For example, pathogens require the substrate para-aminobenzoic acid (PABA)
in order to synthesize folic acid. Folic acid is a coenzyme that is involved in the
synthesis of purine and pyrimidine nucleic acid bases and many amino acids.
The antimetabolite called sulfanilamide, which is a sulfa drug, resembles
PABA. When sulfanilamide is introduced into the pathogenic microorganism,
the enzyme used in making folic acid combines with sulfanilamide instead of
PABA. This then disrupts the formation of folic acid and eventually prevents the
pathogenic microorganism from synthesizing purine and pyrimidine.
Exploring Antimicrobial Drugs
Antimicrobial drugs are classified by their antimicrobial activity. These classifi-
cations are cell wall inhibitors, protein inhibitors, plasma membrane inhibitors,
nucleic acid inhibitors, antimetabolites, and antifungal drugs, antiviral drugs,
and enzyme inhibitors, antiprotozoan drugs, and antihelminthic drugs.
Let’s take a close look at each of these classifications.
CELL WALL INHIBITORS
A cell wall inhibitor is an antimicrobal drug that inhibits the growth or function-
ality of the cell wall of a pathogen. The more common cell wall inhibitors follow:
Penicillin
Penicillin is a group of antibiotics that have a b-lactam ringthe core structure of
penicillins. This core structure is referred to as a “nucleus.” Each member of the
group has a different side chain that is attached to the b-lactam ring. Penicillin
can be natural or semisynthetic.
- Natural penicillin. Natural penicillin is extracted from the Penicillium
mold. The major disadvantage of natural penicillin, except for Penicillin V,
is that it is negatively affected by stomach acid, meaning that the most
effective way to administer natural penicillin is an intramuscular injection.
Another problem is that penicillin itself can be attacked by the enzyme peni-
cillinase. Penicillinases, also known as b-lactamases, are enzymes produced
by many bacteria that attach to the b-lactam ring of the penicillin, render-
ing penicillin ineffective. The more common natural penicillins are:
CHAPTER 16 Antimicrobial Drugs^237