Horizontal Gene Transfer in Bacteria

Introduction to Genetic Information Transfer

Genetic information in bacteria can be transferred both vertically (from parent to offspring) and horizontally (between cells of the same generation). Horizontal gene transfer (HGT) is a major driver of genetic diversity and evolution in prokaryotes.

• Vertical gene transfer: Transmission of genetic material from parent to offspring during reproduction.

• Horizontal gene transfer: Movement of genetic material between organisms of the same generation, not involving reproduction.

Key Point: HGT allows bacteria to rapidly acquire new traits, such as antibiotic resistance or metabolic capabilities, from unrelated organisms.

Genetic Elements Involved in HGT

Plasmids

Plasmids are small, circular, double-stranded DNA molecules that replicate independently of the bacterial chromosome. They often carry genes beneficial to the host, such as antibiotic resistance or virulence factors.

• Fertility (F) plasmids: Carry genes for sex pili and enable transfer of the plasmid via conjugation.

• Dissimilation plasmids: Carry genes for the catabolism of unusual substances.

• Resistance (R) factors: Carry genes that confer resistance to antibiotics.

Transposons

Transposons, or "jumping genes," are segments of DNA that can move from one location to another within a genome or between DNA molecules. They are found in both prokaryotes and eukaryotes.

• Insertion sequences (IS): The simplest transposons, containing only the gene for transposase (an enzyme that catalyzes movement) and recognition sites.

• Complex transposons: Carry additional genes, such as those for antibiotic resistance.

Mechanisms of Horizontal Gene Transfer

Transformation

Transformation is the uptake of "naked" DNA fragments from the environment by a bacterial cell. Some bacteria are naturally competent, while others can be made competent in the laboratory.

• Natural competence: The ability of certain bacteria to take up DNA from their environment without artificial manipulation.

• Artificial competence: Laboratory techniques (e.g., chemical treatment, electroporation) used to induce DNA uptake in bacteria.

Example: Griffith's Experiment

• Demonstrated transformation in Streptococcus pneumoniae using encapsulated (virulent) and non-encapsulated (non-virulent) strains in mice.

• Showed that heat-killed virulent bacteria could transfer genetic material to live non-virulent bacteria, making them virulent.

Steps in Transformation

1. Recipient cell takes up donor DNA from the environment.

2. Donor DNA aligns with complementary bases of the recipient chromosome.

3. Recombination occurs, integrating donor DNA into the recipient genome.

4. Unincorporated DNA is degraded.

Conjugation

Conjugation is the direct transfer of DNA from one bacterial cell (donor) to another (recipient) via cell-to-cell contact, typically mediated by a sex pilus encoded by the F plasmid.

• F+ cells: Contain the F plasmid and can initiate conjugation.

• F- cells: Lack the F plasmid and act as recipients.

• Hfr cells: Have the F plasmid integrated into their chromosome, allowing transfer of chromosomal genes during conjugation.

Key Point: Conjugation is a major mechanism for the spread of antibiotic resistance genes among bacteria.

Transduction

Transduction is the transfer of bacterial DNA from one cell to another via bacteriophages (viruses that infect bacteria).

• Generalized transduction: Random bacterial DNA is packaged into a phage during lytic infection and transferred to a new host.

• Specialized transduction: Specific bacterial genes near the prophage insertion site are transferred by temperate phages during lysogenic infection.

Summary Table: Comparison of HGT Mechanisms

Significance of Horizontal Gene Transfer

• Promotes genetic diversity and rapid adaptation in bacterial populations.

• Facilitates the spread of antibiotic resistance and virulence factors.

• Plays a key role in microbial evolution and the emergence of new pathogens.

Additional info:

• Electroporation is a laboratory technique that uses electrical pulses to increase cell membrane permeability, allowing DNA uptake for transformation.

• DNA uptake specificity can be influenced by sequence motifs recognized by bacterial competence machinery, as seen in Neisseria and Haemophilus species.