Microbial Genetics

Genetic Transfer Overview

• Microbes can receive DNA from donor cells.

• When recipient incorporates donor DNA into its genome, it is called recombinant.

• Unused DNA is degraded by cellular enzymes.

Ways of Genetic Transfer

• There are three main methods of horizontal gene transfer in bacteria:

  1. Conjugation

  2. Transformation

  3. Transduction

Plasmids and Their Roles

• Plasmids are small, circular forms of DNA that can carry additional traits.

• Often not essential for survival but can provide competitive advantages.

Types of Plasmids:

1. F Factor (Conjugative Plasmid):

   • Carries gene for synthesizing sex pili.

   • Enables the transfer of the plasmid from one bacterial cell to another during conjugation.

2. Virulence Plasmid:

   • Contains genes that increase pathogenicity.

   • Example: A strain of E. coli causing severe diarrhea in infants has a virulence plasmid; non-plasmid strains are harmless.

3. Dissimilation Plasmid:

   • Encodes for enzymes that metabolize various sugars and hydrocarbons, aiding survival in different environments.

4. Choleplasmid (Cholechinogenic Factor):

   • Allows microbes to produce cholecines, proteins with antibiotic activity against other microbes.

5. Resistance Factor (R Factor):

   • Carries genes that confer resistance against antibiotics, metals, and toxins.

   • Composed of two major components:

     • RTF (Resistance Transfer Factor) - essential for the transfer of resistance genes.

     • r Determinant - specifies the type of resistance.

Conjugation Process:

• Requires cell-to-cell contact between an F positive (donor) cell and an F negative (recipient) cell.

• The F positive cell forms a conjugation pilus to connect with the F negative cell.

• Plasmids can replicate independently and can transfer a copy to the recipient cell, converting it to F positive.

• Surface Exclusion: Only F positive cells can establish a connection with F negative cells, not other F positive cells.

• Restriction Enzymes in recipients can destroy the incoming plasmid, preventing successful transfer.

Transformation:

• Involves uptake of naked DNA from the environment by competent bacteria (those capable of taking in DNA).

• This process alters the genetic makeup of microbes, potentially introducing new traits.

Transduction:

• Bacterial DNA is transferred from one bacterium to another via a virus (bacteriophage).

• A virus can accidentally package bacterial DNA and transmit it to another bacterial cell during infection.

Summary of Genetic Transfer Mechanisms:

• Conjugation: Direct transfer via physical connection.

• Transformation: Uptake of free DNA from the environment.

• Transduction: Viral-mediated transfer of DNA between bacteria.