Antimicrobial Resistance
Mechanisms of Drug
resistance
• Destruction or inactivation of the
drug
• Prevention of penetration to target
site
• Alteration of target site (mutation)
• Pumping of the drug out of the
bacterial cell
What
resistance
looks like…
What
resistance
looks like…
Pre-exisiting variation
in a population of bacteria is
selected for by the drug.
The variation is inherited
by daughter cells resulting
in a population of bacterial
cells that have adapted
to grow in the
presence of the drug.
Pre-existing
variation
Selection
Inheritance
&
Adaptation
Emerging Antibiotic
Resistance
•
•
•
•
Enterococci
Staphylococcus aureus
Steptococcus pneumoniae
Mycobacterium tuberculosis
How does pre-exisiting
variation happen?
Genetic changes in
bacteria occur via:
-mutations
-gene transfer
What are mutations?
• Change in the base sequence of the
DNA
• Do they always change the genetic
code?
Causes of mutations in
bacteria
• Most are spontaneous
• Errors made by DNA Polymerase
• UV light exposure
Types of Mutations
• Base-pair mutation
– Missense mutation
– Nonsense mutation
– Silent mutation
Base-pair mutation:
missense
Results of base-pair mutations
Types of Mutations
• Frameshift
– Changes the reading
frame
What can cause mutations?
• Chemicals (nitrous acid)
• Physical mutagens (uv light)
• Biological mutagens (transposons)
Barbara McClintock: “jumping
genes” biological mutagen
Nitrous acid as a chemical
mutagen
Nitrosoguanidine alters
guanine bases
Nucleoside analogs are mutagens
Intercalating agents
UV light as a mutagen
Repair mechanisms
• Wrong nucleotide inserted
– Proofreading
– Mismatch repair
Repair: Mismatch
Repair of UV damage
• Two repair mechanisms
– Light repair
– Dark repair
How does pre-exisiting
variation happen?
Genetic changes in
bacteria occur via:
-mutations
-gene transfer
Gene transfer in bacteria
•
1.
2.
3.
There are three types of gene transfer
Transformation
Conjugation
Transduction
All types of gene transfer
• Involve unidirectional transfer of
information (donor-->recipient)
• Require the integration of newly
acquired DNA “homologous
recombination”
• Increases genetic diversity
What happens to DNA that is
not integrated within a
replicon?
DNA integrated can be
inherited
Griffith’s experiments with
Streptococcus pneumoniae
Transformation
• Requires competent
cells to uptake DNA
• Need living cells and
source of DNA
Conjugation
• Transfer of genes
between 2 bacterial
cells
• Gram negative cells
use a sex pilus
• F(+) cells have F
plasmid, F(-) lack F
plasmid
Conjugation between
(F+) and F(-) cells
F plasmid can integrate into
the chromosome creating Hfr
cell
Hfr cell can also become a F’
cell
Conjugation between Hfr
cell and F(-)
• Does transfer of
DNA occur?
• How is it different
from F(+) X F(-)?
• **Note:figure #4 is
wrong, should have
an Hfr cell and a F(-)
recombinant cell
Transduction
• Transfer of genes between a phage and
a bacterial cell
• Generalized transduction: occurs with
lytic or lysogenic phage (section 8.7)
• Specialized transduction: occurs with
lysogenic phage (section 13.3)
Generalized Transduction
Specialized Transduction
• What type of phage
is involved?
• Will it result in
generation of new
phage?
Comparison of mechanisms of DNA
transfer
The Mobile gene pool
• In E. coli, 75% of genes are found in all
strains
• Rest of the genome is made up of the
mobile gene pool or mobilome
• Mobile gene pool made from plasmids,
transposons, or phage DNA
Plasmids
• Found in many
types of organisms
as dsDNA
• Provide the cell new
characteristics
Resistance Plasmids (R plasmids)
Transposons…way to move
genes between organisms
• How did this S.
aureus become
Vancomycin
resistant S. aureus
(VRSA)?
What
resistance
looks like…
Pre-exisiting variation
in a population of bacteria is
selected for by the drug.
The variation is inherited
by daughter cells resulting
in a population of bacterial
cells that have adapted
to grow in the
presence of the drug.
Pre-existing
variation
Selection
Inheritance
&
Adaptation
Mechanisms of Drug
resistance
• Destruction or inactivation of the
drug
• Prevention of penetration to target
site
• Alteration of target site (mutation)
• Pumping of the drug out of the
bacterial cell