细菌转化

Competent E. coli cells are bacteria prepared to take up foreign DNA, such as plasmids. In this experiment, competent cells are used to incorporate the p-GREEN plasmid, allowing researchers to study how bacteria acquire and express new genetic material. The cells' ability to accept external DNA is essential for successful bacterial transformation using plasmids.

The heat shock step, where tubes are submerged in a 42°C water bath for 30 seconds, creates a temperature stress that facilitates plasmid uptake by competent cells. This brief exposure allows the plasmid to cross the bacterial cell membrane more efficiently. Following heat shock, cells are immediately returned to ice to halt the process and stabilize the newly incorporated DNA.

LB plates without antibiotic show bacterial growth from both transformed and untransformed cells, confirming cell viability. LB/ampicillin plates containing antibiotic only grow bacteria that successfully incorporated the resistance plasmid. The minus plasmid LB/ampicillin plate shows no growth, validating that antibiotic selection works. Green fluorescent colonies on the plus plasmid LB/ampicillin plate confirm successful plasmid uptake and expression.

Transformation efficiency is calculated by counting all visible distinct colonies on the plus plasmid LB/ampicillin plate and dividing by the mass of plasmid DNA spread onto that plate. The result is expressed in colony-forming units per microgram of plasmid DNA (cfu/μg). This metric indicates how effectively the competent bacteria took up and expressed the foreign plasmid DNA.

SOC media provides nutrients and osmotic support to help transformed bacteria recover after the stress of heat shock. Adding 950 μL of room temperature SOC media allows cells to stabilize and begin expressing the newly acquired plasmid genes. The subsequent 60-minute incubation at 37°C enables the bacteria to fully recover and replicate before plating.

Always wear appropriate personal protective equipment and keep your face away from suspension cultures to avoid inhaling reagents. Do not touch your face during the experiment. Wash your hands thoroughly before and after every experiment to prevent contamination and exposure to bacterial cultures and chemical reagents.

The experiment uses paired plates to test whether bacteria with the plasmid survive antibiotic exposure while untransformed bacteria do not. The plus plasmid tubes are plated on both LB and LB/ampicillin plates, while minus plasmid tubes serve as controls. If the hypothesis is correct, only the plus plasmid LB/ampicillin plate shows growth, proving that plasmid-conferred resistance enables survival in the presence of ampicillin.