A gene is a fundamental unit of heredity, a specific sequence of DNA that holds the instructions for building a functional product, such as a protein. While genes dictate traits in all organisms, the way they are organized, expressed, and exchanged in single-celled bacteria differs significantly from more complex life forms. These differences provide bacteria with remarkable adaptability and have profound consequences for the environment, human health, and technology.
The Bacterial Genome
Unlike plant and animal cells that store genetic material in a nucleus, a bacterium’s genetic blueprint is in the cytoplasm. The primary structure is the bacterial chromosome, a single, circular molecule of double-stranded DNA. This chromosome is highly compacted into an irregularly shaped region known as the nucleoid, where it is organized by proteins and RNA to fit within the cell.
In addition to the main chromosome, many bacteria harbor one or more plasmids. These are small, circular DNA molecules that exist and replicate independently of the chromosome. Plasmids are not necessary for a bacterium’s daily survival but often carry specialized genes that provide an advantage in certain situations, such as genes conferring antibiotic resistance.
Gene Expression and Regulation
Activating a gene to create a protein involves two stages: transcription and translation. In bacteria, these events occur almost simultaneously within the cytoplasm. This efficiency allows bacteria to respond rapidly to changes in their surroundings, such as the appearance of a new food source.
A feature of bacterial gene regulation is the operon, a cluster of genes controlled by a single promoter or “on-off” switch. This arrangement allows a bacterium to produce a whole set of related proteins at once. The lac operon in E. coli is a well-studied example containing genes that code for enzymes to digest lactose. When lactose is present, it triggers the operon to turn on, and when it is absent, the operon is switched off to prevent wasteful protein production.
Horizontal Gene Transfer
Bacteria can share genetic information with one another through a process known as horizontal gene transfer (HGT). This mechanism is distinct from the vertical inheritance of genes from a parent to its offspring and allows for the rapid spread of genetic traits through a population. HGT is a major driver of bacterial evolution and occurs through several distinct pathways.
- Transformation is a method where a bacterium takes up fragments of DNA that are free-floating in its environment, often released from other bacteria that have died.
- Transduction involves bacteriophages, which are viruses that infect bacteria. During an infection, a piece of the host bacterium’s DNA can be accidentally packaged into a new virus particle and transferred to the next bacterium it infects.
- Conjugation is the most direct method, where two bacteria make physical contact. One bacterium extends a thin, tube-like structure called a pilus to the other, transferring a copy of a plasmid or a portion of the chromosome.
This direct exchange is a highly efficient way to spread advantageous genes, such as those for antibiotic resistance, through a bacterial community.
Impact on Human Health
The transfer of genes between bacteria has significant consequences for human health, primarily by spreading antibiotic resistance and bacterial virulence. When a bacterium acquires a gene that allows it to survive an antibiotic, HGT can quickly transform a susceptible bacterial population into a resistant one. This renders the antibiotic ineffective for treating infections and is a major contributor to the global health challenge of antibiotic resistance.
HGT also facilitates the spread of virulence factors, which are genes that enable bacteria to cause disease. These factors can include instructions for producing toxins or building structures to attach to human cells. A harmless bacterium can become a dangerous pathogen by acquiring a plasmid carrying virulence genes. For example, some strains of E. coli are harmless gut residents, while others cause severe illness because they have acquired toxin-producing genes through HGT.
Harnessing Bacterial Genes in Biotechnology
Beyond their role in disease, the genetic systems of bacteria have been harnessed by scientists to develop powerful biotechnological tools. Bacterial plasmids have become fundamental in genetic engineering. Scientists can insert a gene of interest, such as the human gene for insulin, into a plasmid and then introduce that modified plasmid into bacteria. As the bacteria multiply, they express the inserted gene, turning the microorganisms into factories for producing valuable proteins.
The field of biotechnology has also been advanced by discoveries from the bacterial world, such as the CRISPR-Cas9 system. This technology was originally identified as an adaptive immune system that bacteria use to defend themselves against viruses. Scientists have adapted this natural mechanism into a precise tool that can cut and modify DNA sequences in many organisms, including humans, opening new possibilities for treating genetic diseases.