Chapter4 Heredity and variation of bacteria Xin Gang Department of microbiology and immunology Shantou University Medical College
Genetic materials of bacteria Mechanism of variation outline Concepts Genetic materials of bacteria Mechanism of variation
concepts Heredity Variation Genotypic variation Phenotypic variation
Variation of bacteria Variation in morphology and structure lose capsule (Pneumococcus) H O Lose Spore L-form
Variation Variation in virulence Bovine TB BCG C. diphtheriae 13years, 230 generation C. diphtheriae b-corynephage, diphtheriae toxin
Variation Variation in drug resistance Variation in colony Penicillin resistant strains of staphylococcus aureus Variation in colony S-R Variation in enzyme activity Phenotypic Genotypic
I Genetic materials of bacteria Chromosomes Plasmids Phage Transposable elements
1 Chromosomes DNA forms: ds-DNA, circle
Rolling-circle pattern of replication Size E.coli 1300mm, 4288gene Rolling-circle pattern of replication
2 Plasmids Extrachromosomal genetic elements that are capable of autonomous replication. Small double-stranded DNA molecules, usually circular exist independently of host chromosome autonomously replicating (replicon) may disappear spontaneously or by induction (UV) incompatibility and compatibility
Classification of Plasmids Transfer properties Conjugative 40-100kbp eg. F, R plasmid Nonconjugative <15kbp, transfer by mobilization eg. ColE1 plasmid
Phenotypic effects Fertility plasmid,F plasmid coding sex pilus Resistance plasmid, R plasmid resistance transfer factor resistance determinant Tn 9 Tn 21 Tn 10 Tn 8 RTF Virulence plasmid Coliciogenic plismid R determinant
3 Bacteriophage, phage T4 bacteriophages infecting E.coli.
Bacteriophage (Phage) Definition Bacterial virus. Virus of bacteria, fungi, actinomyces, and spirochete.
Composition and Structure Nucleic acid DNA or RNA Protein Protection Infection
Structure (T4) Head or capsid Tail Head/Capsid Contractile Sheath Tail Tail Fibers Base Plate
Infection of Host Cells Attachment Sheath contraction Nucleic acid injection
Attachment and injection
Types of Bacteriophage Lytic or virulent phage (e.g., T4) Lysogenic or temperate phage (e.g., l)
Virulent Phage Lytic or virulent phage Phage that can only multiply within bacteria and kill the cell by lysis. (e.g., T4)
Lytic Phage Multiplication Cycle
Lytic cycle Attachment Injection and uncoating Biosynthesis Eclipse Early proteins Phage DNA synthesis Late proteins Intracellular accumulation Maturation and releasing
Lysogenic Phage Lysogenic or temperate phage: Prophage Phage that can either multiply via the lytic cycle or enter a quiescent state in the bacterial cell. (e.g., l) Prophage Lysogen,Lysogenic bacterium Lysogenic conversion
Lysogenic phage /temperate phage Lysogenic bacterium /lysogen prophage
Lysogenic or phage conversion Definition: A change in the phenotype of a bacterial cell as a consequence of lysogeny Modification of Salmonella O antigen Toxin production by Corynebacterium diphtheriae
Lytic versus lysogenic infection by phage Lambda
4 Thransposable element Concept Segments of DNA that are able to move around the genome. Properties “Random” movement Jumping genes or movable genes First discovered in the 1940s by Barbara McClintock during her study on maize genetics.(won the Nobel prize in 1983)
Types of Transposon Insertion sequences (IS) Importance A short sequence of DNA containing only the genes for those enzymes required for its transposition. Importance Mutation Plasmid insertion IR: inverted repeat
Types of Transposon Transposons (Tn) or complex Tn Importance contain genes other than those required for transposition (eg. Antibiotic resistance or toxin genes) Importance Antibiotic resistance
II mechanism of variation Mutation Gene transfer and recombination
i Mutations in bacteria Changes in DNA sequences Base substitutions, deletions, insertions, rearrangements Spontaneous mutation: 10-8-10-6 Backword mutation or reverse mutation
ii Gene transfer and recombination Transformation Transduction Conjugation Lysogenic conversion protoplast fusion
General Features of Gene Transfer in Bacteria Unidirectional Donor to recipient Donor does not give an entire chromosome Gene variation can occur between species
1 Transformation Definition: Gene transfer resulting from the uptake of DNA from a donor. Competence of the recipient (Bacillus, Haemophilus, Neisseria, Streptococcus)
Griffith’s transformation experiments
Bacterial Transformation With DNA Fragments
2 Conjugation Donor Definition: Donor DNA transferred to recipient cell through sex pilus. Recipient
F plasmid Mechanism of F+ x F- Crosses Pair formation DNA transfer F+ Conjugation bridge DNA transfer Origin of transfer Rolling circle replication F+ F-
Physiological States F plasmid Hfr Hfr, high frequency of recombination Hfr F’ F’ plasmid
Mechanism of Hfr x F- Crosses
Mechanism of F’ x F- Crosses
Resistance plasmid, R plasmid RTF (resistance transfer factor) transfer genes code pili R determinant (resistance determination ) resistance genes transponsons Tn 9 Tn 21 Tn 10 Tn 8 RTF R determinant
3 Transduction Definition: Gene transfer from a donor to a recipient by way of a bacteriophage
Types of transduction Generalized - Transduction in which potentially any donor bacterial gene can be transferred Specialized (restricted) - Transduction in which only certain donor genes can be transferred
Generalized transduction by bacteriophages
Generalized Transduction Infection of Donor Phage replication and degradation of host DNA Assembly of phages particles Release of phage Infection of recipient Legitimate recombination
specialized transduction by a temprerate bacteriophage
specialized transduction gal bio specialized transduction
lysogenic conversion DNA recombination protoplast fusion
Practical implications Application in diagnosis, treatment and prevention of infectious diseases Eg. L-form PCR Detection of mutagenicity Application in genetic engineering
The Ames Test for mutagenicity