Bacterial Variation
Types of Bacterial Variation
1) Phenotypic variation:
Changes in the bacterial characters in response to
environmental conditions:
o Reversible when the environmental cause is removed.
o Not heritable (no genetic changes).
Examples:
o Penicillin interferes with cell wall synthesis.
o Phenol interferes with development of flagella.
2) Genotypic variation:
Changes in the bacterial characters due to changes
in the genetic constitution of the bacterial cell:
o Irreversible.
o Heritable among bacterial generations.
Causes:
o Mutation.
o Gene transfer.
Gene mutation
Change in the sequence of nitrogenous bases in
the bacterial DNA due to:
o Insertion of new bases.
o Deletion of bases.
o Substitution of one base for another:
• Transition: substitution of purine with purine or
pyrimidine with another pyrimidine.
• Transversion: substitution of purine with pyrimidine or
the reverse.
Mutation may occur spontaneously or induced by agents
like x-rays, UV light, alkylating agents.
Mutation
A
C
G
T
Insertion
A
C
A
G
T
T
A
C
G
T
A
C
G
T
Deletion
Substitution
A
C
G
A
C
A
T
Gene transfer
Transfer of DNA within the bacterial cells by:
 Transposons (jumping genes).
Transfer of DNA between the bacterial cells by:
 Transformation.
 Conjugation.
 Transduction:
 Generalized
 Specialized
Transformation
 When bacterial cells dye, their DNA is released into
the surrounding medium.
 The adjacent bacterial cells will uptake this free
DNA.
Conjugation
 Conjugation occurs between F+ donor cell which is the
bacterial cell carrying F plasmid and F- recipient cell which
does not contain F plasmid.
 The F+ cell will form sex pilus by its F plasmid.
 The pilus arising from F+ cell attaches to a receptor on
the surface of F- cell.
 One strand of F plasmid is transferred through the pilus
from F+ cell to F- cell.
 Each single strand acts as a template for synthesis of the
complementary strand.
 Thus at the end of conjugation, the 2 bacterial cells will
be F+ carrying F plasmid.
Conjugation
Generalized transduction
 During the lytic cycle of bacteriophage replication, the
bacterial DNA becomes fragmented.
 During assembly of the phage, by accident, any fragment
of the bacterial DNA may be enclosed within the phage
head instead of phage DNA.
 The phage will transmit this fragment to other bacterial
cells.
Generalized transduction
Specialized transduction
 During the lysogenic cycle of bacteriophage replication,
the prophage may be induced to detach from the
bacterial chromosome to start a lytic cycle.
 During detachment, the prophage may carry with it a
fragment of the adjacent genetic material of the bacterial
chromosome.
 The prophage will transmit this specific fragment of
bacterial chromosome to other bacterial cells.
Specialized transduction
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