micro sheet # 9 (a correction added)

بسم الله الرحمن الرحيم


Microbiology lec.9
3\10\2010

If all mechanisms to control the microorganisms that cause diseases fail (including sterilization and specific and nonspecific immune responses), a disease will be established. In this case we must treat the microorganisms.

antibacterial agents used to treat infection. Drugs have used for
treatment of infectious diseases since 17th century. In 17th century diseases like Malaria was treated with quinine and Amebiasis was treated with emetine


Paul Ehrlich was the father of chemotherapy and immunology, he formulated the principles of: selective toxicity, the relationship btw pathogens and drugs, and the combined therapy.


Paul Ehrlich was the first to introduce the term "magic pullets" to solve the matter of infection caused by microorganisms but soon it was realized that microbes can develop resistance against drugs.

The first chemotherapy was held up is Arsphenamines
for the treatment of syphilis. This was the first planned chemotherapy that was the result of experiment were done by Paul Ehrlich.

However, chemotherapy started when Domark (a scientist) used sulfonamide in 1935 as "red azo dye protosil" to treat streptococcus pneumonia which is a fatal disease affects mice. protasil was cleaved in the body to be converted into sulfonamide.



Then compounds known now as “antibiotic” produced by microorganisms were eventually discovered to inhibit growth of microorganisms Penicillin as a discovered antibiotic is the first antibiotic to be discovered, it even preceded sulfonamide discovery.

Penicillin was discovered by Alexander Fleming , he cultured staphylococcus in a streak plate and he lift it, and after the weekend he observed the growth of fungus around which there was clear zone.

but this observation didn’t appreciate and didn’t receive the proper attention , that’s why its remain dormant and ignored until 1941 when Shayne and Flory from oxford university expanded on and added on Alexander Fleming observation by production of penicillin in large quantities, to make it available for clinical use; that’s why Alexander.

Then hundreds of unibiotics were produced from specific organisms to affect other organisms. For example , Bacillus syphilis produses bacitracin , and bacillus polynixia with polynixin B and polynexin E.(check a table in the slides)

Before going into details , we must introduce some terms , there
are commonly used in discussion of antimicrobial agents:
.
.selective toxicity

Introduced by Paul Ehrlich and it’s refer to ability
of antimicrobial agent to destroy the organism but doesn’t influence or harm the host. So that, the antimicrobial agents must target the function or structure that is
present in the organisms but not the host.
example : penicillin .. targets the cell wall of the organism, we
don’t have cell walls enclosing our cells, but bacteria have cell walls..that's why penicillin has selective toxicity.

Another example is folic acid synthesis inhibition, bacteria use folic acid to synthesize nucleic acids, but humans don't.. so any drug works in this mechanism won't be toxic for the host but to the bacteria.(selectively toxic)

Antimicrobial agents
They are divided into antibiotics or chemotherapeutic agents.

Sulfonamides are synthetic agent while Penicillin is antibiotic.

Agents that are produced by microorganisms and they act against other known as antibiotic.

Agents that are synthesized in the lab to be used in treatment of infection known as chemotherapeutic agents.


But this difference is not important as even antibiotics can be synthesized in the lab, and actually most of the new advances in antimicrobial researches are the result of synthetic modification


Antibacterial spectrum
Which mean the range organisms that can be affected by
antibacterial agents.

There are certain antimicrobial agents that can kill both gram negative and positive bacteria and some odd microbes(like those who have fagellae) those are referred to broad spectrum antimicrobial agents.

There are agents that kill only one organism or few organisms like those against viruses.those are of narrow spectrum.


Bacteriostatic and bactericidal
The effect of the antimicrobial agents could be in the form of inhibiting further growth, the organism can't multiply anymore and this effect is known bacteriostatic..

Whereas other antimicrobial agents kill the organism ..those are called Bactericidal.

Bacteriostatic agents can be as useful as bactericidal agents.. as they stops the growth of the microbes enabling the immune system of the host to take action.(complementary relationship with immune system)

Combination therapy :
some microorganisms like mycobacterium ,pseudomonas are known to be highly resistant to antimicrobial agents.. so that, if an individual is infected by Pseudomonas aeruginosa (multi resistant) we needs to use more than one antimicrobial agents, this is the most important indication to kill organisms causing severe infection, and to prevent the development of resistance because if we use single antibiotic the bacteria can develop resistance to that organism, and the Combination therapy will act in different mechanism to kill organisms and there is no chance to develop resistance.

we can measure the concentrations of antibiotics required to kill microbes; this is the quantitative measure that can be done using either MIC or MBC. (see the following)

MIC & MBC
MIC refers to (minimal inhibitory concentration) which is the
minimal concentration of an antimicrobial agents that inhibits visual growth of microorganism.because of the growth of organisms inside test tubes, we see these tubes containing turbid solutions .. if we use MIC of an antimicrobial agent to inhibit the growth of organisms that will result in the clearance of the tube from the suspension, being visually clear after being turbid.

However.the concentration of the antibiotic that is needed to kill organisms where no growth is seen is known as Minimal bactericidal concentration (MBC)
we can measure if all organisms are killed or not by dilution techniques.

effect of combination therapy:

there are three types of the effect of combination therapy:

1- synergism: If we combine antimicrobial agents we may have a result that is more of the effect of the two compound a lone.. synergism have been showed when agents that act on the same target or they act on sequential target in the pathway. For example : Gentamycin, is an antibiotic that is given to patients with penicillin. Gentamycin can not penetrate the microorganism well , so that it is given with penicillin which can penetrate the cell wall and then Gentamycin can take action in inhibition of protein synthesis.

2- indifferent effect: the effect of the combination is the same as the sum of the effects of combined aagents.

3- antagonism : the effect is less than the sum of combined effects of both agents.
Antimicrobial mechanisms of action:

There are four major mechanisms of the action that antimicrobial agents act
on the microorganisms:

1. Inhibition of cell wall synthesis
2. Alteration of cell membrane function
3. Inhibition of protein synthesis
4. Inhibition of nucleic acid synthesis and subdivided into
a- Inhibition of DNA
b- Inhibition of RNA
c- Inhibition of ant metabolites

Now we will talk about them in details:

1-Inhibition of cell wall synthesis is

a- the most important as it is specific to bacteria (selective toxicity)
If cell wall synthesis is inhibited, organism will lyses because cell
membrane can’t maintain the osmotic pressure difference between
intracellular and extracellular environment.

Cell wall synthesis can be inhibited by:

betalactams
Vancomycin and Teicoplannin
Bacitracin :it is only antimicobacterial agents. they inhibit mycolic acid
synthesis that is part of micobacteria( not present in other
bacteria).
Isoniazoid ,Ethambutol, and Cycloserine: they work by inhibition of micoic acid synthesis, which is a structure that is part of the cell wall

remember

peptidoglycan of the cell membrane is composed of :
- N-acetylglycosamine(NAG)
- N-acetylmuramic acid (NAM)
- tetrapeptide chain
- cross bridges

2- Alteration of cell membrane function

Some antimicrobial agents work by affecting the cellular membranes of the organisms .. the affect the selective permeability of the cellular membrane that causes oozing of cytoplasm contents into outside the cell.

Polymyxin : they are two types:polymyxin B,polymyxin E:(colestian)
other agents :
Amphotericin: toxic,for systemic use
Imidazoles : a new member of antifungal agent of cell membrane
Triazoles
Polyenes(micostatien)

They are all antifungal agents’ cell membrane and not bacteria

3- Inhibition of protein synthesis

protein synthesis can be inhibited as
consequence of binding these agent of 50 s or 30 s
subunits.
Microbial agents that act on 50 subunit :

-Tetracyclin.
-Aminoglycoside

Microbial agents that act on 30 subunit :

-Lincomycin
-Chloramphenicol
-Erythrymycin

4- Inhibition of nucleic acid synthesis
Quinolones : they inhibit DNA synthesis by enzymatic inhibition, they inhibit :
topoisomerrases
gyrases

these enzymes are needed to supercoil DNA that are involved in
replication of DNA in bacteria.

Metronidazole that produce nitrous producing
substances that destroy DNA.

Sulfonamide substances that are similar to Para – amino benzene
sulfonamide which utilized in bacteria for DNA synthesis.

We will start by inhibitor of cell wall synthesis :

Betalactam antibiotics are composed of:
Penicillin
Cephalosporins (subdivided to 4th generation)
Cephamycin)
Carbapenems
Monobactams
Beta lactamase inhibitors (subdivided into sublactam and
clavulanic acid and tazobactam).

Note :
There are other agent like vancomycin, bacitracins that target the cell wall. But we will start by betalactams:

Mechanism of action :

Betalactams bind to agroup of proteins called "penicillin binding proteins" (PBP). These receptors work like enzymes nessesary to synthesize peptidoglycans of the cell wall. Betalactams bind to it and inhibit it.

Also they activates autolytic enzymes to degrade the peptidoglycans of the cell wall. Because when the cell synthesize new peptidoglycans they will be added to the peptidoglycans that are distructed(because of inhibition of synthesis) this addition will result in autolytic activity of the cell.,

 probably the widely used and the safest
Antimicrobial agents as they have very wide therapeutic index.
whereas other agents like aminoglycoside has a narrow
therapeutic index
Beta lactam is composed of 5 classes:

a. Penicillin(6- aminopenicillanic acid):

It is composed of betalactam ring , athiazolidine ring and side chains that varies btw different penicillins.
b. Cephalosporins (7- aminocephalosporanic acid)
Composed of beta lactam ring adihydrothiazine ring and side chain that varies to give four subtypes.

b. Cephamycins

contain oxygen in place of sulfur in a dihydrothiazine ring
They are similar to cephalosporins but instead of sulfur we have oxygen..(see the structure in the slides please)

d. Carbapenems

e. Monobactams

Carbapenems and Monobactams have only betalactam and side chain without dihydrothiazine or a thiazolidine.

f. Beta lactamase inhibitor:

Both penicillin and cephalosporin are susceptible to beta lactamase in which it opens the betalactam ring this enzyme
In Penicillin converted and transformed from 6-aminopenicillanic acid to6- aminopenicilloic acid.
And in cephalosporin it will converted from 7- amino cephalosporin acid to
7- aminocephalosporoic acid.

So that, there are agents that are given in combination with antibacterial agents, like penicillin and cephalosporin to inhibit the action of Beta lactamases.

These inhibitors are :
Sublactam and Clavulanic acid and Tazobactam

The end

Done by : shadi Jarrar

correction :

اعكسوا بالله المضادات الحيوية إلي بتأثر على
chromosomal subunit 50

مع الأدوية إلي بتأثر على
chromosomal subunit 30

بتصفي هيك
Microbial agents that act on 30 subunit :

-Tetracyclin.
-Aminoglycoside

Microbial agents that act on 50 subunit :

-Lincomycin
-Chloramphenicol
-Erythrymycin