Batch Fermentation and Titer Quantification of Recombinant GFP in E. coli BL21(DE3) Using IPTG Induction and HPLC-Verified Glucose Consumption

Triplicate independent fermentations per induction condition (3 conditions: OD600 0.4, 0.6, 1.2 at induction), run on separate days with fresh inocula. Each fermentation sampled at 0, 1, 2, 3, 4, 6, 8 h post-induction. Two technical replicate assays per sample for OD, glucose, and GFP. Reactor randomization not applicable (single vessel) but condition order counterbalanced across days to control batch effects.

BSL-1 for non-pathogenic E. coli BL21(DE3). Wear lab coat, gloves, goggles. IPTG and kanamycin handled as irritants; sonication requires hearing protection and eye shielding. H2SO4 HPLC mobile phase is corrosive — prepare in a fume hood. Autoclave all biological waste before disposal; collect HPLC solvent waste in labeled containers. Sterilize the bioreactor and decontaminate spills with 70% ethanol or 10% bleach.

Positive control: parallel flask of a previously validated high-titer clone confirms reagent/induction performance. Negative control: BL21(DE3) with empty pET28a (no GFP) shows baseline autofluorescence and confirms titer signal is GFP-specific. Uninduced control: identical culture with no IPTG quantifies leaky expression. Medium blank: sterile medium incubated identically checks contamination and HPLC baseline.

Exponential growth with mu ~0.5-0.7 h^-1 pre-induction. Glucose depletes from 4 g/L to near zero by 6-8 h with modest acetate accumulation (<2 g/L). Soluble GFP titer of 80-250 mg/L; specific yield 30-80 mg/g DCW. Induction at OD 0.6 gives the highest soluble fraction; OD 1.2 shifts product toward inclusion bodies (lower soluble titer).

• E. coli BL21(DE3) transformed with pET28a-GFP (kanamycin resistance)
• LB or defined M9 medium: M9 salts, 4 g/L glucose, 2 mM MgSO4, 0.1 mM CaCl2, trace elements
• Kanamycin, 50 ug/mL
• IPTG, 0.5 mM final (1 M stock, 0.22 um filtered)
• Antifoam 204, 0.05% as needed
• 1 L bioreactor (or baffled shake flask backup) with DO, pH, temperature probes
• 4 M NaOH / 2 M H3PO4 for pH control at 7.0
• HPLC-grade water, 5 mM H2SO4 mobile phase for organic-acid/glucose column
• Bradford reagent and GFP fluorescence standards

To run a controlled 1 L stirred-tank batch fermentation of E. coli BL21(DE3) carrying pET28a-GFP, characterizing the relationship between induction timing (OD600 at IPTG addition), specific growth rate, glucose uptake, and final soluble GFP titer (mg/L) and specific yield (mg GFP / g dry cell weight). The protocol yields a reproducible time-course for process optimization.

1. Inoculate 5 mL LB + kanamycin from a single colony; grow overnight at 37 C, 220 rpm.
2. Dilute 1:100 into 1 L M9 + kanamycin in the bioreactor; set 37 C, pH 7.0, DO >30% via cascade stirring/air.
3. Monitor OD600 hourly until target induction OD (0.4/0.6/1.2).
4. Add IPTG to 0.5 mM; drop temperature to 30 C to favor soluble expression.
5. Sample 2 mL at each time point: measure OD600, pellet for protein, filter supernatant for HPLC.
6. Lyse pellets by sonication in lysis buffer (50 mM Tris pH 8, 300 mM NaCl, lysozyme), clarify at 16,000 x g, 20 min, 4 C.
7. Quantify soluble GFP by fluorescence (ex 488/em 509) against purified GFP standard; total protein by Bradford.
8. Run supernatants on HPLC (Aminex HPX-87H, 5 mM H2SO4, 0.6 mL/min, 60 C, RI detector) for glucose/acetate.
9. Harvest at 8 h; record final dry cell weight (DCW) by drying washed pellet at 105 C to constant mass.

Independent: OD600 at induction (induction timing). Dependent: soluble GFP titer (mg/L), specific yield (mg/g DCW), specific growth rate mu, glucose uptake rate. Controlled: IPTG concentration (0.5 mM), post-induction temperature (30 C), pH 7.0, DO >30%, kanamycin 50 ug/mL, inoculum ratio 1:100, medium composition.

Inducing at mid-exponential phase (OD600 ~0.6) with 0.5 mM IPTG maximizes soluble GFP titer relative to early or late induction, because protein synthesis competes less with biomass formation and avoids inclusion-body formation seen at high induction temperatures. We predict soluble titer at 0.6 OD induction exceeds that at OD 1.2 by >25%.

Compute mu from ln(OD600) slope in exponential phase (linear regression). Convert glucose RI peak areas to g/L via external standard curve. Titer = fluorescence converted to mg/L using purified-GFP standard linear fit (R2>0.99). Specific yield = titer / DCW. Biomass yield Yx/s = DCW formed / glucose consumed. Analyze in Python/pandas or Excel; report time-courses with error bars.

1. No growth: confirm kanamycin not degraded and DE3 lysogen retained — re-streak and verify resistance. 2. Low soluble titer with high total expression: inclusion bodies — lower post-induction temp to 25 C and reduce IPTG to 0.1 mM. 3. Acetate overflow (>3 g/L) stalling growth: reduce glucose feed or improve aeration/DO control. 4. Foaming disrupting DO probe: add antifoam 204 dropwise. 5. HPLC glucose peak co-eluting with organic acids: verify HPX-87H column temperature 60 C and degassed mobile phase.

Compare final soluble titer across induction conditions by one-way ANOVA (n=3 fermentations) with Tukey HSD; alpha=0.05. Specific growth rates compared by two-tailed t-test. Power: detecting a 25% titer difference (SD ~12% of mean) at 80% power, alpha=0.05 needs n=3 per group given the large effect. Report 95% CIs on titer and mu.