Horizontal Gene Transfer in Bacteria

Horizontal Gene Transfer in Bacteria

Flow of Genetic Information

  • Vertical Gene Transfer
    • Transfers genetic information from one generation to the next
    • From parent to offspring
  • Horizontal Gene Transfer
    • Transfers genetic information between cells of the same generation
    • Parent to parent

DNA

  • Information Storage Molecule
    • Acts as a blueprint for organisms
    • Contains genes that encode for proteins
    • Transferred during cell division (Vertical Gene Transfer)
    • Obtained from cells within the same generation (Horizontal Gene Transfer)
    • Expressed by a cell or transferred to another cell through recombination and replication

DNA Expression, Recombination, and Replication

  • Expression
    • Cells utilize the information stored in DNA to synthesize proteins
  • Recombination
    • Genetic information can be exchanged between two DNA molecules
    • Creates genetic diversity
  • Replication
    • Genetic information is vertically transferred from a parent cell to a daughter cell

Plasmids and Transposons

  • Transposons
    • Mobile genetic elements
    • Capable of moving between chromosomes or from one cell to another
    • Found in prokaryotes and eukaryotes
  • Plasmids
    • Circular DNA structures
    • Self-replicating
    • Typically smaller (1-5% of bacterial chromosome size)
    • Often code for virulence factors
Types of Plasmids
  • Conjugative Plasmids
    • Contains genes for sex pili and transfer processes
  • Dissimilation Plasmids
    • Contains genes for enzymes needed to break down unusual compounds
  • Resistance Factors (R Factors)
    • Genes that confer antibiotic resistance

Mechanisms of Horizontal Gene Transfer

  • Transformation
    • Genetic transfer involving uptake of “naked” DNA by bacteria
    • Bacteria can be naturally competent
Transformation Process
  • Recipient cell takes up donor DNA from the environment
  • Donor DNA aligns with complementary bases on the recipient's chromosome
  • Recombination occurs, which can result in genetic diversity
  • Non-recombined DNA may be degraded
Griffith's Experiment
  • Demonstrated transformation in bacteria:
    1. Injected living encapsulated bacteria into a mouse → mouse died.
    2. Injected living non-encapsulated bacteria into a mouse → mouse remained healthy.
    3. Injected heat-killed encapsulated bacteria into a mouse → mouse remained healthy.
    4. Injected living non-encapsulated and heat-killed encapsulated bacteria into a mouse → mouse died.
  • Results from dead mouse:
    1. Isolated colonies of encapsulated bacteria.
    2. A few colonies of non-encapsulated bacteria were also isolated; phagocytes destroyed them, showing the effectiveness of the body's defenses against non-encapsulated bacteria.

Natural Competence of Bacteria

  • Many bacteria can naturally uptake DNA from their environments
  • This process allows for access to genetic material, increasing genetic diversity and survival capability

Conjugation

  • Involves direct DNA transfer from one bacterium to another through physical contact
  • F Factor
    • The plasmid responsible for the ability to conjugate
    • F+ cells (donors) transfer the F factor to F- cells (recipients), converting them to F+
  • High Frequency of Recombination (Hfr) Cells
    • F factor integrates into the bacterial chromosome, increasing the frequency of chromosomal transfer
    • Hfr cells can transfer a portion of their chromosome to F- cells, creating recombinant F- cells
Roles of Pili in Bacterial Conjugation
  1. Initiates contacts between mating pairs
  2. Facilitates transfer of genetic material
  3. Draws mating cells into close contact, which increases the frequency of successful conjugation events

Transduction

  • DNA transfer from a donor to a recipient through a bacteriophage
  • Generalized Transduction
    • Random bacterial DNA is packaged inside a phage and transferred to a new bacterial cell
  • Specialized Transduction
    • Specific bacterial genes are packaged and transferred via a phage
Steps in Generalized Transduction
  1. Phage infects the donor bacterium
  2. Phage proteins are synthesized; the bacterial chromosome is degraded
  3. DNA fragments are packaged into phage capsids
  4. The host cell lyses and releases phages
  5. A phage containing the bacterial DNA infects a new bacterium (recipient)
  6. Recombination can lead to a recombinant bacterial cell

Applications in Bacterial Genetics

  • Natural Transformation
    • Has major effects on bacterial adaptation and ecological diversity
    • Mechanisms similar across Bacteria and Archaea: conjugation, transduction, and transformation
  • Experimental Electroporation
    • Involves using electric fields to increase cell membrane permeability for DNA uptake
    • Used in gene therapy, drug delivery, and in research applications

Bacillus anthracis and Plasmid-encoded Virulence Factors

  • Virulence Factors
    • Necessary for pathogenicity
    • Encoded on two plasmids:
    • pXO1: Exotoxin components
    • pXO2: Capsule formation factor
  • Mechanism of Toxin Action
    • Protective antigen (PA) binds receptors on cells
    • PA serves as an entry mechanism for edema factor (EF) and lethal factor (LF)
    • EF increases intracellular cAMP levels, disrupting cellular processes
    • LF acts as a metalloprotease, disrupting immune responses

Bacillus cereus: Foodborne Illness Causative Agent

  • Responsible for 2-5% of foodborne illnesses
  • Symptoms include severe nausea, vomiting, and diarrhea
  • Caused by enterotoxins released upon bacterial growth in improperly cooked or stored food
  • Types of Illness:
    • Diarrheal: common, related to enterotoxins
    • Emetic: caused by cereulide toxin, which affects potassium channels in gut cells

Bacillus thuringiensis: Insecticidal Properties

  • Produces crystal toxins effective against insects
  • Encoded by cry genes; specific to various insect orders
  • Used in biological pest control due to its specificity and reduced environmental impact

Cry Gene Transgenic Plants

  • Genetic engineering developed in 1985 leading to crops producing Bt toxins
  • Provides internal protection from insect pests, minimizing the need for external pesticide application
  • Especially effective against specific pests like the European corn borer and Colorado potato beetle

Conclusion

  • Horizontal gene transfer mechanisms such as transformation, conjugation, and transduction are essential for bacterial adaptation and evolution
  • Understanding these mechanisms aids in the study of bacterial pathogenicity and the development of treatments and agricultural practices.