Plasmids

1
Plasmids
First plasmid was found in Japan in Shigella
species during an outbreak of dysentary in early
1940's. Usually circular DNA. Exist and
replicate independently. Not essentially for
bacteria.
2
Stringent plasmids e.g. F plasmid 1-10 per
cell Relaxed plasmids e.g. pBR322 gt10 per
cell
oriV
3
Types of Plasmids

• Fertility (F) plasmids
• Conjugation.

4
Types of Plasmids

• Col Plasmids
• Genes encode for colicins
• Degrade plasma membrane
• Degrade bacterial DNA and RNA
• Virulent Plasmids
• Make host bacteria more pathogenic
• Produce toxins----travelers diarrhea.
• Ti plasmid in Agrobacterium

5
Types of Plasmids

• Metabolic Plasmids
• Degrade substances
• Aromatics, pesticides, sugars
• Symbiosis
• Resistance (R) plasmids
• Antibiotic resistance.

6
Engineered Plasmids
oriV
amp
MCS
tet
MCS
7
Antibiotics and Chemotherapeutics
Antibiotics are natural substances produced by
certain groups of microorganisms. e.g.
Streptomyces, Bacillus. Semisynthetic
antibiotics are hybrid substances wherein a
molecular version produced by microbes is
subsequently modified to achieve desired
properties. Chemotherapeutic agents are
chemically synthesized.
8
History of Antibiotics
1929, Fleming's discovery of penicillin. 1935,
Domagk's discovery synthetic chemicals (sulfonami
des) which have the antimicrobial
activity. 1940s, Penicillin was purified and
tested in animals. 1944, Discovery of
streptomycin, chloramphenicol and tetracycline.

9
Killing Effect and Static Effect

• Killing Effect destroy normal cellular
  functions.
• Static Effect prevent cells growth.

10
Broad, Narrow and Limited Spectra of Antibiotics
Broad Spectrum A wide range of Gram-positive
and Gram-negative bacteria. Narrow Spectrum
Mainly against Gram-positive or Gram-negative
bacteria. Limited Spectrum Against a single
type of organism or disease.
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Criterions for Good Antibiotics
1. A wide spectrum of activity. 2.
Nontoxic or side effects. 3. Not allergenic
to the host. 4. Not eliminate the normal
flora of the host. 5. Be able to reach the
part of the body where the infection is
occurring. 6. Inexpensive and easy to
produce. 7. Chemically stable. 8.
Resistance by microbes is and remains uncommon.
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Classification of Antibiotics

• Cell wall synthesis inhibitors
• Produced by Penicillium and Cephalosporium
  moulds. Correspondingly represented by the
  penicillins and ampicillin.

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Penicillin
-lactam
b
Inhibits glycopeptide transpeptidase, an enzyme
for building bacteria cell wall.
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Classification of Antibiotics

• Cell membrane inhibitors
• Bacteriocins, e.g. polymyxin produced by Bacillus
  polymyxis
• Make holes in cell membrane of bacteria
• Antibiotic peptides

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Some Antibiotic Peptides
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Classification of Antibiotics

• Protein synthesis inhibitors
• Most have specific interaction with prokaryotic
  70S ribosome (as opposed to eukaryotic 80S).
• Tetracyclines, chloramphenicol, macrolides,
  streptomycin, kanamycin, tobramycin and
  gentamicin.
• Chloramphenicol inhibits the peptidyl
  transferase.

tetracyline
Chloramphenicol
kanamycin
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Classification of Antibiotics

• Nucleic acid synthesis inhibitors
• Nalidixic acid binds to the DNA gyrase enzyme
  (topoisomerase)
• Rifampicin is a semisynthetic derivative of
  rifamycin. Rifampicin acts quite specifically on
  eubacterial RNA polymerase and binds to the
  b-subunit of the RNA polymerase.

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Classification of Antibiotics

• Enzyme Inhibitors

Competitive inhibitors Non-competitive
inhibitors
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Strategies of Antibiotic Resistance

• Lack a target for antibiotics
• e.g. penicillin resistance in Gram-negative
  bacteria
• Make targets inaccessible for antibiotics
• Efflux pump of tetracyclin

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Strategies of Antibiotic Resistance

• Modify (or mutate) the target for antibiotics
• Mutation in RNA polymerase provides resistance to
  Rifampicin.
• Chemically modify antibiotics
• b-lactamase inactivates penicillin or
  ampicillin.
• chloramphenicol acetyl transferase (CAT).

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-lactamase
b
There are more than 100
-lactamases found
b
in bacteria.
Penicillin
O
O
S
S
N
C
R
N
C
R
C
N
C
N
-lactamase
b
O
O
-lactam
b
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Vertical Evolution and Horizontal Evolution

• Vertical evolution.
• Mutations and selections
• Horizontal evolution.
• Genetic exchange (Conjugation, transformation,
  transduction)