An antibiotic is a drug that kills or slows the growth of bacteria.
Antibiotics are one class of "antimicrobials", a larger group which also
includes anti-viral, anti-fungal, and anti-parasitic drugs. They are relatively harmless
to the host, and therefore can be used to treat infection. The term originally described
only those formulations derived from living organisms, but is now applied also to
synthetic antimicrobials, such as the sulfonamides.
Unlike previous treatments for infections, which included poisons such as
strychnine, antibiotics were labeled "magic bullets": drugs which targeted
disease without harming the host. Antibiotics are not effective in viral, fungal and other
nonbacterial infections, and individual antibiotics vary widely in their effectiveness on
various types of bacteria. Some specific antibiotics target either gram-negative or
gram-positive bacteria, and others are more wide-spectrum antibiotics. The
effectiveness of individual antibiotics varies with the location of the infection and the
ability of the antibiotic to reach this site. Oral antibiotics are the simplest approach
when effective, with intravenous antibiotics reserved for more serious cases. Antibiotics
may sometimes be administered topically, as with eyedrops or ointments.
Following earlier experiments that had demonstrated
interesting anti-bacterial effects from various bacterial secretions, the German scientist
E. de Freudenreich in 1888 isolated a bacterial secretion and noted its antibacterial
properties. Pyocyanase, secreted by Bacillus pyocyaneus, retarded the growth of other
bacteria in situ and was toxic to many disease-causing bacteria. Unfortunately,
pyocyanase's own toxicity and unstable character prevented its use as an effective, safe
antibiotic within the human body.
The first effective antibiotic discovered was penicillin.
French physician Ernest Duchesne noted in his 1896 thesis that certain Penicillium molds
killed bacteria. Duchesne died within a few years, and his research was forgotten for a
generation, until an accident intervened. Alexander Fleming had been culturing bacteria on
agar plates, one of which was ruined by an accidental fungal contamination. Rather than
discarding the contaminated plate, Fleming noticed a clear zone surrounding the colony of
mold. Having previously studied the ability of the enzyme lysozyme to kill bacteria,
Fleming realized that the mold was secreting something that stopped bacterial growth. He
knew that this substance might have enormous utility to medicine. Although he was unable
to purify the compound (the beta-lactam ring in the penicillin molecule was not stable
under the purification methods he tried), he reported it in the scientific literature.
Since the mold was of the genus Penicillium, he named this compound penicillin.
In the 1930s German scientists investigated the antibacterial
properties of certain dyes. One of these was a sulfonamide, prontosil, which was
used to treat infections in humans, where its effect was found to be due to its conversion
in the host to the active form, sulfanilimide. By today's more broad definition,
this would likely qualify as the first successful use of an oral antibiotic. During the
same era, Rene Dubos isolated tyrothricin, an antibiotic used topically for skin
infections, from soil bacteria.
With the increased need for treating wound infections in
World War II, resources were poured into investigating and purifying penicillin, and a
team led by Howard Walter Florey succeeded in producing usable quantities of the purified
active ingredient which were quickly tested on clinical cases. Physicians were exhilarated
at the rapid and reliable cure of conditions which had, until then, been difficult to
treat, terrible to endure, and frequently fatal. Observation of other species of mold and
other organisms revealed a hitherto unknown level of chemical warfare being carried out
against bacteria. New antibiotics were rapidly discovered and came into widespread use,
and a new era of research into the possibility of similarly "magic"
chemotherapeutic cures for other diseases eventually led to successes in the field of
The discovery of antibiotics, along with anesthesia and the
adoption of hygienic practices by physicians (for example, washing hands and using
sterilized instruments) revolutionized medicine. It has been said that this is the
greatest advance in health since modern sanitation. People in developed countries now find
it hard to imagine that a simple scratch once always carried the risk of infection and
Side effects range from slight headache to a major allergic
reaction. One of the more common side effects is diarrhea, which results from the
antibiotic disrupting the balance of intestinal flora, the "good bacteria" that
dwell inside the human digestive system. Other side effects can result from interaction
between the antibiotic and other drugs, such as elevated risk of tendon damage from
administration of a quinolone antibiotic with a systemic corticosteroid.
Common forms of antibiotic misuse include taking an
inappropriate antibiotic, in particular the use of antibacterials for viral infections
like the common cold, and failure to take the entire prescribed course of the antibiotic,
usually because the patient feels better before the infecting organism is completely
eradicated. In addition to treatment failure, these practices can result in antibiotic
In the United States, a vast quantity of antibiotics is
routinely included as low doses in the diet of healthy farm animals, as this practice has
been proved to make animals grow faster. Opponents of this practice, however, point out
the likelihood that it also leads to antibiotic resistance, frequently in bacteria that
are known to also infect humans, although there has been little or no evidence as yet of
such transfer of antibiotic resistance actually occurring.
One side effect of misusing antibiotics is the development of
antibiotic resistance by the infecting organisms, similar to the development of
pesticide resistance in insects. Evolutionary theory of genetic selection requires that as
close as possible to 100% of the infecting organisms be killed off to avoid selection of
resistance; if a small subset of the population survives the treatment and is allowed to
multiply, the average susceptibility of this new population to the compound will be much
less than that of the original population, since they have descended from those few
organisms which survived the original treatment. This survival often results from an
inheritable resistance to the compound, which was infrequent in the original population
but is now much more frequent in the descendants thus selected entirely from those
originally infrequent resistant organisms.
Antibiotic resistance has become a serious problem in both
the developed and underdeveloped nations. By 1984 half the people with active tuberculosis
in the United States had a strain that resisted at least one antibiotic. In certain
settings, such as hospitals and some child-care locations, the rate of antibiotic
resistance is so high that the normal, low cost antibiotics are virtually useless for
treatment of frequently seen infections. This leads to more frequent use of newer and more
expensive compounds, which in turn leads inexorably to the rise of resistance to those
drugs, and a never-ending ever-spiraling race to discover new and different antibiotics
ensues, just to keep us from losing ground in the battle against infection. The fear is
that we will eventually fail to keep up in this race, and the time when people did not
fear life-threatening bacterial infections will be just a memory of a golden era.
Another example of selection is Staphylococcus aureus,
which could be treated successfully with penicillin in the 1940s and 1950s. At present,
nearly all strains are resistant to penicillin, and many are resistant to nafcillin,
leaving only a narrow selection of drugs such as vancomycin useful for treatment. The
situation is worsened by the fact that genes coding for antibiotic resistance can be
transferred between bacteria, making it possible for bacteria never exposed to an
antibiotic to acquire resistance from those which have. The problem of antibiotic
resistance is worsened when antibiotics are used to treat disorders in which they have no
efficacy, such as the common cold or other viral complaints, and when they are used widely
as prophylaxis rather than treatment (as in, for example, animal feeds), because this
exposes more bacteria to selection for resistance.
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