Microbial genetics

1. Microbial genetics

2. Mutations in Bacteria • Mutations arise in bacterial populations • Induced • Spontaneous • Rare mutations are expressed • Bacteria are haploid • Rapid growth rate • Selective advantage enriches for mutants • Gene transfer occurs in bacteria

3. General Features of Gene Transfer in Bacteria • Unidirectional • Donor to recipient • Donor does not give an entire chromosome • Merozygotes • Gene transfer can occur between species

4. Transformation • Definition: Gene transfer resulting from the uptake of DNA from a donor. • Factors affecting transformation • DNA size and state • Sensitive to nucleases • Competence of the recipient (Bacillus, Haemophilus, Neisseria, Streptococcus) • Competence factors • Induced competence

5. Transformation • Steps • Uptake of DNA • Gram + • Gram - • Recombination • Legitimate, homologous or general • recA, recB and recC genes • Significance • Phase variation in Neiseseria • Recombinant DNA technology

6. Transduction • Definition: Gene transfer from a donor to a recipient by way of a bacteriophage • Bacteriophage (phage): A virus that infects bacteria

7. Head/Capsid Contractile Sheath Tail Tail Fibers Base Plate Phage Composition and Structure • Composition • Nucleic acid • Genome size • Modified bases • Protein • Protection • Infection • Structure (T4) • Size (80 X 100 nm) • Head or capsid • Tail

8. Infection of Host Cells by Phages • Adsorption • Tail fibers • Receptor is LPS for T4 • Irreversible attachment • Base plate • Sheath Contraction • Nucleic acid injection • DNA uptake

9. Microbe Library, American Society for Microbiology www.microbelibrary.org

10. Types of Bacteriophage • Lytic or virulent – Phage that multiply within the host cell, lyse the cell and release progeny phage (e.g. T4) • Lysogenic or temperate phage: Phage that can either multiply via the lytic cycle or enter a quiescent state in the bacterial cell. (e.g.,) • Expression of most phage genes repressed • Prophage – Phage DNA in the quiescent state • Lysogen – Bacteria harboring a prophage

11. Cohesive Ends Ligase Linear Double Stranded Closed Circle Opened Circle Events Leading to Lysogeny • Circularization of the phage chromosome • Cohesive ends

12. Transduction • Definition: Gene transfer from a donor to a recipient by way of a bacteriophage • Resistant to environmental nucleases

13. Transduction • Types of transduction • Generalized - Transduction in which potentially any donor bacterial gene can be transferred

14. Generalized Transduction • Infection of Donor • Phage replication and degradation of host DNA • Assembly of phages particles • Release of phage • Infection of recipient • Homologous recombination Potentially any donor gene can be transferred

15. Transduction • Types of transduction • Generalized - Transduction in which potentially any dornor bacterial gene can be transferred. • Specialized - Transduction in which only certain donor genes can be transferred

16. Transduction • Definition • Types of transduction • Significance • Common in Gram+ bacteria • Lysogenic (phage) conversion • e.g. Corynebacterium diptheriae toxin • Toxin derived from lysogenic phage

17. Donor Recipient Conjugation • Definition: Gene transfer from a donor to a recipient by direct physical contact between cells • Mating types in bacteria • Donor • F factor (Fertility factor) • F (sex) pilus • Recipient • Lacks an F factor

18. Conjugation • Significance • Gram - bacteria • Antibiotic resistance • Exponential increase under selective pressure • Gram + bacteria • Production of adhesive material by donor cells

19. Transposable Genetic Elements • Definition: Segments of DNA that are able to move from one location to another • Properties • “Random” movement • Not capable of self replication (not a replicon) • Transposition mediated by site-specific recombination • Transposase • Transposition may be accompanied by duplication

20. Conjugation • Significance • Gram - bacteria • Antibiotic resistance • Exponential increase under selective pressure • Gram + bacteria • Production of adhesive material by donor cells

21. Transposable Genetic Elements • Definition: Segments of DNA that are able to move from one location to another • Properties • “Random” movement • Not capable of self replication (not a replicon) • Transposition mediated by site-specific recombination • Transposase • Transposition may be accompanied by duplication

22. GFEDCBA ABCDEFG Transposase Types of Transposable Genetic Elements • Insertion sequences (IS) • Definition: Elements that carry no other genes except those involved in transposition • Nomenclature - IS1 • Structure (flanking inverted repeats) • Importance • Insertional Mutation • Plasmid insertion • Phase variation

23. IS Resistance Gene(s) IS IS Resistance Gene(s) IS Types of Transposable Genetic Elements • Transposons (Tn) • Definition: Elements that carry other genes in addition to those involved in transposition • Nomenclature - Tn10 • Structure • Composite Tns • Importance • Antibiotic resistance

24. Plasmids • Definition: Extrachromosomal genetic elements that are capable of autonomous replication (replicon) • Episome - a plasmid that can integrate into the chromosome

25. Classification of Plasmids • Transfer properties • Conjugative • Nonconjugative • Phenotypic effects • Fertility • Bacteriocinogenic plasmid • Resistance plasmid (R factors)

26. RTF Tn 10 Tn 8 Tn 9 Tn 21 R determinant Structure of R Factors • RTF • Conjugative plasmid • Transfer genes • R determinant • Resistance genes • Transposons