Bacteria are single-celled organisms that contain the genetic instructions for life encoded within a main structure called the chromosome. Many bacteria also possess additional, smaller loops of genetic material called plasmids. These extra genetic elements are separate from the main chromosome and significantly influence the bacterium’s capabilities. Plasmids provide insight into how bacteria survive, adapt, and evolve in the face of environmental pressures.
What Plasmids Are and How They Differ From Chromosomal DNA
Plasmids are small, circular pieces of double-stranded DNA that exist separate from the bacterial chromosome within the cell’s cytoplasm. They are a form of extrachromosomal DNA, meaning they are not integrated into the main genetic structure. Unlike the large bacterial chromosome, which carries the genes required for fundamental survival, plasmids typically carry a small number of genes. Their size is highly variable, ranging from less than one kilobase pair to over 400 kilobase pairs in length.
A defining characteristic of plasmids is their ability to replicate autonomously, possessing their own origin of replication. This allows them to make copies of themselves independently of the chromosome’s replication cycle. Genes encoded on a plasmid are considered non-essential for basic cell function, but they often provide the bacterium with a selective advantage. This might include traits that help the host cell survive in a challenging environment.
The Quantitative Answer: Variability in Plasmid Copy Number
The number of plasmids a bacterium has is highly variable, depending on the specific plasmid and the host cell. Some bacteria contain no plasmids, while others can harbor multiple different types simultaneously. Furthermore, a single type of plasmid can be present in a wide range of identical copies within one cell.
Scientists use the term “plasmid copy number” (PCN) to describe the average number of a particular plasmid found inside a host cell. This number can vary significantly, typically ranging from a single copy to well over 100 copies per cell. This range is divided into two categories based on the mechanism controlling their replication.
Low-copy number plasmids are maintained at a limited number, often between one and 15 copies per cell. Their replication is tightly controlled and linked to the cell division cycle. High-copy number plasmids, conversely, can exist in 20 to 100 or more copies within the same bacterium. These plasmids have a relaxed control over their replication, allowing them to accumulate to higher levels.
Genetic Superpowers: Functions Plasmids Provide
The significance of plasmids lies in the specialized functions their genes provide, offering a genetic advantage for the host bacterium. These accessory genes enable the bacteria to thrive in conditions that would otherwise be restrictive.
The most widely recognized function is antibiotic resistance, encoded on R-plasmids. These resistance genes allow the bacterium to produce enzymes that inactivate an antibiotic, alter the cellular target the drug binds to, or pump the drug out of the cell.
Plasmids also frequently carry genes that enhance the bacterium’s ability to cause disease, known as virulence factors. A virulence plasmid might encode a toxin that damages host tissues or proteins that allow the bacterium to evade the host’s immune system. The presence of these genes can transform a harmless bacterium into a pathogenic one.
A third major function involves metabolic capabilities, often referred to as degradative functions. Some plasmids carry instructions for enzymes that allow the bacterium to break down unusual or toxic compounds, such as petroleum components, herbicides, or heavy metals. This enables the bacteria to utilize these substances as a food source or survive in polluted environments.
How Plasmids Spread Between Bacteria
The impact of plasmids is amplified by their ability to move freely between individual bacteria, even those of different species. This process, called horizontal gene transfer (HGT), is a major driver of bacterial evolution and the rapid spread of advantageous traits like drug resistance.
The most common method for plasmid transfer is conjugation, often described as bacterial mating. Conjugation involves direct physical contact between a donor bacterium, which possesses the plasmid, and a recipient cell. The donor cell extends a specialized protein appendage known as a sex pilus to attach to the recipient. A channel then forms between the cells, and a copy of the plasmid DNA is transferred from the donor to the recipient. This highly efficient process is the primary mechanism for disseminating antibiotic resistance genes.
Plasmids can also be transferred through two other mechanisms: transformation and transduction.
Transformation
Transformation occurs when a bacterium takes up naked DNA, including plasmid DNA, that is freely floating in the environment. This DNA is often released from dead bacterial cells.
Transduction
Transduction involves a bacteriophage, a virus that infects bacteria, which accidentally packages a fragment of plasmid DNA instead of its own genetic material. The phage then injects this plasmid DNA into a new host cell, facilitating the transfer.