Chapter 8-9 Bacterial Genetics

Bacterial DNA is compacted into a nucleoid by nucleoid-associated proteins (NAPs).
The nucleoid allows efficient packaging and gene accessibility.

* Circular or linear DNA: Most bacteria have a single circular chromosome, though some have linear chromosomes or plasmids.
* Gene density: High; minimal non-coding regions compared to eukaryotes.
* Operons: Genes in the same pathway are often co-regulated.

*The dtxR gene is repressed by Fe²⁺ ions.
*This controls the production of diphtheria toxin in Corynebacterium diphtheriae.

Chloramphenicol: Blocks translation.
Rifampin: Inhibits RNA polymerase.

1. DNA sequence changes: Some bacteria alter DNA (e.g., phase variation to switch gene expression).
2. Transcriptional control: Repressors or activators bind specific DNA sequences, and sigma factors direct RNA polymerase to promoters.
3. Translational control: Repressor proteins can block ribosome binding to mRNA.
4. Post-translational control: Proteins can be activated, deactivated, or degraded after synthesis.

A cluster of genes regulated together by a shared promoter and operator, often part of the same metabolic pathway (e.g., the lac operon). Is a DNA sequence including a promoter, operator, and multiple structural genes.
*Example: The lac operon regulates lactose metabolism.

*Without lactose: The repressor binds the operator, blocking transcription.
*With lactose: Lactose binds the repressor, preventing it from blocking transcription, enabling expression of lactose-digesting enzymes.
*Positive regulation: High cAMP levels activate CAP, enhancing RNA polymerase binding when glucose is scarce

1. Transformation: Uptake of free DNA from the environment.
2. Conjugation: Transfer of DNA through direct cell-to-cell contact, often involving plasmids.
3. Transduction: DNA transfer via bacteriophages, which can integrate host DNA into new cells.

- Conjugative plasmids: Carry genes for transfer (e.g., sex pili).
- Dissimilation plasmids: Encode enzymes for unusual compound metabolism.
- R factors: Carry antibiotic resistance genes.

Mutations are changes in DNA that can be neutral, beneficial, or harmful.

- Spontaneous mutations: Occur naturally (1 in 10⁶ genes for E. coli).
- Induced mutations: Caused by mutagens like radiation or chemicals.

They generate genetic variation, enabling adaptation to new environments or antibiotics through horizontal gene transfer.

A method to identify bacterial mutants by comparing growth on selective and non-selective media.

A test that detects mutagens by exposing bacteria with known mutations to chemicals and identifying reversions to normal function.

*In E. coli: 1 mutation every 216 cells.
*Mutation rate increases significantly in the presence of mutagens.

*Uptake of naked DNA from the environment.
*Common in Gram-positive bacteria like Streptococcus.

Transfer of DNA between bacteria via direct cell-to-cell contact and often mediated by conjugative plasmids, which carry genes for sex pili and DNA transfer.

DNA is transferred between bacteria by a bacteriophage (phage) and Phage DNA can integrate into the host genome, transferring host genes during later infections.

The exchange of DNA between two molecules and Facilitates genetic diversity.

Mobile genetic elements that can move within the genome.
Example: Tn10, which carries tetracycline resistance.

Luke was infected with a "nontypeable" Haemophilus influenzae strain, which the Hib vaccine does not cover.

Oriel developed diphtheria from Corynebacterium diphtheriae, whose toxin inhibits protein synthesis.
The dtx toxin gene is carried by a beta phage integrated into the bacterial chromosome.

*Harrison's Staphylococcus aureus infection developed resistance to multiple antibiotics, requiring long-term combination therapy.
*The infection persisted due to small colony variants resistant to antibiotics.

*Multiple strains of S. pneumoniae recombined within Eva’s nasopharynx, producing new strains via horizontal gene transfer.
*Undetected bacterial strains contributed DNA, illustrating the adaptability of pathogens through genetic exchange.