substance that a microorganism produces, in small amounts, that inhibits the
growth of another microorganism. Therefore, penicillin is an antibiotic produced
by the Penicillium notatummicroorganism that inhibits the growth of the
Staphylococcus aureusbacterium.
It took ten years since Fleming discovered penicillin before the first clinical
trials were successful. These clinical trials proved to everyone that penicillin
cured diseases caused by the Staphylococcus aureusbacterium. The next chal-
lenge was to mass-produce penicillin. This required a highly productive strain of
Penicillium. The break-through came with the isolation of a highly productive
strain of Pennicilliumisolated from a cantaloupe fruit.
Many of the antibiotics in use today are produced from the Streptomyces
species. Streptomycesare bacteria that live in the soil. Other antibiotics come
from the genus Bacillus bacteria and the genera Cephalosporium and
Penicillium which are molds.
Antimicrobial Activity: Who to Attack?
The way in which an antibiotic attacks a pathogenic microorganism is called the
antimicrobial activity. You might say this is the way the antibiotic identifies the good
guys from the bad guys. The good guys are eukaryotic cells and the bad guys are
prokaryotic cells (bacteria). Human cells do not chemically resemble bacteria
cells,this is the reason why the antibiotic can differentiate between the good guys
and the bad guys.
Eukaryotic cells and prokaryotic cells are different in a number of ways, such
as the presence or absence of cell walls and their chemical make up. There are
also differences in their respective metabolisms and structures of their organelles,
such as the ribosome. It is these differences that antibiotics target so that only the
prokaryotic cells are destroyed.
Sometimes there can be a problem, especially when the disease-causing
agents are other eukaryotic cells. While bacteria cells are dissimilar to human
cells, the same cannot be said of other pathogenic microorganisms such as
helminths, fungi, and protozoa. These microorganisms are comprised of eukary-
otic cells that resemble human cells. Therefore antibacterial agents are not effec-
tive against helminths, fungi, and protozoa. Likewise, antibiotics are of no use
against viruses because a virus invades a human cell and reprograms the human
cells with the virus’ genetic information to create new viruses. Since the virus is
inside the human cell, the antibiotic is ineffective.
CHAPTER 16 Antimicrobial Drugs^233