Bacterial Genetics Tutorial Notes

Bacterial Genetics Bacterial Genome: Chromosome Extra-Chromosomal Bacterial Chromosome: cccDNA Almost naked DNA No Introns Very few repetitious sequences Haploid Single chromosome Chromosome Size: 5,800 kbp Salmonella typhi 4,800 kbp Escherichia coli 4,600 kbp Staphylococcus aureus 2,900 kbp Heamophilus influenzae 1,830 kbp Helicobacter pylori 1,700 kbp Campylobacter jejuni 1,640 kbp Mycoplasma genitalium 580 kbp Pseudomonas aeruginosa Exceptions: Linear chromosome: Streptomyces (7,800 kbp) Borrelia (1,000 kbp) Multiple cccDNA chromosome: 2 chromosomes: Brucella (2,100 kbp & 1,200 kbp) Vibrio cholerae classic strain (2,400 kbp & 1,600 kbp) El tor strain (2,960 kbp & 1,070 kbp) 3 chromosomes: Burkholderia cepacia (3,600 3,200 & 1,100 kbp) Gene Transfer among Bacteria 1- Transformation 2- Transduction 3- Conjugation Transformation The most common method Entry of naked DNA directly into bacterial cells cells must be made competent Natural transformation ( cells are naturally competent) Artificial transformation ( cells are brought into competence state) CaCl2-heat shock treatment Electroporation Biolistic methods (Gene Gun) Transduction Entry of foreign DNA into bacterial cells via bacteriophages (virus particles) Two types of bacteriophage: Lytic Temperate Lytic phages: All RNA phages are lytic Most DNA phages are also lytic They lyse their host bacterial cells following entry of their genome The entire lytic infectious cycle takes about 30 min Following infectious cycle, many new progeny phage particles are released The host bacterial cells are lysed Therefore; these phages are not suitable for transduction examples: T-even (T2, T4, …) phages Temperate phages: examples: P1, P15, Mu, Lamda (λ) Following entry of their genome into bacterial cells, their initial goal is integration of their genome into bacterial genome (either chromosome or plasmid) The introduced phage genome is now called “Prophage” The bacterial cell is called “Lysogene” The phenotypic and genotypic change is referred to as “Lysogenic Conversion” There is now a coexistence of phage and bacteria Both benefit from this process (Mutualism) The bacterial cells usually gains new traits ( ability to produce toxins, enzymes ,…) examples: Diphtheria toxin Cholerae toxin Erythrogenic toxin The lysogenic state usually comes to end upon encounter of the bacterial cells with UV, antibiotics, mutagenic agents,… A lytic cycle will start, releasing many new phage particles. Therefore, the temperate phages will eventually become Lytic Conjugation entry of DNA from a bacterial cell (Donor or Male) into another cell (Recipient or Female) through mating (conjugation) bridge “a sexual DNA transfer “ In Gram negative cells: The Donor must have sex pilus and Fertility factor The fertility factor genes (tra) are either within plasmid or within chromosome Three types of male cell (F+ , Hfr, F´ ) Female cell is called Fsingle stranded DNA gets transferred into female cell through the mating bridge following “rolling circle replication” process. The initial replication site is called oriT The enzyme relaxase helps to insert the single stranded DNA into the bridge In female cell, the ssDNA gets replicated into double stranded form In Gram Positive cells Conjugation is mediated by production of PHREMONS It is less understood