Bacterial Genetics
Vocabulary
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Binary fission
Exponential growth
Gram positive
Gram negative
Pathogen
Antibiotic
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Selection pressure
Adaptation
Mutation
Conjugation
Transduction
Transformation
Antibiotic resistance
Ideal conditions: Population doubles
every 20 minutes
• Bacteria reproduce exponentially:
• Population may double every 20 minutes
during optimal growth!
• Why don’t they take over the world?!
How do bacteria reproduce?
• Single circular
chromosome
replicates
• Cell divides by binary
fission
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http://www.bact.wisc.edu/Microtextbook/images/text
book/growth/bactdivision.swf
Replication of DNA
Gram Stains & Bacteria
• Gram Negative
• Thin wall stains
pink
• Outer capsule
• More pathogenic
• Gram Positive
• Thick cell wall of
peptidoglycan stains purple
• Less severe infections
• Susceptible to penicillin
How do antibiotics work?
• Selective poison;
• Targets specific bacterial enzymes,
proteins, ribosomes (that we don’t have)
• Penicillin – interferes with bacteria’s ability
to build a cell wall (Gram +)
•
http://cellsalive.com/qtmovs/penpop_mov.htm
Bacteria fight back!
• Bacteria can become
resistant to antibiotics
• Bacteria survive and
pass trait to offspring
• Can also transfer trait
to other bacteria
• Result: population of
resistant bacteria
Methicillin resistant staph. aureus
Selection Pressure & Resistance
How do bacteria become
drug resistant?
1. Change the target
2. Destroys antibiotic
3. Acquire drug resistant genes from other
bacteria (phone a friend)
Mutation
• Target in bacteria for
antibiotic changes so
bacteria is no longer
affected by antibiotic.
Destruction
• Bacteria with
adaptation that allows
them to destroy
antibiotic will survive
and pass this
adaptation to all
offspring
Genetic Transfer
• Bacteria can
(randomly) acquire
genes from other
bacteria which may
allow them to become
resistant to antibiotic.
Mechanisms of Genetic Exchange
• DNA containing resistant gene is passed
from one host to another
• DNA can insert into new host chromosome
or remain separate as a circular piece of
DNA (plasmid )
• 3 processes transfer genetic material
(including resistance to antibiotics)
Conjugation
• Conjugation from HHMI
biointeractive
Conjugation
• “Mating”
• Donor bacterium has a
plasmid with gene for
drug resistance; forms
pillus to connect to
recipient
• Recipient bacterium is not
drug resistant (yet)
• Contact required
• Plasmid transferred
• Recipient now resistant
Transduction
Transduction
• Phage picks up
resistant gene in one
bacterial host and
transfers that gene to
another host cell
when it infects the
new host.
Transformation
Transformation
• Griffith showed that DNA in a broth could be
picked up by bacteria who then acquired new
traits
Summary
• Conjugation – Circular Plasmid (DNA)
transferred to recipient cell through sex
pillus
• Transduction – DNA from one bacteria
picked up by phage and transferred to
another bacteria
• Transformation – naked DNA in the
environment is taken up by bacterial cells
What doesn’t
kill us makes us
stronger!
Advantage* of Drug Resistance
Drug resistant bacteria quickly become
dominant population –
but now they are more dangerous
* Advantage to bacteria, that is. You are sicker than ever. 
 The development of bacterial resistance to
antibiotics provides direct evidence for evolution.
Rise of the “Super Bugs”
Gene transfer increases the
spread of antibiotic resistance
“Normal” vs. Drug Resistant Bacteria