the same cultures.

HPLC analysis first confirmed that the system worked as intended. A peak with a retention time of approximately 8.0 minutes, matching an authentic resveratrol standard, grew progressively over the course of cultivation, and the time-course profile showed accumulation during the early stages followed by a plateau at later time points. That pattern demonstrated that the engineered biosynthetic pathway was functionally active and provided the platform against which the biosensor could be judged. For the biosensor side, the team used E. coli BL21 cells carrying a reporter plasmid with eGFP under the control of the ttgABC promoter and a regulatory plasmid expressing wild-type TtgR. Dried culture extracts redissolved in DMSO were added to biosensor cultures at an optical density of about 0.3, incubated for three hours, and fluorescence was read at excitation and emission wavelengths of 480 and 510 nanometers and normalized to cell density.

The quantitative comparison hinged on calibration curves generated with authentic resveratrol standards for both methods. Biosensor fluorescence responses were converted into estimated concentrations, and these were placed side by side with the HPLC-determined values for the same samples. The biosensor-derived figures showed relative agreement of 92.7 to 93.8 percent with the chromatographic measurements, and the biosensor consistently estimated slightly higher concentrations than HPLC, with differences remaining within roughly seven percent of the chromatographic values. Concentrations at the two-hour and four-hour sampling points were excluded from the quantitative comparison because the amounts produced fell below the reliable quantification range of the calibration curves, although the overall production profiles from both methods tracked the same time-dependent trend.

The authors attribute the small discrepancies to differences in detection sensitivity, extraction efficiency and the signal transduction steps of the biosensor system. Even so, they argue that the level of agreement shows the TtgR-based biosensor can provide reliable estimates of resveratrol production and that transcription factor biosensors can serve not only for compound detection but also for quantitative evaluation of biosynthetic performance. The team notes one important boundary: direct intracellular sensing remains to be investigated, so in its current form the approach measures extracted compound rather than reporting from within the producing cells themselves.

Beyond the resveratrol application, the study also profiled engineered TtgR variants against three flavonoids. Biosensors carrying wild-type or variant TtgR were exposed to 0.5 mM resveratrol, quercetin or naringenin for three hours, and responses were expressed as induction coefficients, the ratio of fluorescence in ligand-exposed cells to that of unexposed controls. Wild-type TtgR showed the strongest response toward resveratrol, and all tested variants responded significantly less, at P < 0.001 in one-way ANOVA with Dunnett’s multiple-comparison test using wild type as the control. The N110F variant, by contrast, responded significantly more strongly than wild type to both quercetin and naringenin while responding substantially less to resveratrol, illustrating that the effects of TtgR mutations are strongly ligand dependent.

One double mutant produced a more puzzling result. N110F/F168W retained a significantly enhanced response toward quercetin compared with wild type, yet its response to naringenin was not significantly different from wild type, meaning that adding the F168W substitution to N110F reshaped the ligand-response profile in a way the authors could not yet explain. They state that the molecular basis remains unclear and will require further investigation using structural or direct ligand-binding analyses. Although the primary objective of the study was to test the wild-type biosensor for resveratrol monitoring, the variant data suggest that protein engineering could tailor biosensor selectivity for other flavonoids and extend the platform to additional biosynthetic pathways, including derivatives of resveratrol such as pterostilbene, pinostilbene and piceatannol, which have been biosynthesized in engineered microbes in earlier work.

The practical upshot is a faster, more accessible route to quantifying microbial metabolite production. The authors position the work as a demonstration that a sensing platform previously validated only against authentic standards can move into quantitative assessment of biosynthetically produced metabolites in engineered microbial systems, a step they argue supports the use of transcription factor biosensors in pathway optimization, strain screening and broader synthetic biology applications.

Subject of Research: A transcription factor-based E. coli biosensor for quantitative monitoring of resveratrol produced by an engineered microbial biosynthesis pathway

Article Title: Validation of ttg operon system-based E. coli biosensor for quantitative monitoring of resveratrol biosynthesis

Article References: Song, K., Ji, H., Bae, J., Jang, G., & Yoon, Y. (2026). Validation of ttg operon system-based E. coli biosensor for quantitative monitoring of resveratrol biosynthesis. Applied Biological Chemistry, 69(1), Article 70. https://doi.org/10.1186/s13765-026-01141-x

Image Credits: AI Generated

DOI: 10.1186/s13765-026-01141-x

Keywords: resveratrol, biosensor, Escherichia coli, TtgR, ttg operon, Pseudomonas putida, HPLC, metabolic engineering, synthetic biology, flavonoids, eGFP reporter, whole-cell biosensing