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Summary Molecular Bacteriology of Infectious Diseases (16/20)
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This is a summary of the course that is given in the Master Infectious and Tropical Diseases.
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Molecular Microbiology1. Antibiotics: introduction and classification schemes
What is an antibiotic:
An antibiotic is a drug that kill or prevent the multiplication of a bacteria. Respectively they are called
bactericidal and bacteriostatic. A bacteriostatic antibiotic will leave some bacteria behind but they
are so few that the immune system can take care of it. The specificity of an antibiotic resides in its
ability to damage bacteria and not the host. The first antibiotic was penicillin. To see if an antibiotic is
working you plate your target bacteria out on an agar plate and then insert disks with the antibiotic
inside of them. Then the antibiotic will diffuse into the agar. And depending on the range they clear
out you can see how good an antibiotic is.
Antibiotic classifications:
The antibiotic classifications are based on the targets
where they work. These targets are different from the
same mechanics in humans.
Metabolic analogues:
Metabolic analogues will function by molecular
mimicry. They will look like important molecules for
the bacteria. The AB mimic some metabolite or
molecule that is required by the bacteria.
One of these molecules is the PABA molecule which is an important molecule in the folic acid
pathway. Folate acid is a product that humans don’t make we get it from green vegetables or pills.
Bacteria can make folate acid by their self. This molecule pathways get mimicked by the antibiotics
trimethoprim and sulphonamide. They both work on a different stage in the pathway and are often
given together. It is then called co-trimoxazole and it is heavily usen in HIV.
Protein inhibitors:
Protein synthesis is an important target for antibiotics. We can target this specific because the
Svedberg unit of their ribosomal subunits is different than that from humans. These subunits have
rRNA in them namely 16S and 23S rRNA. Furthermore they have three sites namely A site, P site and
E site. These sites are important for the synthesis of an amino acid chain. In A site the tRNA enters
with the amino acid on it, the P site is for the elongation of the poly amino acid chain and at the E
site the tRNA leaves the ribosome again. The amino acid chain leaves via a small channel in the
ribosome. The way of working from the antibiotics can be very different.
• Tetracycline will bind on 30S subunit and block protein synthesis
• Aminoglycosides will bind the 30S subunit on the A site and this inhibits peptide chain transfer
and interfere with the formation of the initiation complex.
• Mupirocine is used in the nose against MSRA binds isoleucyl tRNA so isoleucyl cannot be
incorporated in the peptide chain.
• Fernicolen these binds the 50S subunit and will prevent the ribosomal translocation and inhibits
the peptide bond formation
• Lincosamiden will also bind the 50S subunit an inhibits the transpeptidase reaction and peptide
chain elongation.
• Fusidindeacid will bind and inhibit peptide chain elongation factors, so it will prevent peptide
chain elongation.
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, MASK = macrolides, Azalides,
Streptogramins and Ketolides.
Macrolides have a macrolactol ring.
Ketolides are used against bacteria
that have a resistance against
macrolides. These group have specific
arms standing out which helps them to
bind even when the bacteria has
developed a macrolides resistance.
Some examples of amino-glycoside. Don’t
learn them all. Gentamicin is the most
commonly used against gram – and
Pseudomonas. Amikacine can work against
bacteria that are resistant against
gentamicin.
Some example of tetracyclines. Doxy and
Minocycline are the new ones.
Cell membrane inhibitors:
The bacterial cell was is also an important target. The Gram+ cell wand is verry thin because it has a
one layer of peptidoglycan. These make the cell membrane porous so the cell will not hold because
water will get in and the cell will be destroyed. An example of antibiotic that can do this is
polymixines. A subclass of this antibiotic is colistin which is used to treat multiple resistant G-
bacteria.
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, Glycosyl bond between NAG and NAM. The
NAMs also have peptide chains hanging out.
This make the peptidoglycan stable. PBPs =
penicillin binding proteins have 2 functions;
transpeptidation and transglycosylation.
Cell wall inhibitors:
The cell wall is different for gram positive and gram negative bacteria. We can make the difference
between them based on gram staining. So the gram staining is as follow, we first add cristalviolet
then we add iodine and then wash them and then add safranine. After this we will see G- is pinked
coloured and G+ will be coloured purple. This difference in staining is because of the difference in the
cell wall.
G+ has a more simple cell wall then G- . G- has a cell membrane then a thin peptidoglycan layer and
then an inner cell membrane. This gives an extra problem to conquer if we want to target G- then we
have to have an antibiotic that can be uptake by the porins. The porin will only pass molecules that
are hydrophilic and small in size. The peptidoglycan layer is a layer that contains NAG and NAM and
peptide chains. The layer is build due to two reactions namely the transpeptidation and
transglycosylation reaction.
The most important antibiotic for this class is β lactams. Other antibiotics in this class are bacitracine
and glycopeptiden. Bacitrancine will interfere with the carrier molecule and inhibits this. The
glycopeptiden have a subclass vancomycine this antibiotic is used as a last line of defence against
MRSA. It is a large molecule so it is not effective against G- bacteria. This antibiotic will prevent the
transpeptidation reaction by binding D-Ala-D-Ala and will prevent the binding of two peptidoglycan
sheets. The β lactam is the most studied class of antibiotics. This class of antibiotics will prevent the
transpeptidation reaction. And it targets the bifunctional enzymes penicillin binding proteins (PBP).
An example of β lactam using antibiotics is cephalosporines. Cephalosporines have 4 generations
with the 1th generation best against G+ bacteria and the 4th generation best against G- bacteria. An
example for each generation has to be known.
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, Another antibiotic that uses β lactam is penicillin. Penicillin G is a small molecule with a narrow
spectrum. In the modern penicillin we have β lactamase inhibitors to prevent β lactamase to destroy
the β lactam ring and destroys the working of the antibiotic.
DNA inhibitors:
DNA inhibitors can work on two levels namely the transcription level and on the replication level. For
the transcription we have rifampicin which prevent transcription because it forms a stable drug-
enzyme complex with RNA polymerase. And for replication we can use the nitro imidazole which are
against anaerobe bacteria. Furthermore we can use quinolones. The quinolones has already three
generations.
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