Bacterial Genetics in Microbiology: Conjugation, Transduction & Transformation

Bacteria are remarkable organisms that continuously fascinate scientists and students alike. They have an incredible ability to alter and share their genetic information, resulting in rapid adaptation and survival. These adaptations often lead to important phenomena such as antibiotic resistance and the creation of novel metabolic pathways.

When we talk about bacterial genetics in microbiology, we focus on how bacteria inherit and exchange their genetic material. Besides the straightforward passing of genes from a parent cell to its offspring (known as vertical transmission), there are also types of bacterial genetics that allow for gene transfer between two unrelated bacterial cells—referred to as horizontal gene transfer. This horizontal transfer primarily happens by:

1. Conjugation in bacteria

2. Transduction in bacteria

3. Transformation in bacteria

In the sections below, we will dive into each method of horizontal gene transfer, explore their mechanisms in detail, and highlight some unique aspects that make them central to understanding bacterial evolution.

Vertical vs Horizontal Gene Transfer

• Vertical Gene Transfer: This occurs when bacteria reproduce by binary fission, creating two identical daughter cells that inherit the mother cell’s genes.

• Horizontal Gene Transfer: Involves passing genetic material between two existing (and sometimes unrelated) bacterial cells. This process is crucial because it allows for the rapid spread of new traits such as antibiotic resistance.

Conjugation in Bacteria

Conjugation is a form of horizontal gene transfer that relies on direct physical contact between two bacterial cells. It often involves a plasmid (like the F-factor in Escherichia coli), although certain chromosomal genes can also be transferred.

Key Steps in Bacterial Conjugation

1. Formation of Sex Pilus

• The donor cell (often termed F+) produces a sex pilus—a hair-like structure that attaches to the recipient (F–) cell.

• This pilus then retracts, pulling the two cells closer together.

2. Creation of Conjugation Bridge

• A bridge or channel forms between the donor and recipient cells, establishing a direct route for DNA transfer.

3. Transfer and Replication of Plasmid DNA

• The F-factor (plasmid) is nicked at its origin of transfer.

• One strand migrates into the recipient cell while the donor retains the other strand.

• Both donor and recipient synthesise a complementary DNA strand, ensuring each cell ends up with a copy of the plasmid.

4. Completion and Separation

• After the plasmid is fully replicated in both cells, the conjugation bridge is dismantled.

• The recipient cell now becomes F+ and gains the ability to transfer genetic material to other cells.

Also Read: Microbiology

Hfr Conjugation

In certain strains, the F-factor can integrate into the bacterial chromosome, creating what is termed an Hfr (High-frequency recombination) strain. During Hfr conjugation, parts of the donor’s chromosomal DNA are also transferred to the recipient cell. This process can introduce new traits more extensively than simple plasmid transfer.

Transduction in Bacteria

Transduction in bacteria involves the transfer of genetic information through bacterial viruses called bacteriophages (phages). Phages infect bacterial cells, replicate within them, and can package bacterial DNA into new virus particles, distributing it to other bacteria.

Types of Transduction

1. Generalised Transduction

• Occurs during the lytic cycle of a bacteriophage.

• When phage particles are assembled, fragments of the host bacterial DNA can accidentally get packaged into the phage capsid.

• These “defective” phage particles can inject bacterial DNA into a new host, allowing for the exchange of genes.

2. Specialised Transduction

• Occurs with temperate phages that undergo the lysogenic cycle.

• The phage genome integrates into the bacterial chromosome and can remain dormant for many generations.

• Upon induction into the lytic cycle, the phage sometimes excises incorrectly, taking specific bacterial genes along with its own genome.

• These genes are then transferred to the next bacterium infected by the phage.

Why It Matters: Transduction is significant because it often moves genes responsible for toxin production or antibiotic resistance from one strain to another, contributing to the rapid evolution of pathogenic bacteria.

Transformation in Bacteria

Transformation in bacteria is the process by which a bacterial cell picks up free DNA from its surroundings. This free DNA might be from dead cells that lysed and released their genetic contents.

Explore: Genetic Engineering

Competence: The Ability to Take Up DNA

Not all bacteria can naturally take up DNA. Those that can are termed “naturally competent,” such as Streptococcus pneumoniae. Others are induced to become competent through laboratory techniques:

1. Chemical Competence

• Cells are treated with calcium salts (e.g., calcium chloride) at cold temperatures to create transient pores.

• A brief heat shock can then drive the external DNA into the bacterial cells.

2. Electroporation

• Bacterial cells are exposed to an electrical pulse, which temporarily disrupts the cell membrane.

• DNA enters through these transient pores.

Screening for Transformed Cells

To confirm successful transformation, scientists often place the bacteria on a medium containing antibiotics. Only those cells that have taken up the antibiotic resistance gene (part of the newly acquired DNA) will survive and grow, indicating successful transformation.

Beyond the Basics: Unique Insights on Bacterial Genetics

Here are a few additional points to deepen your understanding:

• CRISPR Systems: Many bacteria possess CRISPR (Clustered Regularly Interspaced Short Palindromic Repeats) as part of their immune systems against phages. By recognising specific phage DNA sequences, bacteria can destroy them.

• Multiple Plasmids: Some bacteria carry more than one plasmid simultaneously, each with its own origin of replication and unique functions (e.g., resistance to multiple antibiotics).

• Biofilm Communities: In biofilms, bacteria live in close contact. This increases the rate of conjugation in bacteria, as physical proximity makes gene transfer events more frequent.

• Medical Relevance: Understanding how antibiotic resistance genes spread through transduction in bacteria and transformation in bacteria is crucial for developing better control measures against multidrug-resistant strains.

Interactive Quiz: Test Your Knowledge

1. Which form of gene transfer requires direct contact between two bacterial cells?
a) Transduction
b) Conjugation
c) Transformation
d) Vertical Gene Transfer

2. What term describes bacteria capable of taking up DNA from their surroundings?
a) Transductive
b) Lysogenic
c) Competent
d) Resistant

3. In which transduction type can phages transfer only specific bacterial genes?
a) Generalised
b) Specialised
c) F-factor
d) Hfr

4. During conjugation, the donor bacterial cell is often referred to as:
a) F–
b) Hfr–
c) F+
d) R+

5. What is the main difference between vertical and horizontal gene transfer?
a) Vertical transfer involves plasmids; horizontal transfer involves chromosomes
b) Vertical transfer is from parent to offspring; horizontal transfer is among coexisting cells
c) Horizontal transfer only happens in viruses
d) Horizontal transfer requires identical cells

Check Your Answers

1. b) Conjugation

2. c) Competent

3. b) Specialised

4. c) F+

5. b) Vertical transfer is from parent to offspring; horizontal transfer is among coexisting cells