Plasmids

Q: What types of pathogenicity plasmids are identified in Escherichia coli, and what do they code for?

Specific pathogenicity plasmids are identified in Escherichia coli that encode the synthesis of virulence factors. Ent-plasmid — determines enterotoxin synthesis.Hly-plasmid — determines hemolysin synthesis. Additionally, E. coli harbors the Col-plasmid, which is responsible for bacteriocinogeny. It encodes the synthesis of colicins—substances that kill bacteria of other strains or closely related species, providing carrier bacteria with a competitive advantage when colonizing ecological niches.

For medical students2 min readUpdated 2026-10-10

Plasmids are extra-chromosomal, autonomous genetic elements in bacteria, represented by double-stranded DNA molecules. They are not essential for basic cell viability, but they encode auxiliary functions that provide bacteria with selective advantages, such as antibiotic resistance or pathogenicity.

StructureDouble-stranded DNA, size $10^3$ – $10^6$ base pairs

LocalizationBacterial cytoplasm (autonomous or integrated into the chromosome)

SignificanceEncode non-essential traits beneficial for survival

TransferCan be transferred between cells (conjugation)

Plasmid Functions and Phenotypic Traits

Although a bacterium can survive and reproduce without plasmids, their presence provides the microorganism with significant advantages in the struggle for existence. Plasmids encode several important properties:

Resistance: the ability to withstand the action of antibacterial drugs.
Pathogenicity: production of toxins, adhesion factors, and other virulence molecules that help cause disease.
Antagonism: production of bacteriocins (e.g., colicins) that inhibit the growth of competing microorganisms.
Metabolic features: the ability to ferment or degrade complex organic molecules.
Defense systems: synthesis of restriction and modification enzymes that protect DNA.

Replication, Copy Number, and Compatibility

The duplication of plasmid DNA proceeds independently of the bacterial chromosome, although it utilizes the enzymatic machinery of the host cell itself.

Based on replication control mechanisms, two types of plasmids are distinguished:

Strictly controlled: their duplication is synchronized with the chromosomal division cycle. As a result, a low copy number (only 1–2 plasmid copies) is maintained in the cell.
Relaxed control: replicate relatively freely, due to which their copy number is high and can reach 10 to 200 copies per bacterium.

An important concept is replicon compatibility. Two different plasmids are considered incompatible and cannot stably coexist in the same cell if their replication regulation mechanisms are very similar (they begin to compete). If the control mechanisms are different, the plasmids are compatible and can reside in the cytoplasm simultaneously.

Horizontal Gene Transfer (Transmissibility)

Plasmids play a key role in the exchange of genetic information between bacteria. According to their transfer capacity, they are divided into two groups:

Transmissible (conjugative): these are usually large DNA molecules containing a special tra-operon. These genes are responsible for the synthesis of sex pili and the formation of the conjugation bridge, through which a copy of the plasmid is transferred to the recipient cell.
Non-transmissible (mobilizable): small plasmids lacking the tra-operon. They are incapable of independent transfer. Their transfer is possible (mobilization) only if a transmissible plasmid is present in the same cell, providing its machinery for conjugation.

Medical Significance of Plasmids

In clinical practice, the presence of plasmids in bacteria determines the severity of the infection and the complexity of its treatment.

R-plasmids (from Resistance): carry genes for enzymes that inactivate antibiotics. Acquisition of such a plasmid can make a bacterium resistant to multiple classes of drugs simultaneously. It is R-plasmids, often being transmissible, that are responsible for the rapid spread of multidrug resistance and the emergence of nosocomial (hospital-acquired) pathogens.
Pathogenicity plasmids: determine virulence factors. They are characteristic of the causative agents of plague, anthrax, dysentery (Shigella), yersiniosis, and borreliosis. In Escherichia coli, for example, the Ent-plasmid (responsible for enterotoxin) and the Hly-plasmid (hemolysin synthesis) are distinguished.
Bacteriocinogenic (e.g., Col-plasmids): encode toxins against bacteria of other strains, providing an advantage in colonizing a niche.

Frequently asked questions

What types of pathogenicity plasmids are identified in Escherichia coli, and what do they code for?

Specific pathogenicity plasmids are identified in Escherichia coli that encode the synthesis of virulence factors.

Ent-plasmid — determines enterotoxin synthesis.
Hly-plasmid — determines hemolysin synthesis.

Additionally, E. coli harbors the Col-plasmid, which is responsible for bacteriocinogeny. It encodes the synthesis of colicins—substances that kill bacteria of other strains or closely related species, providing carrier bacteria with a competitive advantage when colonizing ecological niches.

What is the mechanism of action of restriction and modification enzymes encoded by plasmids?

The mechanism of action of these enzymes consists in protecting the bacterial cell from foreign genetic material by destroying or modifying it.

Endonucleases (restriction enzymes) — cleave foreign DNA that has entered the cell. They cut DNA molecules into fragments at strictly defined sites.
Methylases (modification enzymes) — modify the invading DNA, making it resistant to the destructive action of endonucleases.

Which classes of antibiotics are most frequently inactivated by enzymes encoded in R-plasmids?

R-plasmids contain genes encoding enzymes that destroy or inactivate antimicrobial drugs, often conferring resistance to multiple families of antibiotics simultaneously.

Enzymatic inactivation is listed as the key resistance mechanism for:

beta-lactams;
tetracyclines;
macrolides;
chloramphenicol.

Furthermore, the main groups of enzymes inactivating antibiotics include:

β-lactamases — destroy the β-lactam ring;
aminoglycoside-modifying enzymes;
chloramphenicol acetyltransferase.

Genes for such enzymes can be localized both in the chromosome and in plasmids.

What is the F-plasmid (F-factor) and what is its role in conjugation?

The F-plasmid (fertility plasmid) is an extrachromosomal genetic element with a molecular mass of about $60 \cdot 10^6$ Da that controls the process of conjugation. Its role is as follows:

Encodes the synthesis of sex pili (F-pili) in donor cells, which are necessary for the transfer of genetic material.
In its autonomous state, ensures its own transfer into plasmid-free cells.
In its integrated chromosomal state (in Hfr strains), initiates directed transfer of donor chromosomal genes into the recipient cell, while the transmissible plasmid itself is transferred last, completing the circuit.

What is the difference between an episome and a regular plasmid?

An episome is a plasmid capable of reversible integration. It can exist autonomously in the cytoplasm or integrate into the bacterial chromosome and replicate along with it as a single replicon.

How do R-plasmids protect bacteria against antibiotics?

They contain genes whose templates are used to synthesize enzymes capable of destroying or chemically inactivating antimicrobial drug molecules.

What is the tra-operon and what is its purpose?

This is a gene complex present in conjugative plasmids. It encodes the transfer machinery (including sex pili) required to transmit DNA from one bacterium to another.

Go deeper

Prokaryotic DNA replication mechanisms
The process of bacterial conjugation
Restriction and modification enzymes
Types of bacterial toxins and virulence factors
Use of plasmids as vectors in genetic engineering