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Antibiotic Resistance-MRSA
Fungi –chitin(glucosamine)
Pectin- structural heteropolysaccharide contained in the primary
cell walls of terrestrial plants. Used as the jelling agents in jam and
jelly.
Bacterial Cell Wall
Peptidoglycan-sugars and amino acids that forms a mesh-like layer
outside the plasma membrane of bacteria, forming the cell wall
Alexander Fleming’s Other
Experiment
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In 1945, a few years after Penicillin was
introduced to the world, Fleming created a
strain of Staphylococcus aureus that was
resistant to penicillin. This was
accomplished by exposing s. aureus to suboptimal doses of Penicillin. Fleming
warned the world about antibiotic resistance
during an interview with the New York
Times.
But the world did not listen…
Between 1945 and 1955, Penicillin was available
to the public over the counter, without a
prescription.
 During those ten years the public did in vivo what
Fleming did in vitro:
People stopped taking Penicillin as soon as they
began to feel better, they took Penicillin for viral
infections, some reports even say Penicillin was
even used to treat male pattern baldness…
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Resistance
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By the end of the 1950’s 50% or more of all
Staphylococcus aureus strains were resistant to
penicillin.
1975 MRSA (methicillin resistant Staphylococcus
aureus).
1988 VRE (Vancomycin resistant enterococci) .
2002 VRSA (Vancomycin resistant
Staphylococcus aureus).
Today, 70% of all health care facility infections
are resistant to one or more antibiotics.
Antibiotic Diversity
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There is great diversity among antibiotics and among their
molecular targets, here are a few examples:
1. Penicillins (including methicillin),
Cephalosporins, and Vancomycin- Target the
bacterial cell wall.
2. Sulfa drugs: growth factor analogues.
3. Quinolones: effect DNA girase.
4. Tetracycline: effects the bacterial ribosome.
5. Rifampin: targets nucleic acid metabolism.
All target processes specific to the bacteria without harming the host
Eukaryote vs Prokaryote
Contents of Cell Wall May Protect
Bacterial from Antibiotic
Schematic Diagrams Of The Two Bacteria Cell Wall Types
 A Gram Positive Bacteria Cell and a Gram Negative Bacteria Cell
A: Peptidoglycan layer Polymer of sugars and amino acids for structure
and support. Note the difference in thickness between the two cells.
This difference is what allows gram-positive and gram-negative
bacteria to stain in separate colors.
 B: Cytoplasmic Membrane Encases the cell’s cytoplasm.
 C: Cytoplasm Living cell substance holding all of the cell’s “organs.”
 D: Outer membrane Found only in gram-negative bacteria. It holds
special chemicals toxic to animals. This membrane is highly resistant
to many antibacterial chemicals.
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Crosslinking of Petidoglycans
performed by transeptidases aka
penicillin binding proteins (PBP)
Amino acids
and amino sugars
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Penicillin binds PBP’s to
prevent them from
functioning in crosslinking
Mechanism of Penicillin
The Beta Lactam (4 membered ring with
carbonyl) binds to the active site of the
transpeptidase enzyme
 Once inside the enzyme’s active site,
penicillin doesn’t leave, so it acts as a
‘suicide substrate.’

Beta Lactamases (from bacteria) have evolved to
break beta lactam ring in penicillins for resistance
Methicillin
Vancomycin has no
beta- lactam ring
Mechanism of Vancomycin
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Forms multiple hydrogen bonds to the Dalanyl-D-alanine amino acids of the Nacetyl muramic acid, and N-acetyl
glucosamine monomers that are the ‘bricks’
of the cell wall. This prevents the formation
of the crosslinking in the walls of the
bacteria. They are then unable to produce a
wall and they will die.
Cell Wall Synthesis
3 Stages of cell wall synthesis
 1. Synthesis of N-acetylmuramic acid
peptide synthesis.
 2. Synthesis of N-acetylmuramic acid-Nacetylglucosamine repeating chains
 3. Cross linking of the MurNAc-GlcNac
chains by transpeptidase.
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Vancomycin Resistance
Vancomycin Resistance is conferred by a
change from D-alanyl D-alanine to Dalanyl D-lactate or D-alanyl D-serine.
 This decreases the number of hydrogen
bonds vancomycin can form with the Nacetylmuramic acid and N-acetyl
glucosamine. In effect, increasing the Kd.
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Types of Antibiotic Resistance
1. Chemical Warfare, aka the beta
lactamase.
 2. Change in target protein structure.
 3. Antibiotic Efflux mechanism
 4. Inaccessibility.
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Efflux Mechanisms
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Pump antibiotics out of the cell, decreasing
the intercellular concentration of drug, and
increases the changes that the few
molecules of the drug that remain inside the
cell can be degrade by enzymes (especially
beta lactamses).
Inaccessibility
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If a drug can’t access it’s target, it can’t work.
Example: Gentamicin effects the bacterial
ribosome, and is actively transported across the
gram negative cell membrane through porin
channels. A mutation to one of the proteins
involved in this pathway slows entrance of
gentamicin into the cell, therefore resulting in
resistance.
Combating Antibiotic Resistance
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CDC’s approach to combat antibiotic resistance:
1. Vaccination
2. More appropriate use of and attention to indwelling
catheters.
3. Early involvement of Infectious Disease Specialists
4. Choosing more appropriate/specialized antibiotics
(using ‘broad spectrum drugs as little as possible’)
5. Appropriate use of prophylactic antibiotics for surgery.
6. Stricter infection control, especially handwashing!
What you can do:
Don’t ask your doctor for antibiotics.
 If prescribed antibiotics take them for the
full course of treatment, and at equally
spaced time intervals as prescribed by your
doctor.
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WASH
YOUR HANDS!!!!
Questions?
References:
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Medical Biochemistry, N.V. Bhagavan. Jones and Bartlett Publishing,
1992. Pages 345-350.
Goodman and Gilman’s The Pharmacological Basis of Therapeutics
11th edition. L. Brunton, J. Lazo, and K. Parker. McGraw-Hill
Medical Publishing Division, 2006. Chapters 42, 43, 44.
Cdc.gov
Textbookofbacteriology.net
Also, special thanks to Debra Dunaway-Mariano, PhD professor of
chemistry at UNM. (if you’re interested in this topic you might want
to take her ‘boilogical chemistry’ course, it was a big help in preparing
for this presentation, as well as being a really cool class.)
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Additional Slides
Beta Lactamase
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Hydrolyzes the Carbonyl portion of the beta
lactam ring. The ring breaks because of strain,
and the business end of penicillin is no longer able
to bind the transpeptidase enzyme. Kd actually
approaches infinity.
Examples: Haemophylis influenzae secretes a beta
lactamase, making these strains resistant to all but
methicillin.
However, it has been discovered that clavulanic
acid acts as a beta-lactamase inhibitor. The
antibitoic Augmentin is actually amoxicillin
combined with clavulanic acid.
Clavulanic acidBeta-lactamase
inhibitor.