Plasmid: Characteristics, Types, Functions, and Host Range

Plasmid

• Plasmids are the extrachromosomal genetic elements found in bacteria.
• They are circular pieces of DNA that are extra genes.
• About 1-20 copies of plasmids are present in one bacterial cell.
• Episomes are the type of plasmid that can be inserted into the bacterial chromosome and can replicate with it.
• For normal life and functioning, a plasmid is not required in the bacteria. But their presence confers new properties in the bacteria. Example: Drug resistance, toxigenicity

Properties/Characteristics of bacterial plasmids:

Physical properties:

• Circular DNA: Plasmids take the form of double-stranded circular DNA, often supercoiled.
• Autonomous Existence: They can exist autonomously within a cell, capable of replicating independently of the bacterial chromosome.
• Molecular Weight: Plasmids typically have a molecular weight ranging from 10⁶-10⁸ , encoding anywhere from 40 to 50 genes.
• Size Variability: These genetic elements can vary in size, ranging from 1500-400,000 base pairs. Plasmid as large as 2 million base pairs can occur in some bacteria.

Replication:

• Plasmids are not just passive passengers; they carry their own self-replication genes, ensuring their survival and propagation.

Curing:

• Interestingly, plasmids can be spontaneously lost or eliminated by specific curing agents.

Incompatibility:

• Two plasmids of the same group cannot coexist within the same cell, demonstrating a unique form of bacterial competition.

Transferability:

• While some plasmids prefer to keep to themselves, others are quite social. Self-transferable plasmids have the ability to move from one bacterial cell to another.

Recombinations:

• certain plasmids, known as episomes, can integrate with the host chromosome, blurring the lines between bacterial and plasmid DNA.

Mobilisation:

• Self-transferable plasmids can go a step further by mobilizing not only themselves but also chromosomal genes or other plasmids through the process of integration.

Types of plasmid:

• Based on their function, plasmids are of five types:
• Resistance ( R ) plasmid
• Fertility (F) plasmid
• Bacteriocinogen or Col plasmid
• Degradative plasmid
• Virulence plasmid

 1. R-plasmid (R-factor):

• They are circular with double-stranded plasmid.
• They come in two sizes: large and small, each with its specific role.
• Large R-Plasmids: These molecular heavyweights, with a molecular weight of 60 million, are conjugative ‘R’ factors. They code for the conjugation process and carry extra DNA.
• Small R-Plasmids: With a molecular weight of 10 million, these plasmids contain only the ‘r’ genes and are not conjugative. They play a crucial role in carrying resistant genes for various drugs.
• It consists of two components.
  • Resistance transfer factor (RTF): carries the genes that govern the process of intercellular transfer.
  • Resistant determinant ( R-determinant): carries resistant genes for each of the several drugs.
• The drug resistance is not transferrable in the case when RTF dissociates from the R-determinant.
• For the spread of the multiple drug resistance in the bacteria, R factor plays a vital role.
• Antibiotics can be destroyed and the membrane transport system can be modified.
• R-factor may carry the resistance genes either one, two, or more than these.
• They may also carry the gene resistance for the metal ions.
• They also carry resistance to certain bacteriophages by coding for the enzymes.

2. F-plasmids:

• Also known as F-factors, these plasmids are all about bacterial mating. They contain essential genetic information for extra-chromosomal existence, self-transfer, and the synthesis of sex-pilus.
• F-plasmid carries some fourteen genes which include the structural gene for the pilin.
• Pilin is the pilus protein that functions in sex pilus formation.
• Strains of bacteria having the F plasmid are called F₊ and function as donors.
• Strains of bacteria lacking the F plasmid are called F- and function as recipients.
• It is also called the conjugative plasmid.
• The conjugative function is determined by the cluster of at least 25 transfer (tra) genes.
• These genes determine: Expression of pili, synthesis and transfer of DNA during mating

3. Bacteriocinogen or Col plasmid:

• Coliform bacteria produce deadly toxins known as colicins. Col plasmids carry the genes responsible for producing these toxins, which target closely related species or even different strains of the same species.
• Some bacterial substances are produced not only by the coliforms but also by the other bacteria.
• This group of substances is called bacteriocins.
• Colicins are produced by coli
• Pyocin are produced by Pseudomonas aeruginosa.
• Marscesins are produced by Serratia marcescens.
• Diphthericin is produced by Corynebacterium diphtheria.
• Bacteriocin produced by the different bacterial strains helps in the interspecies typing of organisms.

4. Degradative plasmids:

• They help break down and digest dead organic matter, recycling it into energy and nutrients.
• It is then used in the biosynthesis process.

5. Virulence plasmids:

• With the help of virulence plasmids, harmless bacteria can transform into formidable pathogens. These plasmids carry genes responsible for causing disease, making them a potential threat in the microbial world.

Based on the role in conjugation, plasmids are of two types:

• Conjugative plasmid
• Non-conjugative plasmid

i. Conjugative plasmids:

• These large plasmids (F plasmids) carry genes that are responsible for transferring themselves to other cells. It includes the genes that direct the synthesis of sex pilli.

ii. Non-conjugative plasmids:

• These plasmids are present in Gram-positive bacteria, especially in the Gram-positive cocci. It is also present in the Gram-negative organism. Example: Haemophilus influenza, Neisseria gonorrhoeae. They are usually small, 1-10 dal. In each bacterium, multiple copies (more than 30 ) may be present. When the same bacterium carries both the conjugative and non-conjugative plasmids, they can be mobilized for transfer to another cell. When the conjugation is established then the donor can transfer non-conjugative plasmids.

Functions/Applications of Plasmids:

• Plasmids play a crucial role in spreading antibiotic resistance genes among bacteria, contributing to the development of antibiotic-resistant strains.
• They are widely used in recombinant DNA technology for gene cloning and expression.
• Plasmids are employed in gene therapy to introduce therapeutic genes into the body to treat diseases.
• Plasmids can carry genes involved in metabolic activities, aiding in the degradation of environmental pollutants.
• They can produce antibacterial proteins.
• Some plasmids enhance the pathogenicity of bacteria.
• Plasmids can fix nitrogen and degrade organic compounds, benefiting bacteria when nutrients are scarce.

Host Range of plasmid

• A plasmid’s host range refers to the types of bacteria in which it can replicate. This characteristic is often determined by the ori (origin of replication) region, which serves as the starting point for replication.
• Narrow Host Range Plasmids:
• Examples include ColE1 plasmids like pBR322, pET, and pUC.
• These plasmids primarily replicate in Escherichia coli (E. coli) but can also find a home in related bacteria like Salmonella and Klebsiella.
• Broad Host Range Plasmids:
• Examples include RK2, RSF1010 plasmids, and RC plasmids like pBBR1MCS.
• Plasmids with the ori region of RK2 can replicate in most types of Gram-negative proteobacteria.
• RSF1010-derived plasmids can even replicate in Gram-positive bacteria like Firmicutes.

Determining the Host Range

• It is sometimes difficult to ensure the particular plasmid will replicate on the other host or not. So, The actual host ranges of most plasmids are unknown.
• Initially, plasmids need to be introduced to the other bacteria.
• Experimental methods like transformation, electroporation, and conjugation are used to introduce plasmids into different bacteria and observe their replication.
• However, it’s essential to note that not all plasmids can thrive in every bacterial neighborhood due to various factors, including gene expression compatibility.
• Sometimes the selected gene can be introduced into the different bacteria.
• A bacterium might possess resistance to any antibiotic due to the presence of a particular gene. Such resistance property can be transferred to other bacteria too when they will uptake those resistant genes.
• For example, the kanamycin resistance gene, which is first found in the Tn5 It can be expressed in most Gram-negative bacteria. Then, it will make resistant to kanamycin antibiotic.
• By this property, a marker gene can be cloned in the plasmid. i.e making numerous copies.
• A transposon carrying a selectable marker into the plasmid can also be introduced by this method.
• Care must also be taken to ensure that the plasmid has not recombined into the host chromosome.
• Determining the host range of a plasmid is laborious too. Many barriers hinder the transfer of plasmid into the host. The same method can’t be approached to all sorts of plasmids and bacteria.

FAQs

1. Can plasmids be found in all types of bacteria?
   • Plasmids are more common in certain bacterial groups, but their presence is not universal. Some bacteria lack plasmids altogether.
2. How do plasmids contribute to antibiotic resistance?
   • Plasmids often carry genes that confer resistance to antibiotics. These genes can be transferred from one bacterium to another, spreading antibiotic resistance.
3. What is the significance of a plasmid’s host range?
   • A plasmid’s host range determines the types of bacteria in which it can replicate. This knowledge is crucial for various applications, including biotechnology and genetic engineering.
4. Can plasmids move between bacterial species?
   • Yes, plasmids can sometimes transfer between different bacterial species, especially if they carry genes that provide a survival advantage.
5. Are plasmids always beneficial for bacteria?
   • While plasmids can provide advantages such as antibiotic resistance, they can also be a burden on bacteria due to the energy required for their replication and maintenance.

Types of Plasmids: The Vital Players in Molecular Biology Research