Bacterial transformation is a process where a bacterium alters its genetic makeup by incorporating foreign DNA directly from its surrounding environment. This mechanism involves the direct uptake of DNA that is not contained within another cell or virus. Transformation is one of the primary ways bacteria achieve horizontal gene transfer, allowing them to rapidly acquire new traits, such as antibiotic resistance or increased virulence.
Natural Competence in Bacteria
For transformation to occur naturally, bacteria must first enter a specialized physiological state known as competence. This term describes the ability of the bacterial cell to actively bind and transport external DNA across its cell envelope. Only certain species, including Streptococcus pneumoniae, Haemophilus influenzae, and Bacillus subtilis, are naturally competent and possess the necessary genetic machinery for this process.
The induction of competence is a highly regulated event, often triggered by specific environmental cues like nutrient limitation or high cell density. Bacteria employ a signaling system, such as quorum sensing, to gauge population size and coordinate the expression of competence genes. The purpose of this DNA uptake is multifaceted, providing the cell with raw nucleotides for nutrition, a template for DNA repair, or a source of new genes for adaptation.
The Step-by-Step Process of DNA Uptake
Uptake begins when a double-stranded DNA fragment binds to specific protein receptors on the surface of the competent bacterial cell. This external DNA is then met by a specialized protein complex, often referred to as the transformasome, which spans the cell’s membranes. As the DNA is pulled toward the cell interior, a nuclease enzyme degrades one of the DNA strands.
Only the remaining single strand of DNA is actively transported into the cytoplasm through a dedicated channel. The other strand is broken down into its nucleotide components. Once inside, the single-stranded DNA is protected from degradation by binding to specialized proteins. If the imported DNA shares a similar sequence with the host cell’s chromosome, it can be integrated into the main genome through homologous recombination. If the DNA is a circular piece, like a plasmid, it may replicate independently within the cytoplasm.
Inducing Transformation in the Laboratory
Many common laboratory strains, such as Escherichia coli, are not naturally competent, or their natural efficiency is too low for practical use. Scientists must artificially induce competence to force these bacteria to take up engineered DNA, typically a circular plasmid. These artificial methods bypass the cell’s natural, protein-driven uptake machinery by temporarily altering the permeability of the cell membrane.
The chemical method involves treating the cells with cold calcium chloride solution. Positively charged calcium ions shield the negative charges of the DNA and the cell surface, facilitating proximity to the membrane. A brief temperature increase, known as heat shock, then temporarily destabilizes the membrane, creating transient pores that allow the DNA to pass into the cell.
Electroporation relies on a physical force rather than chemical treatment. The bacterial cells are mixed with the DNA and subjected to a high-voltage electrical pulse. This electrical charge causes a momentary and reversible breakdown of the cell membrane’s structure, opening temporary nanoscale pores. The electrical potential gradient drives the DNA into the cell through these pores.
Identifying Successful Transformations
Following the introduction of foreign DNA, scientists must distinguish the few cells that successfully incorporated the DNA from the vast majority that did not. This identification relies on selection, a technique that uses a specific genetic component engineered into the introduced DNA molecule, known as a selectable marker gene.
The most common selectable marker is an antibiotic resistance gene, such as resistance to ampicillin or kanamycin. The foreign DNA, typically a plasmid, carries both the gene of interest and this resistance marker. After the transformation procedure, the bacteria are cultured on a solid growth medium that contains the corresponding antibiotic.
Only bacterial cells that successfully took up and are expressing the resistance gene can survive and multiply on the selective plate. Non-transformed cells, which lack the resistance gene, are unable to grow and die off. The resulting colonies are confirmed to be successful transformants, ready for further study or use.