Section III. PENICILLINS AND CEPHALOSPORINS; ERYTHROMYCIN
Antibiosis is an association between two or more organisms that is detrimental to
one of them. A chemical agent responsible for this antagonistic relationship is called an
antibiotic. An example of this is the relationship between certain bacteria and various
plants (fungi and molds). The bacteria are unable to live and grow in the presence of
these plants or their extracts. The extracts are called antibiotics. There are numerous
official and nonofficial antibiotics. Their powerful antibacterial action is extremely
valuable in combating diseases caused by pathogenic organisms. Antibiotics, then, are
products of living microorganisms (or similar synthetic substances) that kill or inhibit the
growth of other microorganisms. Penicillin is the classic example.
5-12. SPECTRUM OF ACTIVITY
The antibiotics can be classified as either "narrow" or "broad" spectrum
antibiotics, depending upon the different organisms against which they are effective.
Penicillin, for example, is effective against most gram-positive bacteria but against only
a few gram-negative cocci. For this reason, it is called a narrow spectrum antibiotic.
Tetracycline, on the other hand, is effective against the common infecting gram-positive
and gram-negative organisms; it is a broad spectrum antibiotic.
The individual antibiotics can best be studied pharmacologically by first
discussing penicillin and then comparing the other agents with it. Penicillin is the
antibacterial substance derived from the mold Penicillium notatum or Penicillium
chrysogenum. Several forms have been identified and designated as penicillin F, G, K,
O, V, and X. Of these, penicillin G was found to be the best, and the manufacture of the
others has been discontinued.
a. Absorption. When administered by intramuscular injection, potassium
penicillin G is very rapidly absorbed into the circulation. Since it is also rapidly excreted
by the kidneys, repeated doses must be given at frequent intervals to maintain blood
levels, or a more slowly absorbed oil dispersion or sparingly soluble salt must be used.
b. Excretion. Penicillin is mainly excreted by the kidneys in urine. Most of a
therapeutic intramuscular dose of potassium penicillin G is excreted in 5 hours, but
lessening amounts are detected for as long as 7 to 12 hours.
c. Mechanism of Action. The mode of action of penicillin has long been in
dispute. The present concept, arrived at after long study by several researchers, is that
penicillin's antibacterial activity is due to an inhibition of vital metabolic functions within