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What is an antibiotic?
Antibiotics, also known as antimicrobial drugs, are drugs that fight infections
caused by bacteria. Alexander Fleming discovered the first antibiotic, penicillin,
in 1927. After the first use of antibiotics in the 1940s, they transformed medical
care and dramatically reduced illness and death from infectious diseases.
The term "antibiotic" originally referred to a natural compound
produced by a fungus or another microorganism that kills bacteria which cause
disease in humans or animals. Some antibiotics may be synthetic compounds (not
produced by microorganisms) that can also kill or inhibit the growth of microbes.
Technically, the term "antimicrobial agent" refers to both natural
and synthetic compounds; however, many people use the word "antibiotic"
to refer to both. Although antibiotics have many beneficial effects, their use
has created the new problem of antibiotic resistance.
Antibiotics may be informally defined as the subgroup of anti-infectives
that are derived from bacterial sources and are used to treat bacterial infections.
Other classes of drugs, most notably the sulfonamides, may be effective antibacterials.
Similarly, some antibiotics may have secondary uses, such as the use of demeclocycline
(Declomycin, a tetracycline derivative) to treat the syndrome of inappropriate
antidiuretic hormone (SIADH) secretion. Other antibiotics may be useful in treating
To minimize risk of adverse reactions and development of resistant strains
of bacteria, antibiotics should be restricted to use in cases where there is
either known or a reasonable presumption of bacterial infection. The use of
antibiotics in viral infections is to be avoided. Avoid use of fluroquinolones
for trivial infections.
In severe infections, presumptive therapy with a broad-spectrum antibiotic
such as a 3rd generation cephalosporin may be appropriate. Treatment should
be changed to a narrow spectrum agent as soon as the pathogen has been identified.
After 48 hours of treatment, if there is clinical improvement, an oral antibiotic
should be considered.
Antibiotic resistance is the ability of bacteria or other microbes to resist
the effects of an antibiotic. Antibiotic resistance occurs when bacteria change
in some way that reduces or eliminates the effectiveness of drugs, chemicals,
or other agents designed to cure or prevent infections. The bacteria survive
and continue to multiply causing more harm.
Antibiotic resistance has been called one of the world's most pressing public
health problems. Over the last decade, almost every type of bacteria has become
stronger and less responsive to antibiotic treatment when it is really needed.
These antibiotic-resistant bacteria can quickly spread to family members, schoolmates,
and co-workers - threatening the community with a new strain of infectious disease
that is more difficult to cure and more expensive to treat. For this reason,
antibiotic resistance is among CDC's top concerns.
Antibiotic resistance can cause significant danger and suffering for children
and adults who have common infections, once easily treatable with antibiotics.
Microbes can develop resistance to specific medicines. A common misconception
is that a person's body becomes resistant to specific drugs. However, it is
microbes, not people, that become resistant to the drugs.
If a microbe is resistant to many drugs, treating the infections it causes
can become difficult or even impossible. Someone with an infection that is resistant
to a certain medicine can pass that resistant infection to another person. In
this way, a hard-to-treat illness can be spread from person to person. In some
cases, the illness can lead to serious disability or even death.
Antibiotic use promotes development of antibiotic-resistant bacteria. Every
time a person takes antibiotics, sensitive bacteria are killed, but resistant
germs may be left to grow and multiply. Repeated and improper uses of antibiotics
are primary causes of the increase in drug-resistant bacteria.
While antibiotics should be used to treat bacterial infections, they are
not effective against viral infections like the common cold, most sore throats,
and the flu. Widespread use of antibiotics promotes the spread of antibiotic
resistance. Smart use of antibiotics is the key to controlling the spread of
Antibiotic resistance occurs when bacteria change in some way that reduces
or eliminates the effectiveness of drugs, chemicals, or other agents designed
to cure or prevent infections. The bacteria survive and continue to multiply
causing more harm. Bacteria can do this through several mechanisms. Some bacteria
develop the ability to neutralize the antibiotic before it can do harm, others
can rapidly pump the antibiotic out, and still others can change the antibiotic
attack site so it cannot affect the function of the bacteria.
Antibiotics kill or inhibit the growth of susceptible bacteria. Sometimes
one of the bacteria survives because it has the ability to neutralize or evade
the effect of the antibiotic; that one bacterium can then multiply and replace
all the bacteria that were killed off. Exposure to antibiotics therefore provides
selective pressure, which makes the surviving bacteria more likely to be resistant.
In addition, bacteria that were at one time susceptible to an antibiotic can
acquire resistance through mutation of their genetic material or by acquiring
pieces of DNA that code for the resistance properties from other bacteria. The
DNA that codes for resistance can be grouped in a single easily transferable
package. This means that bacteria can become resistant to many antimicrobial
agents because of the transfer of one piece of DNA.
Although there are several classification schemes for antibiotics, based
on bacterial spectrum (broad versus narrow) or route of administration (injectable
versus oral versus topical), or type of activity (bactericidal vs. bacteriostatic),
the most useful is based on chemical structure. Antibiotics within a structural
class will generally show similar patterns of effectiveness, toxicity, and allergic
PENICILLINS. The penicillins are the oldest class of antibiotics, and have
a common chemical structure which they share with the cephalopsorins. The two
groups are classed as the beta-lactam antibiotics, and are generally bacteriocidal-that
is, they kill bacteria rather than inhibiting growth. The penicillins can be
further subdivided. The natural pencillins are based on the original penicillin
G structure; penicillinase-resistant penicillins, notably methicillin and oxacillin,
are active even in the presence of the bacterial enzyme that inactivates most
natural penicillins. Aminopenicillins such as ampicillin and amoxicillin have
an extended spectrum of action compared with the natural penicillins; extended
spectrum penicillins are effective against a wider range of bacteria. These
generally include coverage for Pseudomonas aeruginaosa and may provide the penicillin
in combination with a penicillinase inhibitor.
CEPHALOSPORINS. Cephalosporins and the closely related cephamycins and carbapenems,
like the pencillins, contain a beta-lactam chemical structure. Consequently,
there are patterns of cross-resistance and cross-allergenicity among the drugs
in these classes. The "cepha" drugs are among the most diverse classes
of antibiotics, and are themselves subgrouped into 1st, 2nd and 3rd generations.
Each generation has a broader spectrum of activity than the one before. In addition,
cefoxitin, a cephamycin, is highly active against anaerobic bacteria, which
offers utility in treatment of abdominal infections. The 3rd generation drugs,
cefotaxime, ceftizoxime, ceftriaxone and others, cross the blood-brain barrier
and may be used to treat meningitis and encephalitis. Cephalopsorins are the
usually preferred agents for surgical prophylaxis.
FLUROQUINOLONES. The fluroquinolones are synthetic antibacterial agents,
and not derived from bacteria. They are included here because they can be readily
interchanged with traditional antibiotics. An earlier, related class of antibacterial
agents, the quinolones, were not well absorbed, and could be used only to treat
urinary tract infections. The fluroquinolones, which are based on the older
group, are broad-spectrum bacteriocidal drugs that are chemically unrelated
to the penicillins or the cephaloprosins. They are well distributed into bone
tissue, and so well absorbed that in general they are as effective by the oral
route as by intravenous infusion.
TETRACYCLINES. Tetracyclines got their name because they share a chemical
structure that has four rings. They are derived from a species of Streptomyces
bacteria. Broad-spectrum bacteriostatic agents, the tetracyclines may be effective
against a wide variety of microorganisms, including rickettsia and amoebic parasites.
MACROLIDES. The macrolide antibiotics are derived from Streptomyces bacteria,
and got their name because they all have a macrocyclic lactone chemical structure.
Erythromycin, the prototype of this class, has a spectrum and use similar to
penicillin. Newer members of the group, azithromycin and clarithyromycin, are
particularly useful for their high level of lung penetration. Clarithromycin
has been widely used to treat Helicobacter pylori infections, the cause of stomach
OTHERS. Other classes of antibiotics include the aminoglycosides, which are
particularly useful for their effectiveness in treating Pseudomonas aeruginosa
infections; the lincosamindes, clindamycin and lincomycin, which are highly
active against anaerobic pathogens. There are other, individual drugs which
may have utility in specific infections.
All antibiotics cause risk of overgrowth by non-susceptible bacteria. Manufacturers
list other major hazards by class; however, the health care provider should
review each drug individually to assess the degree of risk. Generally, breastfeeding
is not recommended while taking antibiotics because of risk of alteration to
infant's intestinal flora, and risk of masking infection in the infant. Excessive
or inappropriate use may promote growth of resistant pathogens.