Plasmid functions and phenotypic traits
Although bacteria can survive and reproduce without plasmids, their presence gives microorganisms significant advantages in the struggle for existence. Plasmids encode a number of important properties:
- Resistance: the ability to withstand antibacterial drugs.
- Pathogenicity: production of toxins, adhesion factors, and other virulence molecules that help cause disease.
- Antagonism: production of bacteriocins (e.g., colicins) that suppress 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 process of plasmid DNA duplication proceeds independently of the bacterial chromosome, yet utilizes the enzymatic machinery of the host cell itself.
Based on the replication control mechanism, two types of plasmids are distinguished:
- Strict control: their duplication is synchronized with the chromosome division cycle. As a result, a low copy number is maintained in the cell (only 1–2 plasmid copies).
- Relaxed control: replicate relatively freely, due to which their copy number is high and can reach 10 to 200 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 typically large DNA molecules containing a special tra operon. These genes are responsible for the synthesis of sex pili and the formation of a conjugation bridge through which a plasmid copy is transferred to the recipient cell.
- Non-transmissible (mobilizable): small plasmids without a tra operon. They are incapable of independent transfer. Their transfer is possible (mobilization) only if a transmissible plasmid providing its own conjugation machinery is present in the same cell.
Medical significance of plasmids
In clinical practice, the presence of plasmids in bacteria determines the severity of the infection and the difficulty of its treatment.
- R plasmids (from Resistance): carry genes for enzymes that inactivate antibiotics. Acquisition of such a plasmid can render a bacterium resistant to multiple drug classes simultaneously. It is R plasmids, often being transmissible, that are responsible for the rapid spread of multiple drug resistance and the emergence of nosocomial infection pathogens.
- Pathogenicity plasmids: determine virulence factors. They are characteristic of the causative agents of plague, anthrax, dysentery (Shigella), yersiniosis, and borreliosis. In E. coli, for instance, the Ent plasmid (responsible for enterotoxin) and Hly plasmid (hemolysin synthesis) are distinguished.
- Bacteriocinogenic (e.g., Col plasmids): encode toxins against bacteria of other strains, providing an advantage in niche colonization.
Briefly: answers to topical questions
What types of pathogenicity plasmids are identified in Escherichia coli (E. coli) and what do they code for?
Specific pathogenicity plasmids encoding the synthesis of virulence factors are identified in Escherichia coli.
- 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 allow DNA molecules to be cleaved into fragments at strictly defined sites.
- Methylases (modification enzymes) — modify the invading DNA, rendering 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 degrade or inactivate antibacterial drugs, and frequently confer resistance to multiple families of antibiotics simultaneously.
Enzymatic inactivation is indicated as a key resistance mechanism for:
- beta-lactams;
- tetracyclines;
- macrolides;
- chloramphenicol.
Also, among the main groups of enzymes inactivating antibiotics are:
- β-lactamases — destroy the β-lactam ring;
- aminoglycoside-modifying enzymes;
- chloramphenicol acetyltransferase.
The 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 weight of approximately $60 \cdot 10^6$ Da that controls the conjugation process. Its role is as follows:
- Codes for the synthesis of sex pili (F pili) in donor cells, which are necessary for the transfer of genetic material.
- In the autonomous state, ensures its own transfer to plasmid-less cells.
- When integrated into the chromosome (in Hfr strains), it initiates directed transfer of donor chromosomal genes to the recipient cell, while the transmissible plasmid itself is transferred to the latter, closing the circuit.
Class and exam questions
What is the difference between an episome and a regular plasmid?
An episome is a plasmid capable of reversible integration. It can reside 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 from antibiotics?
They contain genes serving as templates for the synthesis of enzymes capable of degrading or chemically inactivating antibacterial drug molecules.
What is the tra-operon and what is its function?
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.
What else to study on this topic
- 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
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