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)
F. Griffith (1928) using pneumococci
Original Transformation Exp.
DNA-Mediated Transformation
DNA-Mediated Transformation
Intermicrobial exchanges by
vectors
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
7
Transduction
(madiated by phage)
Microbe Library, American Society for Microbiology
www.microbelibrary.org
Lytic versus Lysogenic
Generalized Transduction
Generalized Transduction
Transduction
• Types of transduction
– Generalized - Transduction in which potentially
dornor bacterial gene can be transferred.
– any
Specialized:
Transduction in which only certain donor
genes can be transferred.
Transduction
(specialized)
Specialized Transduction
(Lysogenic Phage)
bio
gal
gal
gal
bio
gal
bio
bio
bio
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
Donor
• F factor (Fertility factor)
– F (sex) pilus
– Recipient
• Lacks an F factor
Recipient
Conjugation
• Significance
– Gram - bacteria
• Antibiotic resistance
– Gram + bacteria
• Production of adhesive material by donor cells
Conjugation
Conjugation: Sex or F Pilus
Plasmids
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
RTF
– Conjugative
plasmid
– Transfer genes
• R determinant
– Resistance genes
R determinant
Self-Transmissible R Plasmid
Conjugation: F Plasmid Transfer
Conjugation
F+ and HFr cells
F+
Hfr
Integrated (Hfr)
(High Frequency of Recombination)
Hfr and F’ cells
Hfr
F’
Mechanism of Hfr x F- Crosses
Hfr
Hfr
F-
F-
Hfr
Hfr
F-
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
Resistance Gene(s)
IS
IS
Resistance Gene(s)
IS