Bacterial genetics /certified fixed orthodontic courses by Indian dental academy

INDIAN DENTAL ACADEMY

Chapter 8:
Bacterial Genetics
Leader in continuing dental education
www.indiandentalacademy.com


Important Point:


Bacterial Genetics

 “Acquiring genes through gene transfer provides
new genetic information to microorganisms,
which may allow them to survive changing
environments.”
 “The major source of variation within a bacterial
species is mutation.”
 “In mutations, usually only a single gene changes
at any one time.”
 “In contrast, gene transfer results in many genes
being transferred simultaneously, giving the
recipient cell much more additional genetic
information.”

Bacterial Genetics Overview

 Most bacteria are haploid which means that there is
no such thing as dominance-recessive relationships
among bacterial alleles.
 Bacteria don’t have sex in the animal/plant sense of
sex (i.e., mating followed by recombination of whole
genomes).
 Instead, bacteria acquire DNA from other bacteria
through three distinct mechanisms:
 Transformation
 Transduction
 Conjugation
 This DNA may or may not then recombine into the
recipient’s genome.
 We use phrases like “Lateral” or “Horizontal” Gene
Transfer to describe these sexual interactions.
 Bacterial DNA is also subject to mutation, damage
(not the same thing as mutation), and natural
selection. www.indiandentalacademy.com

Mutation: Terms & Concepts

 Wild Type refers to the microorganism as isolated
from the wild.
 A mutated microorganism that has lost a metabolic
function, particularly an ability to synthesize a specific
growth factor, is called an Auxotroph.
 The wild-type parent to an auxotroph is called a
Prototroph.
 A Mutation is found in a gene; a mutant is an
organism harboring a Mutation.
 We designate mutant phenotypes using three-letter
abbreviations; the phenotype of a tryptophan-requiring
auxotroph would be described as Trp-.
 A bacterium that has mutated to resistance to an
antibiotic (or other substance) is given the superscript
“R”; thus, the phenotype ampicillin resistance is
indicated as AmpR.
 Mutants can be spontaneous or induced by a
Mutagen; an agent that causes DNA to mutate.

Types of Mutations

 Base Substitution
 Point mutation = single base is substituted.
 Missense mutation = base change changes single
amino acid to different amino acid.
 Nonsense mutation = base change changes single
amino acid to stop codon.
 Null or Knockout mutation = mutation that totally
inactivates a gene.
 Deletion or insertion mutation = change in number of
bases making up a gene.
 Frameshift mutation = insertion or deletion of
something other than multiples of three bases.
 Frameshifts typically radically change downstream
codons, generating stop codons, and typically
knocking out gene function.
 Reversion mutation = mutated change back to that of
wild type. www.indiandentalacademy.com

Rates of Mutation

 The mutation rate of different genes usually varies
between 10-4 and 10-12 mutations per cell division
(essentially equivalent to per cell).
 10-4 = one in 10,000; 10-12 = one in one trillion.
 To calculate the probability of two independent
mutations we multiple the two mutation rates.
 Thus, if streptomycin resistance occurs at a rate of 10-6
mutations per cell division and the rate of mutation to
resistance to penicillin is 10-8 then the rate of mutation to
both antibiotics is 10-6 * 10-8 = 10-14 (note that the
exponents are added).
 That is, we would have to have a population of onehundred trillion cells to have one double mutant, which
even for bacteria is a lot of cells.
 This is the basis for Combination Therapy, e.g., the use
of more than one chemotherapeutic against
tuberculosis, HIV, cancer, etc.
 The odds of sufficiently multiply resistant mutants drops
with each new chemotherapeutic added to the mix.

Direct Selection for Mutants



Indirect Selection: Replica
Plating


Indirect Selection:
Penicillin
Enrichment

Indirect Selection:
Isolation of ts
Mutants
This is one example of isolation of mutants carrying
conditionally lethal mutations found in essential genes.

Ames Salmonella Test


DNA-Mediated Transformation

Note that DNA
is taken up
naked from
the
environment.


DNA-Mediated Transformation



F. Griffith (1928) using pneumococci

Original Transformation Exp.

Artificial Competence
by Electroporation

Competence
denotes the
ability to take
up DNA naked
from the
environment.

Most bacteria
are not naturally
competent but
many can be
made artificially
so.

Artificially
induced
competence is
very important
to gene cloning.

Generalized Transduction

Bacteriophages
are viruses that
only infect (and
can kill)
bacteria.


Generalized Transduction


Conjugation: Sex or F Pilus


Conjugation: F Plasmid Transfer


F and Other Plasmids

 F plasmids encode genes that allow both their
replication and transfer.
 They are thus known as Self-Transmissible Plasmids.
 There are other plasmids that can take advantage of
conjugation but don’t encode the the necessary
genes. These are non-self transmissible plasmids.
 Transduction is also capable of transferring smaller
plasmids.
 R plasmids are named not for their mode of
transmission but instead for the resistance genes that
they encode such as to antibiotics.
 Some plasmids are present in bacteria in low copy
numbers (1 or 2/bacterium) whereas other plasmids
are present in high copy numbers (such 100s/bact.).
 Plasmids, R and otherwise, can have very wide host
ranges allowing easy transfer of already evolved
genes between bacterial species.

Self-Transmissible R Plasmid

Note
multiple
resistance
genes.

Resistance Transfer Factor
(conjugation genes)

Transfer of non-R Virulence Factors

 Genes that can make bacteria more virulent
(able to cause disease) are called Virulence
Factor genes.
 Virulence factors include fimbriae that allow
attachment to host tissues, exotoxins, etc.
 Virulence factor genes may be transferred by
transformation, transduction, or conjugation.
 Virulence factor genes tend to congregate on
bacterial chromosomes in regions known as
Pathogenicity Islands.
 New bacterial pathogens can emerge via the
uptake of entire pathogenicity islands transferred
intact from unrelated bacteria.

Transfer Protection: R-M
Systems

 Not all incoming DNA is necessarily good for the
receiving bacterium (i.e., DNA can be parasitic).
 Bacteria employ Restriction Enzymes to protect
themselves from the foreign DNA.
 Restriction enzymes recognize specific,
palindromic (same spelling backward and
forward) DNA sequences of 4 to 8 base pairs in
length that are known as Recognition
Sequences.
 Bacteria also employ Modification Enzymes that
modify DNA to protect it from Restriction
Enzymes.
 Together these are called RestrictionModification Systems.
 Restriction enzymes are crucial components of
genetic engineering.


Restriction Endonuclease
Action

Restriction Endonuclease
Action

Note in
particular that
DNA is cut at
palindromic
regions.


DNA Modification: RE Protection


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Bacterial genetics /certified fixed orthodontic courses by Indian dental academy

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