Chapter4Heredity and variation

of bacteria

Xin GangDepartment of microbiology and immunolog

y Shantou University Medical College

outline

Concepts Concepts

Genetic materials of Genetic materials of bacteriabacteria

Mechanism of variationMechanism of variation

concepts

•Heredity•Variation

– Genotypic variation– Phenotypic variation

Variation of bacteria

i. Variation in morphology and structure

– lose capsule (Pneumococcus)– H O– Lose Spore– L-form

Variation

ii. Variation in virulence– Bovine TB BCG

13years, 230 generation

– C. diphtheriae-corynephage, diphtheriae toxin

Variation

iii. Variation in drug resistance– Penicillin resistant strains of staphyl

ococcus aureusiv. Variation in colony

– S-Rv. Variation in enzyme activity

– Phenotypic– Genotypic

I Genetic materials of bacteria

1. Chromosomes2. Plasmids3. Phage4. Transposable elements

DNA forms: ds-DNA, circle

1 Chromosomes1 Chromosomes

• Size– E.coli 1300m, 4288gen

e

• Rolling-circle pattern of replication

2 PlasmidsExtrachromosomal genetic elements tha

t 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 indu

ction (UV)• incompatibility and compatibility

Classification of Plasmids

• Transfer properties– Conjugative

40-100kbp eg. F, R plasmid

– Nonconjugative<15kbp, transfer by mobilizationeg. ColE1 plasmid

• Phenotypic effects

Fertility plasmid,F plasmid coding sex pilus

Tn 9

Tn

21

Tn 10

Tn 8

RTF

R determinant

Resistance plasmid, R plasmid resistance transfer factor resistance determinant

Virulence plasmidColiciogenic plismid

3 Bacteriophage, phage

T4 bacteriophages infecting T4 bacteriophages infecting E.coliE.coli..

Bacteriophage (Phage)

•Definition –Bacterial virus.–Virus of bacteria, fungi, actino

myces, and spirochete.

•Composition– Nucleic

acid•DNA or RNA

– Protein•Protection•Infection

Composition and Structure

Structure (T4)– Head or caps

id– Tail

Tail

Tail Fibers

Base Plate

Head/Capsid

Contractile Sheath

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., )

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:– Phage that can either multiply via

the lytic cycle or enter a quiescent state in the bacterial cell. (e.g., )

• Prophage• Lysogen,Lysogenic bacterium• Lysogenic conversion

Lysogenic bacterium/lysogen

prophage

Lysogenic phage /temperate phage

Lysogenic or phage conversion– Definition: A change in the phenoty

pe 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)– A short sequence of DNA containing

only the genes for those enzymes required for its transposition.

IR: inverted repeat

•Importance–Mutation–Plasmid insertion

Types of Transposon

• Transposons (Tn) or complex Tn– contain genes other than those required fo

r transposition (eg. Antibiotic resistance or toxin genes)

•Importance

–Antibiotic resistance

II mechanism of variation

i. Mutationii. Gene transfer and

recombination

i Mutations in bacteria

• Changes in DNA sequences• Base substitutions, deletions, insertion

s, rearrangements• Spontaneous mutation: 10-8-10-6

• Backword mutation or reverse mutation

ii Gene transfer and recombination

1. Transformation2. Transduction3. Conjugation4. Lysogenic conversion5. 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 TransformationWith DNA Fragments

2 Conjugation

Definition: Donor DNA transferred to recipient cell through sex pilus.

Donor

Recipient

F plasmid

F+Mechanism of F+ x F- Crosses

Pair formation Conjugation bridge

DNA transfer

Origin of transfer

Rolling circle replication

F+ F- F+ F-

F+ F+F+ F+

Hfr F’

F’ plasmid

Hfr,high frequency of recombination

F+ Hfr

Physiological States F plasmid

F plasmid

Mechanism of Hfr x F- Crosses

Hfr F- Hfr F-

Hfr F-Hfr F-

Mechanism of F’ x F- Crosses

F’ F’F’ F’

F’ F- F’ F-

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 transductionby 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 transductionby a temprerate bacteriophage

gal

bio

gal bio

gal bio

gal

bio

bio

gal

specialized transduction

iv.lysogenic conversionv. DNA recombination

protoplast fusion

Practical implications

1. Application in diagnosis, treatment and prevention of infectious diseases

Eg. L-form PCR

2. Detection of mutagenicity

3. Application in genetic engineering

The Ames Test for mutagenicity