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An antibiotic
is a chemical compound that inhibits or abolishes the growth of
microorganisms, such as bacteria, fungi, or protozoans. The term
originally referred to any agent with biological activity against living
organisms; however, "antibiotic" now is used to refer to substances with
anti-bacterial, anti-fungal, or anti-parasitical activity. The first
antibiotic compounds used in modern medicine were produced and isolated
from living organisms, such as the penicillin class produced by fungi in
the genus Penicillium, or streptomycin from bacteria of the genus
Streptomyces. With advances in organic chemistry many antibiotics are
now also obtained by chemical synthesis, such as the sulfa drugs. Many
antibiotics are relatively small molecules with a molecular weight less
than 2000 Da.
Points of attack on bacteria by
antibiotics unlike previous treatments for infections, which often
consisted of administering chemical compounds such as strychnine and
arsenic, with high toxicity also against mammals, antibiotics from
microbes had no or few side effects and high effective target activity.
Most anti-bacterial antibiotics do not have activity against viruses,
fungi, or other microbes. Anti-bacterial antibiotics can be categorized
based on their target specificity: "narrow-spectrum" antibiotics target
particular types of bacteria, such as Gram-negative or Gram-positive
bacteria, while broad-spectrum antibiotics affect a wide range of
bacteria.
The effectiveness of individual antibiotics varies with the location of
the infection, the ability of the antibiotic to reach the site of
infection, and the ability of the microbe to inactivate or excrete the
antibiotic. Some anti-bacterial antibiotics destroy bacteria
(bactericidal), whereas others prevent bacteria from multiplying (bacteriostatic).
Oral antibiotics are simply ingested, while
intravenous antibiotics are used in more serious cases, such as
deep-seated systemic infections. Antibiotics may also sometimes be
administered topically, as with eye drops or ointments.
In the last few years, three new classes of antibiotics have been
brought into clinical use. This follows a 40-year hiatus in discovering
new classes of antibiotic compounds. These new antibiotics are of the
following three classes: cyclic lipopeptides (daptomycin),
glycylcyclines (tigecycline), and oxazolidinones (linezolid).
Tigecycline is a broad-spectrum antibiotic, while the two others are
used for gram-positive infections. These developments show promise as a
means to counteract the growing bacterial resistance to existing
antibiotics.
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