Assessment of pUC19 Plasmid Stability in Escherichia coli BL21 During Serial Passaging.

Background: Plasmid stability is an important factor in molecular biology and biotechnology because it determines whether recombinant plasmids can be reliably maintained during bacterial growth and experimental procedures. Although the high-copy-number cloning vector pUC19 is widely used in molecular biology research, limited information is available regarding its stability during prolonged cultivation in Escherichia coli BL21. Objective: This study aimed to evaluate the stability and retention of the pUC19 plasmid in E. coli BL21 during serial passaging under laboratory conditions. Methods: Chemically competent E. coli BL21 cells were transformed with the pUC19 plasmid using the heat shock method, and successful transformation was confirmed by growth on ampicillin-containing agar plates. Plasmid stability was evaluated over a seven-day period using serial passaging and replica plating under both selective and non-selective conditions. Results: Plasmid retention remained at 100% during the first 48 hours of cultivation. However, a gradual decline in plasmid stability was observed during extended passaging in the absence of antibiotic selection, with plasmid retention decreasing progressively to 66% after 168 hours. Conclusion: These findings indicate that pUC19 is highly stable in E. coli BL21 during short-term cultivation but gradually loses stability during prolonged growth without selection pressure. Maintaining appropriate selective conditions is therefore important to ensure plasmid retention during extended bacterial cultivation.