Bacterial Variation Types

Bacterial Variation

• Definition: Temporal or permanent changes in bacterial general characters during rapid growth and multiplication.
• 2 Types:
  • Phenotypic variation
  • Genotypic variation

Phenotypic Variation

• Cause: Adaptation of bacteria to external conditions, activating genes and leading to temporal changes in bacterial characters.
• Characters: Changes are usually reversible, unstable, and non-heritable.
• Examples:
  • Spore formation
  • Change in pigment production

Genotypic Variation

• Cause: Modification of a character due to changes in genes caused by mutations or gene transfer, leading to permanent changes in bacterial characters.
• Characters: Changes are heritable, permanent, irreversible, and stable.
• Consequences:
  • Non-functional genes
  • Acquiring functional genes with appearance of new characters, such as:
    • Antibiotic resistance
    • Toxin production

Bacterial Genetics

• Advantages of studying bacterial genetics:
  • Discovery of mechanisms of antimicrobial drug resistance
  • Facilitation of genetic engineering
• Bacteria have:
  • Chromosomal and extra-chromosomal genetic elements
  • Ability of horizontal transfer
  • High rate of division

Genome

• Definition: Genetic material and information in a cell, including chromosomes and plasmids.
• Chromosomes: Structures containing DNA that physically carry hereditary information, containing genes.
• Genes: Functional segments of DNA that code for functional products.

Mutation

• Definition: Genotypic variation due to change in the sequence of bases in the DNA, either by substitution, deletion, or insertion of bases.
• Characters:
  • Spontaneity: Natural
  • Discontinuity: Sudden
  • Rarity: Infrequent, but rate increases with exposure to mutagens
  • Stability: Inherited
• Types:
  • Substitutions
  • Deletions
  • Insertions
  • Inversions

Genetic Transfer

• Definition: Genotypic variation due to transfer of genes, either within the cell (transposition) or between cells (conjugation, transduction, and transformation).
• Types:
  • Transformation
  • Transduction
  • Conjugation

Importance of Genetic Transfer

• To organisms: As indicated below
• To research: As a vector for transferring genes into cells

Types of Plasmids

• According to function:
  • Fertility factor plasmids (F plasmid): Carry genes for transfer to another bacterium via conjugation
  • R plasmids: Carry drug resistance genes
  • Col plasmids: Carry genes for bacteriocins
  • Virulence plasmids: Carry virulence genes, e.g. toxin production
• According to structure:
  • Transmissible (conjugative) plasmids:
    • Can be transferred from cell to cell by conjugation
    • Contain transfer (tra) genes
    • Large in size
    • Usually present in a few copies per cell
    • Common in Gram negative bacteria
  • Non-transmissible (non-conjugative) plasmids:
    • Cannot be transferred from cell to cell
    • Do not contain transfer genes
    • Small in size
    • Frequently present in many copies per cell
    • Common in Gram positive bacteria

Bacterial Chromosome

• Definition: Large, covalently closed, circular DNA molecule
• Importance: Responsible for replication and protein synthesis
• Chromosomal replication:
  • Definition: Process of cell multiplication
  • Steps:
    1. Entire two strands of chromosome separate
    2. Each strand is attached to a mesosome and acts as a template on which a complementary strand is formed by the action of DNA polymerase enzyme
  • New cell is formed, each cell will contain a copy of the chromosome
  • DNA replication occurs only in dividing cells

Extra-chromosomal Genetic Elements

Transposon

• Definition: Extra-chromosomal elements that are generally different from chromosomes
• Characteristics:
  • Much smaller than the chromosome
  • Carry only genes not essential for the life of bacteria
  • Can be transferred within and between bacteria species

Plasmid

• Definition: Small, circular DNA molecules that can exist independently of the chromosome
• Characteristics:
  1. Much smaller than the chromosome
  2. Carry only genes not essential for the life of bacteria
  3. Can be transferred within and between bacteria species

Gene Expression

• Definition: Process of protein synthesis in which the sequence of nucleotides in a gene determines the sequence of amino acids in a protein
• Steps:
  • Transcription:
    • Separation of the DNA strands of certain gene in the chromosome
    • One strand acts as a template to form a complementary strand (messenger RNA; mRNA) by the RNA polymerase enzyme
  • Translation:
    • The entire mRNA is translated into corresponding sequences of amino acids forming proteins on the surface of the ribosome