Horizontal gene transfer (HGT) describes the movement of genetic material between organisms through means other than direct parent-to-offspring inheritance. This process allows bacteria to acquire new genetic information from their environment or other bacteria. Unlike vertical gene transfer, where genes are passed down through reproduction, HGT enables a rapid and flexible exchange of traits, even between different bacterial species.
Mechanisms of Gene Exchange
Bacteria employ several distinct mechanisms for the lateral exchange of genetic material.
Transformation
One such mechanism is transformation, where bacteria directly pick up free-floating DNA from their surroundings. This DNA often originates from dead bacterial cells that have released their contents into the environment. For a bacterium to take up this “naked” DNA, it must be in a state called “competence,” which involves specific cellular machinery that allows DNA to pass through its membrane. Once inside, the acquired DNA can be integrated into the recipient bacterium’s own genetic material, potentially giving it new capabilities.
Transduction
Another method is transduction, which involves bacteriophages, viruses that specifically infect bacteria. During a phage infection, the virus replicates inside the bacterial cell, and sometimes, fragments of the host bacterium’s DNA are accidentally packaged into new viral particles instead of viral DNA. When these “loaded” phages then infect another bacterium, they inject the bacterial DNA from the previous host into the new recipient cell. This transfer allows genetic material to move between bacteria without direct cell-to-cell contact.
Conjugation
Conjugation represents a third common pathway, often described as bacterial “sex” due to the direct physical contact involved. A donor bacterium extends a specialized structure called a pilus, or sex pilus, to connect with a recipient bacterium. Through this bridge, a copy of genetic material, often in the form of a small circular DNA molecule called a plasmid, is transferred from the donor to the recipient cell. These plasmids can carry various genes, and their transfer can rapidly disseminate traits through a bacterial population.
The Role in Bacterial Evolution
Horizontal gene transfer significantly influences bacterial evolution, enabling bacteria to adapt to new conditions far more quickly than through random mutations alone. For instance, a bacterium might gain the ability to metabolize a different food source or survive in extreme temperatures by acquiring genes through HGT.
This genetic flexibility contributes to the immense diversity seen in bacterial populations. It allows for the swift spread of beneficial traits, promoting survival and proliferation in various ecological niches. The exchange of genetic material reshapes microbial genomes. Estimates suggest that a substantial portion of a bacterium’s genome, possibly around 20% in species like Escherichia coli, may have originated from horizontal gene transfer events.
Consequences for Medicine and Disease
Horizontal gene transfer significantly impacts human health, particularly concerning infectious diseases. Its most significant concern is its role in the rapid spread of antibiotic resistance genes among bacteria. A bacterium that acquires a gene conferring resistance to an antibiotic can quickly transfer this gene to other bacteria, even those of different species, rendering once-effective drugs powerless. This widespread dissemination contributes to the global challenge of drug-resistant infections.
For example, resistance genes for antibiotics like tetracycline and β-lactams have spread horizontally into various bacterial strains, including methicillin-resistant Staphylococcus aureus (MRSA) strain CC398. Beyond antibiotic resistance, HGT also facilitates the spread of virulence factors, which are genes that enhance a bacterium’s ability to cause disease. These factors can include genes for producing toxins that harm host cells, adhesins that help bacteria stick to tissues, or invasion factors that enable bacteria to enter host cells. The transfer of such genes can transform a harmless bacterium into a pathogen or increase the severity of an existing infection.
Applications in Biotechnology
Scientists have learned to harness the natural processes of horizontal gene transfer to develop various biotechnological applications. Genetic engineering techniques frequently employ principles similar to HGT to introduce new genetic material into organisms for specific purposes. This controlled transfer allows for the modification of bacteria to produce valuable compounds.
For instance, bacteria have been engineered to produce human insulin for medical use, a process that relies on inserting the human insulin gene into bacterial cells. Similarly, HGT-inspired methods are used to modify bacteria to produce enzymes for industrial applications, such as those found in laundry detergents. These techniques also extend to bioremediation, where bacteria are engineered to break down pollutants in the environment.