Lecture-3

PENICILLIN

Previous lecture was about introduction and basic classification of antibiotics.
Have a look. LECTURE-2
As far as penicillin is concerned it is safest antimicrobial drug.
Remember 4 things about penicillin.

1. Penicillin has beta-lactam ring in its structure.
2. It acts by inhibiting bacterial cell wall synthesis (last step i.e.transpeptidation)
3. It is bactericidal.
4. It is mianly effective against gram positive bacteria.

Here comes the concept.

As bacteria grow and divide they constantly breakdown and rebuild portions of their peptidoglycan cell wall.

For synthesis of cell wall, an enzyme transpeptidase required which catalyzes peptidoglycan cross links formation.

But beta-lactam ring of penicillin binds with transpeptidase and hence prevent cross links formation.

The enzymes that hydrolyze the peptidoglycan cross-links continue to function, even while those that form such cross-links do not. means there is imbalance between cell wall breakdown and formation.

This weakens the cell wall of the bacterium, and osmotic pressure becomes increasingly uncompensated—eventually causing cell death (cytolysis). 

In addition, the build-up of peptidoglycan precursors triggers the activation of bacterial cell wall hydrolases and autolysins, which further digest the cell wall's peptidoglycans. 

The small size of the penicillins increases their potency, by allowing them to penetrate the entire depth of the cell wall.

As gram positive bacterial cell wall is mainly composed of peptidoglycan so penicillin is mainly effective against gram positive bacteria.

Hence all the 4 points about penicillin justified.

hope you got it.if not here is in simple words:
bacteria when divide, it definitely need to double its cell wall and an enzyme transpeptidase is required to do so.
At the same time other enzymes like hydrolases and autolysins break old cell wall.
In the presence of penicillin this process fails because beta-lactam ring of penicillin binds transpeptidase hence no cell wall formation but hydrolases and autolysins keep on working.As a result bacteria losses its cell wall whenever it tries to divide and atlast die.




Bacteria that attempt to grow and divide in the presence of penicillin fail to do so, and instead end up shedding their cell walls.
Thats all.