Bacteria exchange genetic information directly with one another, a process that allows for their rapid adaptation. This differs from the reproductive strategies of complex organisms. Instead of relying solely on inheritance from a parent cell, bacteria can acquire new genes from their neighbors, including those from different species. This capacity for genetic exchange underlies their evolutionary success and resilience.
Horizontal Versus Vertical Gene Transfer
The transmission of genetic material in bacteria occurs in two primary ways. Vertical gene transfer is analogous to inheritance in animals. A parent bacterium divides through binary fission, and its genetic material is passed down to its offspring. This ensures the continuation of the species’ genetic lineage from one generation to the next.
Horizontal gene transfer (HGT) involves the movement of genetic material between existing, unrelated bacteria. A bacterium can acquire new genes from a neighbor, even one from a different species. This direct exchange allows for much faster acquisition of new traits compared to the slower process of random mutations accumulating through vertical transfer. The primary distinction is that vertical transfer is generational, while horizontal transfer occurs between individuals of the same generation.
The Three Pathways of Horizontal Gene Transfer
Bacteria employ three main strategies for horizontal gene transfer. Each pathway uses a different method to move DNA from a donor cell to a recipient, allowing for genetic recombination and the creation of hybrid genomes.
One method is transformation, where a bacterium takes up “naked” DNA from its environment. This genetic material comes from bacterial cells that have died and broken apart, releasing their contents. For a recipient to absorb this DNA, it must be in a state of “competence,” where its cell wall and membrane are permeable. The new DNA can then be integrated into the recipient’s genome.
A second pathway is transduction, which involves viruses that infect bacteria, called bacteriophages. During their replication cycle inside a host, a bacteriophage might accidentally package a fragment of the host’s DNA into a new virus particle instead of its own viral DNA. When this phage infects another bacterium, it injects the DNA from the previous host, transferring genes between them.
The third mechanism is conjugation, which requires direct physical contact between two bacterial cells. Often described as “bacterial mating,” a donor cell extends a tube-like structure called a pilus to a recipient cell. Through this bridge, the donor transfers a copy of its DNA. Conjugation is the most common mechanism for HGT and can transfer genes between different species.
Genetic Material on the Move
The genetic information transferred between bacteria is carried on mobile genetic elements. These are segments of DNA that can move within a genome or be transferred between cells, which is a factor in the rapid spread of new traits.
Plasmids are common vehicles for gene transfer. These are small, circular DNA molecules that exist separately from the main bacterial chromosome. Plasmids replicate independently and are transferred from one bacterium to another during conjugation. They contain genes that provide an advantage in specific environments.
Another mobile element is the transposon, or “jumping gene.” Transposons are DNA sequences that can move from one location to another within a cell’s genome, aided by an enzyme called transpoase. A transposon can jump from the bacterial chromosome to a plasmid, which increases its chances of being transferred to another bacterium during conjugation.
Impact on Antibiotic Resistance and Adaptation
The transfer of genes among bacteria has significant consequences, most notably the spread of antibiotic resistance. When a bacterium develops a gene to survive an antibiotic, HGT can pass this resistance gene to other bacteria. This process explains how “superbugs” capable of resisting multiple antibiotics can emerge and spread in healthcare settings and the environment.
Plasmids are the primary carriers of these antibiotic resistance genes, and conjugation is a main method for their transfer. When a recipient bacterium acquires a resistance plasmid, it becomes resistant and can become a new donor. The use of antibiotics can accelerate this process, as it eliminates susceptible bacteria and allows resistant ones to thrive and share their genes.
This gene sharing allows bacteria to adapt in other ways. HGT enables bacteria to acquire genes to break down new food sources or survive in harsh environments. A bacterium might gain the ability to metabolize industrial pollutants or to colonize a new host. This exchange of genetic information drives the diversification and resilience of bacteria.