Changes in bacterial traits

Caused by:

• Changes in environmental conditions (only phenotypic changes)

• Changes in the genetic codes 1- Intermicrobial exchange 2- Mutations (point mutations, insertions, deletions)

Intermicrobial exchange

• Transformation (Capturing DNA from solution)

• Transduction (Phage-mediated)

• Conjugation (Bacterial Sex)

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Intermicrobial exchanges by Intermicrobial exchanges by vectorsvectors

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Characteristics of genetic vectors

• must be capable of carrying a significant piece of donor DNA

• must be readily accepted by the host

• plasmids – small, well characterized, easy to manipulate & can be transferred into appropriate host cells through transformation

• bacteriophages – have the natural ability to inject their DNA into bacterial hosts through transduction

Transduction(madiated by phage)

Microbe Library, American Society for Microbiology

www.microbelibrary.org

Lytic versus Lysogenic

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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.

Transduction(specialized)

Specialized Transduction(Lysogenic Phage)

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Transduction

• Definition• Types of transduction• Significance

– Common in Gram+ bacteria– Lysogenic (phage) conversion

• e.g. Corynebacterium diptheriae toxin– Toxin derived from lysogenic phage

Conjugation

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

Donor

Recipient

– Recipient• Lacks an F factor

Conjugation

• Significance– Gram - bacteria

• Antibiotic resistance

– Gram + bacteria• Production of adhesive material by donor cells

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Plasmids

• Definition: Extrachromosomal genetic elements that are

capable of autonomous replication (replicon)• Episome - a plasmid that can integrate into

the chromosome

Classification of Plasmids• Transfer properties

– Conjugative– Nonconjugative

• Phenotypic effects– Fertility– Bacteriocinogenic plasmid (or encoding some other toxins)

– Resistance plasmid (R factors)

Structure of R Factors

• RTF– Conjugative

plasmid– Transfer genes Tn 9

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Tn 10

Tn 8

RTF

R determinant

• R determinant– Resistance genes

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F+ and HFr cells

Integrated (Hfr)(High Frequency of Recombination)

F+ Hfr

Hfr and F’ cells

Hfr F’

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-

Transposable Genetic Elements

• Definition: Segments of DNA that are able to move from one location to another (across the genome or from one genome to another)

• Properties– “Random” movement

• Transposase– Transposition may be accompanied by duplication

Types of Transposable Genetic Elements

• Transposons (Tn)/ Insertion elements (IS)– Definition: Elements that carry other genes in

addition to those involved in transposition– Nomenclature - Tn10, IS6110– Structure

• Composite Tns– Importance

• Antibiotic resistance

•Epidemiology and evolutionary studies

IS ISResistance Gene(s)

IS ISResistance Gene(s)