Whole-Cell Lactose
Continuous Monitoring
Biosensor
An E. coli-based whole-cell biosensor engineered with a modified lac promoter driving EGFP expression — enabling efficient, continuous, and low-cost lactose detection for dairy quality control and biomedical applications.
This study constructs an Escherichia coli-based whole-cell biosensor responsive to lactose by linking a modified lac promoter with the Enhanced Green Fluorescent Protein (EGFP) reporter gene. The biosensor enables high-efficiency, continuous, and low-cost monitoring, making it suitable for dairy quality control and biomedical research. The recombinant plasmid was constructed by replacing the native lacZ gene with EGFP in the pET28a vector, then transformed into competent E. coli BL21(DE3). Fluorescence was observed exclusively in lactose-exposed cultures, confirming the biosensor's lactose selectivity.
Why Lactose Monitoring Matters
Lactose plays a critical role across multiple industries — from dairy and food production to pharmaceuticals. Understanding lactose concentration and its various forms is essential for managing product quality, characteristics, and production efficiency (Portnoy & Barbano, 2021).
Globally, a significant proportion of the population experiences lactose malabsorption. Lactase activity naturally declines with age (Li et al., 2023), and long-term lactose intolerance can lead to deficiencies in calcium and zinc, affecting patient recovery. Yet even intolerant individuals can typically consume up to 12 g per serving, and moderate lactose intake positively shapes gut microbiome composition (Romero-Velarde et al., 2019).
Given the global prevalence of lactose malabsorption and the individual variability in tolerable intake, developing systems capable of precise, economical, and real-time dietary lactose detection is critical for effective dietary monitoring and medical diagnosis.
Existing Detection Methods
| Method | Principle | Advantage | Limitation |
|---|---|---|---|
| β-Gal Assay | β-galactosidase + X-gal → blue product | Measures protein interactions | Insoluble product; difficult spectrophotometric quantification |
| Chromatography | GC / UHPLC-MS/MS separation | High precision, low LOD | Expensive equipment; not suitable for continuous monitoring |
| Enzyme Activity | Enzymatic colorimetric reaction | Simple operation | Substrate-dependent; limited quantitative accuracy |
| Whole-Cell Biosensor ★ This Study | lac promoter → EGFP fluorescence | Continuous, low-cost, scalable, visual | Currently qualitative; signal stability under investigation |
Experimental Design
Host Organism
The host organism selected for biosensor construction is Escherichia coli BL21(DE3). This strain belongs to the domain Bacteria, with prokaryotic and unicellular characteristics. For this experiment, the bacteria were chemically treated to become competent — capable of taking up exogenous plasmid DNA from the environment through the process of transformation.
Fig. — E. coli BL21(DE3) cells with EGFP expression (3D scientific render)
Recombinant Plasmid Construction
Linearized pET28a vector serves as the biosensor scaffold, carrying a kanamycin resistance marker for bacterial selection.
The EGFP gene is amplified via polymerase chain reaction (PCR) to produce sufficient DNA copies for downstream insertion.
Restriction enzymes cut at two sites flanking the lacZ operon, excising the entire β-galactosidase gene from the plasmid.
A recombination kit ligates the PCR-amplified EGFP fragment into the linearized pET28a, placing EGFP under lac promoter control.
No Lactose — Repressor blocks operator
Transformation & Selection
After plasmid construction, the recombinant DNA is introduced into competent E. coli BL21(DE3) via heat-shock transformation. Bacteria are cultured in LB medium at 37°C with shaking at 220 rpm for one hour to allow recovery.
Cells are then plated on agar supplemented with kanamycin A. Only bacteria that have successfully incorporated the plasmid — and thus express the kanamycin resistance gene — survive this selection step. Colonies that grow confirm successful transformation.
Biosensor Response
Fig. 1 — Biosensor detection under blue excitation light. Tubes (L→R): fructose, lactose, sucrose, glucose. Only the lactose tube exhibits green fluorescence.
Observations
After 6–7 hours of incubation with four different sugars, colonies exposed to lactose produced vivid green fluorescence visible under blue excitation light. Cultures exposed to fructose, sucrose, and glucose showed no significant fluorescence.
This pattern confirms the biosensor's lactose-specific response, consistent with the natural regulatory function of the lac promoter controlling reporter gene expression.
| Sugar | Fluorescence | Interpretation |
|---|---|---|
| Fructose | None | lac promoter not activated |
| Lactose | Strong ✓ | lac promoter activated → EGFP expressed |
| Sucrose | None | lac promoter not activated |
| Glucose | None | Catabolite repression; lac promoter suppressed |
The absence of signal in the presence of fructose, sucrose, and glucose demonstrates the biosensor's selectivity for lactose, consistent with the natural regulatory mechanism of the lac operon in E. coli. Weak background signal observed under non-lactose conditions is likely attributable to leaky expression.
Performance, Limitations & Outlook
Instrument-Free Detection
The experiment demonstrates that a visible fluorescence signal can be obtained without specialized fluorescence measurement instruments, making the biosensor observable in simple laboratory settings.
Qualitative Nature
Current results remain qualitative. Future improvements should incorporate quantitative fluorescence measurement to establish detection limits (LOD) and sensitivity parameters.
Signal Stability
Potential signal instability should be addressed in subsequent testing. Long-term stability and reproducibility across multiple experimental runs require further characterization.
Comparison with Prior Research
Compared to previously reported lactose biosensors (Lin et al., 2023; Gao et al., 2021), this design offers a simpler and lower-cost visual detection approach. While it may be less precise for measuring concentration changes, the observed fluorescence pattern confirms the effectiveness of the lac promoter–EGFP construct as a lactose-responsive system.
The whole-cell biosensor approach offers distinct advantages over conventional β-galactosidase assays: lower cost, continuous monitoring capability, and high scalability. These properties make it highly adaptable to complex environments where traditional analytical methods are impractical.
Future Directions
This study successfully demonstrates the construction and functional validation of a whole-cell lactose biosensor based on engineered E. coli. By replacing the native lacZ gene with EGFP in the pET28a plasmid under lac promoter control, the biosensor produces a specific, visually detectable fluorescent signal in the presence of lactose. The selectivity observed — with no significant signal from fructose, sucrose, or glucose — validates the design principle and establishes a foundation for future quantitative development. This work contributes to the field of synthetic biology and advances biosensor applications for dairy quality control and biomedical monitoring.
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