- Penicillin Gis the prototype compound for natural penicillin as is used
in defense of staphylococci, streptococci, and several spirochetes.
Natural penicillin has a narrow spectrum of activity. Penicillin G lasts
up to six hours when injected intramuscularly. - Procaine penicillinis a combination of the drugs procaine and penicillin
G and lasts up to 24 hours, although the concentration of procaine peni-
cillin diminishes after four hours. - Benzathine penicillinis a combination of benzathine and penicillin G
and can last for up to four months. - Penicillin Vis not inhibited by acid in the stomach and can be taken orally.
- Semisynthetic penicillin. Semisynthetic penicillins are chemically altered
natural penicillins that are designed to overcome disadvantages of natural
penicillins. There are two ways in which scientists modify natural peni-
cillins. They stop the natural synthesis of a Penicilliummolecule and use
the penicillin nucleus. The other way is to modify the side chains from the
natural penicillin and insert a new side chain that overcomes the disadvan-
tage of the original side chain. This is the technique used to combat peni-
cillinase. Common types of semisynthetic penicillin are: - Methicillinwas the first semisynthetic penicillin developed to resist
penicillinase. - Oxacillinis a newer semisynthetic penicillin that resists penicillinase
and has replaced methicillin. - Ampicillinis a semisynthetic penicillin that overcomes the narrow spec-
trum of activity of natural penicillin by attacking gram-negative and
gram-positive bacteria. Ampicillin is not resistant to penicillinases. - Amoxicillinis similar to ampicillin.
- Carbenicillinis a member of the carboxypenicillin group and has a
broad spectrum of activity against gram-negative bacteria and is used to
fightPseudomonas aeruginosa.
•Ticarcillinis similar to carbenicillin. - Mezlocillinis a member of the ureidopenicillins group of penicillins and
is a modification of ampicillin, giving it a broader spectrum of activity
against bacteria. - Azlocillinis similar to mezlocillin.
- Augmentin (which is the trade name)is a combination of amoxicillin
and potassium clavulanate, which is produced by streptomycete. Amoxi-
cillin attacks gram-negative and gram-positive bacteria and potassium
clavulanate resists penicillinase.
(^238) CHAPTER 16 Antimicrobial Drugs