Characteristics of Bacterial Genetics
Bidirectional circular DNA replication
Transcription
- RNApol + Sigma Factor = Holoenzyme
Translation
- 1st amino acid is always fMet (Soluble PAMP)
Operon Concept
Gene regulation to conserve energy (switched on or off in specific situations)
Quorum Sensing
Bacterial population produces signal molecules (Inducers) that can cause adverse effects
Harmless type: low bacterial population density = low levels of inducers = inducers diffuse away before they can act
Aggressive type: high bacterial population density = high levels of inducers = inducers cross cell membranes and activate manufacture of specific products, namely BIOFILM
Biofilm
Living Layers that protect against:
- Phagocytosis
- Dehydration
- Antibiotic penetration
Provide Adhesion
Composed of Mucopolysaccharides
Bacterial Gene Transfer Types
Transposition = move a transposon to different locations on a genome (aka jumping genes)
Transformation = uptake of “naked” DNA
Transduction = transfer genetic information via bacterial viruses
Conjugation = Transfer of DNA through cell-to-cell contact
Phase Variation
Bacteria can switch between distinct flagellin proteins (typically targeted by antibodies)
Ex: Salmonella and H2/H1 + H1 Repressor
- Phase 2 = H1 repressed, H2 Expressed
- Phase 1 = Promoter switches direction: H1 no longer repressed, H2 no longer expressed
Antibiotic Resistance
Bacteria acquires mobile genetic elements (Plasmids or Transposons)
Must stabilize DNA that is taken up to prevent cellular degradation; accomplish this via recombination into bacterial chromosome (homologous/nonhomologous) or maintained as plasmid
Genetic Recombination
Homologous Recombination
- Gene REPLACEMENT
- Some DNA degraded (non-recombined DNA
- Requires: RecA (recombination enzyme) and high sequence homology between transfer DNA and target
Nonhomologous Recombination
- Gene INTEGRATION
- No DNA degraded
- Only a small region of homology needed
Pathogenicity Islands (PAIs)
Consequence of Gene Integration
- contain virulence-related genes that may encode toxins, adhesins, antibiotic resistance genes, etc.
Properties of Transposable Genetic Elements
- Random Movement
- can move from any DNA molecule to other DNA molecules; not entirely random, some preference involved - Incapable of Self Replication
- Are not autonomous elements - Transposition mediated by site-specific recombination
- requires limited sequence homology; mediated by enzyme Transposase (encoded by transposon itself) - Transposition can be accompanied by duplication
- Cut and paste or Copy and paste mechanism
Transformation
- found in Gram Positive bacteria
- Cells need to be Competent to do this (able to take up DNA)
Conjugation
- found in Gram Negative bacteria
- Donor Cells (Male) require a tra operon, which codes for an F pili/Sex Pili
- Recipient Cells (Female) have no F factor (no pili)
Every cross must have one F positive and one F negative cell; after cross, both cells will be donor cells
Curing
Conjugation = F- becoming F+
Curing = F+ becoming F-
- removal of plasmids of bacteria in order to reduce spread of antibiotic resistance/virulence plasmids
HFR Integration
- Same mechanism as normal conjugation, except with an entire bacterial chromosome
- you will never reach the tra operon; you’ll only get some of the bacterial genome into recipient cell before pore collapses.
- Recipient REMAINS F NEGATIVE
Transduction
Bacteriophage used to transfer bacterial genetic information
Generalized Transduction
- Mediated by Lytic Bacteriophages (cell lysed to release more phages)
- any gene from disrupted cell transferred
Specialized Transduction
- Mediated by Temperate/Lysogenic Phages (cell is not lysed)
- Specific genes (prophages/provirus) integrated into a target’s chromosome (target cell = Lysogen), and cell will multiply, carrying the gene
Lysogenic Conversion
Lysogenic phages can encode virulence factors; once a prophage of a lysogenic gene is integrated into bacterial chromosome, the cell may begin producing the encoded virulence factors, resulting in disease.
