(Mode of Action of Drugs)

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1.4

Chapter

9.3 Beta Lactam Antibiotics

Beta Lactam

Antibiotics

PENICILLINS

Penicillin was originally extracted from the
mould Penicillium notatum but now it is
extracted from its related mould Penicillium
chrysogenum due to its high yield. Penicillin
consists of thiazolidine ring fused with a
beta lactam ring which is essential for its
antibacterial activity. These two rings forms
a nucleus named as 6-aminopenicillanic
acid.

Mechanism of Action

The bacterial cell wall is a rigid outer
layer that completely surrounds the
cytoplasmic membrane. Penicillin and other
betalactam antibiotics inhibit bacterial
growth by interfering with a specific step
in bacterial cell wall synthesis. Penicillins
are classified as in table 9.3.1.

BENZYL PENICILLIN

It is the most potent β-lactam antibiotic
and inhibits the growth of susceptible
microorganism in vitro in lowest
concentration and is available in water
soluble sodium and potassium salts.

Penicillin is effective against gram posi-

tive and negative cocci and some gram

positive bacilli. Among the cocci, strepto-

cocci are highly sensitive. Gonococci, pneu-

mococci and meningococci are sensitive to

penicillin.

Among the bacilli, gram positive Bacillus

anthracis, Corynebacterium diphtheriae,

Clostridium species are highly sensitive.

Among the spirochetes, Treponema pallidum

is highly sensitive to penicillin.

Gram negative bacilli, fungi, protozoa,

rickettsiae, chlamydiae, viruses and

Mycobacterium tuberculosis are totally

insensitive to penicillin.

Pharmacokinetics

After oral administration, benzyl

penicillin is destroyed by gastric acid. It

is mainly absorbed from the duodenum.

It is absorbed in aqueous solution rapidly

after intramuscular or subcutaneous

administration. Penicillin is widely

distributed in the body after absorption

and approximately 60% of plasma

penicillin is bound to albumin. The major