Bacteria share genetic material through horizontal gene transfer (HGT), a process distinct from vertical inheritance where traits pass directly from parent to offspring. HGT allows bacteria to acquire new genetic information from other bacteria, even different species, rather than solely through reproduction. This mechanism is a significant force in bacterial evolution and adaptation, enabling rapid changes in their characteristics and contributing to their survival and diversity.
Transformation
Transformation is a process where a bacterium takes up “naked” DNA directly from its environment, often from dead bacterial cells. For a bacterium to take up this foreign DNA, it must be in a specific physiological state called “competence,” which refers to the ability to bind and import extracellular DNA across its cell membrane.
Many bacteria are naturally competent, actively pulling DNA fragments into their cells using specialized proteins that form channels. Once inside, the foreign DNA can integrate into the recipient bacterium’s own chromosome or exist as a separate genetic element, such as a plasmid. This acquisition of new genetic material can introduce novel traits, leading to rapid adaptation.
Conjugation
Conjugation involves the direct transfer of genetic material between two bacterial cells through physical contact. This process begins when a donor bacterium extends a specialized, tube-like pilus to connect with a recipient, pulling the cells closer to facilitate DNA transfer.
During conjugation, small, circular plasmids are frequently transferred from the donor to the recipient cell. These plasmids exist separately from the main bacterial chromosome and can carry various genes, including those that confer new abilities. After DNA transfer, both cells synthesize a complementary DNA strand, resulting in both having a complete copy of the transferred material. This direct cell-to-cell contact effectively spreads genetic traits within bacterial populations.
Transduction
Transduction is a process where bacterial DNA is transferred from one bacterium to another by a bacteriophage, a virus that specifically infects bacteria. During its replication cycle within a host, a bacteriophage can accidentally pick up fragments of the host’s DNA.
When this phage infects a new bacterium, it injects the acquired bacterial DNA into the new recipient cell. This newly introduced DNA can then integrate into the recipient’s genome, transferring genetic traits. This mechanism allows for genetic exchange without direct cell-to-cell contact, relying on the viral vehicle.
Why Bacterial Genetic Transfer Matters
These methods of genetic transfer profoundly impact bacterial populations and human health. They are primary drivers of bacterial evolution, enabling rapid adaptation to changing environments. The ability to acquire new genes quickly allows bacteria to thrive in diverse niches and develop new capabilities, contributing to the vast diversity and adaptability observed in microbial life.
One significant consequence of bacterial genetic transfer is the spread of antibiotic resistance. Genes conferring resistance can be readily transferred between bacteria through transformation, conjugation, and transduction. This horizontal dissemination means that once a resistance gene emerges, it can rapidly spread to other bacterial species, including those causing human infections. The widespread transfer of resistance genes poses a serious global health challenge, limiting treatment options and contributing to the rise of multidrug-resistant strains.