Penicillin (only good for gram-positive since gram negative cell wall is buried inside)
Bacteria are constantly remodelling their peptidoglycan cell wall. Penicillin inhibits the
formation of peptidoglycan cross-links. It does this by binding its B-lactam ring portion (the
square ring) to the enzyme DD-transpeptidase. As a result, DD-transpeptidase cannot catalyze
formation of these cross-links and an imbalance between cell wall production and degradation
develops causing the cell to rapidly die (enzymes that hydrolyze the peptidoglycan cross-links
continue to function). As a result the osmotic pressure becomes very high and the cell wall
weakens which causes the death. In addition, the build-up of peptidoglycan precursors triggers
the activation of bacterial cell wall hydrolases and autolysins which further destroys the cell
Penicillin shows a synergistic effect with aminoglycosides (another antibiotic) since the
inhibition of peptidoglycan synthesis allows aminoglycosides to penetrate the bacterial cell wall
more easily, allowing their disruption of bacterial protein synthesis within the cell.
The chemical structure of penicillin is triggered with a very precise, pH-dependent directed
mechanism, affected by a unique spatial assembly of molecular components, which can activate
by protonation. Penicillin can protect itself from spontaneous hydrolysis in the body in its
anionic form, while storing its potential as a strong acylating agent, activated only upon approach
to the target transpeptidase enzyme and protonated in the active centre. This targeted protonation
neutralizes the carboxylic acid moiety, which is weakening of the β-lactam ring N–C(=O) bond,
resulting in a self-activation. Specific structural requirements are equated to constructing the
perfect mouse trap for catching targeted prey.
Ampicillin (good against both gram positive and gram negative)
It is similar in structure to Penicillin in that it has the B-lactam ring and everything else but
Ampicillin in addition has an amino group attached to a benzene ring. The amino group helps the
drug penetrate the outer membrane of gram-negative bacteria. Ampicillin acts as an irreversible
inhibitor of the enzyme transpeptidase. It inhibits the third and final stage of bacterial sell wall
synthesis in binary fission (which leads to lysis).
Polymyxin B (used mainly for all gram-negative infections except Proteus and Neisseria genera.
Bad for gram-positive because cell wall is too thick to get to the membrane)
Polymyxin B has positively charged amino groups in its cyclic peptide portion and uses those to
attach to a negatively charged site in LPS.