Horizontal Gene Transfer and Antibiotic Resistance

Samuel E. Kelly Ethnic Cultural Center

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Introduction to Horizontal Gene Transfer (HGT)

  • Core Concepts: Horizontal gene transfer (HGT), also known as lateral gene transfer, refers to any process through which genetic material is transferred between organisms.

  • Distinction from Vertical Transfer:     * Vertical Transfer: This is the traditional transmission of genes from a parental generation to its offspring.     * Horizontal Transfer: This involves the acquisition of DNA from other organisms within the same generation, often crossing species boundaries.

  • Metaphorical Representation of Antibiotic Resistance: The transcript includes a scenario depicting the "Antibiotic Resistance" as a group. A character named Albert, while taking a short-cut through medical hospital kitchens, is approached by a member of the Resistance who says: "Pssst! Hey kid! Wanna be a Superbug..? Stick some of this into your genome… Even penicillin won't be able to harm you..!"

  • Learning Objectives:     * Understand the three major mechanisms for DNA exchange between bacteria: DNA-mediated transformation, Transduction, and Conjugation.     * Understand the rapid transfer of antibiotic resistance between bacteria.     * Build a foundational understanding of DNA transfer in bacteria and other species as a basis for understanding cloning and genetic engineering.

Mechanisms for DNA Lasting and Integration

  • Requirement for Replication: For donor DNA to "last" within a recipient cell and its subsequent progeny, it must be in a form that can be replicated by the cell's replication machinery.

  • Forms of Persistent DNA in Bacteria:     * The DNA must be a circular chromosome (or a mini-chromosome/plasmid) that possesses its own origin of replication.     * Alternatively, the donor DNA must be integrated directly into the recipient's existing chromosome.

  • Integration via Homologous Recombination:     * This is an enzyme-catalyzed process.     * Some of the same enzymes utilized in DNA repair facilitate homologous recombination.     * It involves the exchange of genetic material between DNA molecules with similar sequences.

  • Site-Specific Recombination: This is another distinct enzymatic process that integrates DNA into the genome at specific, predetermined sequences.

DNA-Mediated Transformation

  • Definition: Transformation is the process by which a bacterial cell takes up "naked" DNA from the surrounding environment. This DNA is typically released when nearby cells are lysed.

  • Historical Context - Griffith’s Experiments (1928):     * Frederick Griffith demonstrated that the ability to synthesize a capsule (and thus become virulent) could be transferred from heat-inactivated "smooth" (S) strains to living but avirulent "rough" (R) strains of Streptococcus pneumoniae.

  • Historical Context - Avery–MacLeod–McCarty (1944):     * These researchers demonstrated that DNA alone (naked DNA) was the specific component sufficient to cause the phenotypic change observed in Griffith's experiments.

  • Competence: This refers to the specific physiological state or ability of a cell to take up extracellular or naked DNA. Only certain species are naturally competent.

  • Process Detail:     * The intake of naked DNA occurs.     * The process involves the replication of a heteroduplex (a double-stranded DNA molecule with different sequences on each strand).     * This replication generates one wild-type cell and one recombinant cell.

  • Environmental Sensitivity: Transformation is known to be sensitive to the addition of DNase (an enzyme that degrades DNA) in the extracellular environment.

  • Real-World Example - Seaweed Digestion:     * The human gut hosts thousands of microbial species containing approximately 500,000500,000 genes.     * Some genes break complex carbohydrates into energy-rich fatty acids for human use.     * Gut bacteria in certain populations (e.g., in Asia) have acquired DNA through HGT from marine organisms that allow them to digest complex carbohydrates found in seaweed and marine algae.

Signatures of Horizontally Acquired Genes

  • GC Content: Genes acquired horizontally often have a different ratio of Guanine-Cytosine (GC) pairs compared to the rest of the host genome.

  • Evolutionary History: These genes often display an evolutionary history or phylogenetic lineage that differs significantly from the host organism.

Bacterial Transduction

  • Definition: Transduction occurs when DNA is transferred from one bacterium to another via a bacteriophage (a virus that infects bacteria).

  • Generalized Transduction:     * Occurs due to an error in the phage replication cycle during packaging.     * Can involve either lytic or temperate phages.     * Formation of a Transducing Particle: During the construction of viral particles, bacterial DNA fragments (created when phage enzymes cut up the host genome) accidentally enter a protein coat instead of phage DNA.     * Process: The transducing particle attaches to a new host cell and injects the bacterial DNA. This DNA then integrates into the new host chromosome via homologous recombination.

  • Specialized Transduction:     * Involves only temperate phages (viruses that can choose between lytic and lysogenic pathways).     * Incorrect Excision: Occurs when a prophage (phage DNA integrated into the host genome) is excised incorrectly from the bacterial chromosome. This results in the phage taking adjacent bacterial genes with it.     * Defective Phage: The resulting phage particles are defective because they leave some phage genes behind and carry some bacterial DNA flanking the integration site. Only DNA flanking the specific integration site can be transferred this way.

  • Phage Type Definitions:     * Lytic Phage: A virus that can only replicate and eventually lyse (destroy) the host cell.     * Temperate Phage: A virus that can integrate its DNA into the host genome (lysogeny) or enter the lytic cycle.

Bacterial Conjugation

  • Definition: Conjugation is the transfer of DNA from one bacterium to another requiring direct cell-to-cell contact. It is described as the closest bacteria come to having sex.

  • The F-Plasmid (Fertility Plasmid): Serves as the primary model for understanding conjugation.

  • Donor Classifications:     * F+F^+: Contains the fertility plasmid and acts as the donor ("male").     * F−F^-: Lacks the fertility plasmid and acts as the recipient ("female").     * Hfr: High frequency of recombination cells where the F plasmid has integrated into the chromosome.     * F′F': Mentioned as a third type (though not covered in detail in the course).

  • The Mechanism of Plasmid Transfer:     1. Making Contact: The F+F^+ cell uses an F pilus to contact the F−F^- cell.     2. Initiating Transfer: One strand of the F plasmid is cut at the "origin of transfer."     3. Transfer and Synthesis:         * In the donor cell, replication replaces the strand being transferred.         * In the recipient cell, the complement to the transferred single strand is synthesized.     * Result: The process follows the equation F++F−→F++F+F^+ + F^- \rightarrow F^+ + F^+. Both cells end up as F+F^+ donors.

  • R-Plasmids: These are specific plasmids (Resistance plasmids) that encode resistance to one or more antibiotics. Many R-plasmids are conjugative, allowing for rapid spread of resistance.

The Mobile Gene Pool (Mobilome)

  • Core Genome: Consists of conserved genes that are present in all strains of a particular species.

  • Mobile Genetic Elements (Mobilome): Includes genetic components that can move within or between genomes, contributing to genomic variability. These include:     * Plasmids.     * Transposons.     * Genomic Islands.

  • Implications: The mobile gene pool is a major driver of evolution and allows for the rapid transfer of antibiotic resistance, even between different species of bacteria.

  • Variation Example: The transcript references genomic variability between strains of the bacterium Aggregatibacter actinomycetocomitans (Aa) which can be observed at http://expression.washington.edu/genetable/script/gene_table_viewer?organism=aa_ha&build=12_06_06.

  • Note on Additional Mechanisms: While transformation, transduction, and conjugation are the primary focus, other mechanisms like "Gene Transfer agents" and "Vesiduction" exist but were not discussed in detail.