What is an antibiotic and antibiotic
An antibiotic is a medicine that kills or inhibits the growth of microbes,
such as bacteria and fungi. 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. Alexander Fleming discovered
the first antibiotic, penicillin, in 1927. 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.
What is antibiotic resistance?
Antibiotic resistance is the ability of bacteria or other microbes to
resist the effects of an antibiotic. 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
Why are bacteria becoming resistant to antibiotics?
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 bacteria 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.
What is the Centers for Disease Control and Prevention (CDC) doing
to monitor antibiotic resistance in foodborne bacteria?
To monitor antibiotic resistance, CDC, supported by the Food and Drug
Administration (FDA) and in partnership with state and local health departments
established the National Antimicrobial Resistance Monitoring System for Enteric
Bacteria (NARMS) in 1996. This system detects emerging resistance and guides
studies that evaluate where and how people become infected with resistant
foodborne bacteria. For more information, see the NARMS website at www.cdc.gov/narms
. CDC and state health
departments investigate outbreaks caused by particular bacteria, and conduct
other studies to better understand the circumstances under which they arise and
Why is antibiotic resistance a food safety problem?
Antibiotic resistance is a food safety problem for several reasons.
First, antibiotic resistance is increasing to some antibiotics, such as
fluoroquinolones and third-generation cephalosporins. These antibiotics are
commonly used to treat serious infections caused by bacterial pathogens
frequently found in food, such as Salmonella and Campylobacter.
Each year, several million people in the United States are infected with
Salmonella and Campylobacter, which usually cause diarrhea that lasts about a
week. Antibiotics are not recommended for treatment of most of these diarrheal
illnesses, but are used to prevent complications in infants, persons with
weakened immune systems, and older persons. Antibiotics may be life-saving for
several thousand people each year who have serious invasive infections, such as
bacteremia (infection in the bloodstream) and meningitis (infection of the
lining of the brain and spinal cord). Salmonella infections are treated with
ampicillin, trimethoprim-sulfamethoxazole, fluoroquinolones or third-generation
cephalosporins, but some Salmonella and Campylobacter infections have become
resistant to these medicines.
A second reason that antibiotic resistance is a food safety problem is that
more people may become ill. Ordinarily, healthy persons who consume a few
Salmonella may carry them for a few weeks without having any symptoms, because
those few Salmonella are held in check by the normal bacteria in their
intestines. However, even a few antibiotic-resistant Salmonella in food can
cause illness if the person who consumes the contaminated food then takes an
antibiotic for another reason. The antibiotic can kill normal bacteria in the
gut, letting a few Salmonella that ordinarily would be unlikely to cause
illness, take over and cause illness.
A third possible reason that antibiotic resistance is a food safety problem
is that the food supply may be a source of antibiotic-resistant genes. Harmless
bacteria present in food-producing animals could be resistant, and humans could
acquire these bacteria when they eat meat products from these animals. Once
ingested, resistant genes from these bacteria could be transferred to bacteria
that cause disease. Quantifying the extent to which this contributes to a food
safety problem is difficult.
How do bacteria that are in food become resistant to antibiotics?
Many of the bacteria in food that cause disease are found in the
intestinal tracts of animals or people. Use of antibiotics in food animals and
people can select for resistant strains that end up in the food supply. Healthy
food-producing animals commonly carry bacteria that can cause illness in humans,
including Salmonella and Campylobacter. Humans are the reservoir for some
food-borne bacteria, including Shigella and Salmonella Typhi (the cause of
typhoid fever), and increasing resistance in these bacteria is predominately the
consequence of antibiotic use in humans.
Why are antibiotics used in food-producing animals?
Antibiotics are used in food-producing animals for three major reasons.
First, antibiotics are used to treat sick animals. Second, antibiotics are used
in the absence of disease to prevent diseases during times when animals may be
susceptible to infections. This use affects a larger number of animals, because
it usually involves treating a whole herd or flock, which increases the
likelihood of selecting for organisms that are resistant to the antibiotic. In
these two examples, high doses of antibiotics are used for short periods of
time. Third, antibiotics are commonly given in the feed at low doses for long
periods to promote the growth of cattle, poultry, and swine. In the 1950s
studies showed that animals given low doses of antibiotics gained more weight
for a given amount of feed than untreated animals. Exactly how this occurs is
unknown. This effect appears to be less effective or absent in animal production
settings with good sanitation.
Does the use of antibiotics to promote growth pose a public health
The use of antibiotics to promote growth is widespread in food animal
production. Antibiotics used for growth promotion increase the pressure for
bacteria to become resistant. To address this public health problem, the World
Health Organization (WHO) has recommended that antibiotics not be used for this
purpose. It is determined that this practice is unsafe for the public's health
(World Health Organization).
How does antibiotic use in animals differ from use in
In humans, antibiotics are usually used to treat sick individuals but can
occasionally be used to prevent illness. Sick animals are sometimes treated
individually, but often whole flocks or herds of animals are treated at once,
including animals that are not ill. In humans, antibiotics are sometimes given
to healthy persons to prevent specific infections; this type of use is much more
common in animals. In humans, antibiotics are not given to promote growth, yet
this is a major reason for using antibiotics in animals.
How much is used in food-producing animals?
In the United States, data on the quantity of antibiotics given to food
animals is not available to the public or to government agencies. According to
the Union of Concerned Scientists (www.ucsusa.org
), about 25 million pounds of
antibiotics and related drugs are used every year in livestock for
nontherapeutic purposes. The Animal Health Institute (www.ahi.org
) estimates that 20.2 million pounds
of antibiotics were produced for use in farm and companion animals in 2003.
Which antibiotics used in food-producing animals are related to
antibiotics used in humans?
The majority of antibiotics used in food animals belong to classes of
antibiotics which are also used to treat human illness; these include
tetracyclines, sulfonamides, penicillins, macrolides, fluoroquinolones,
cephalosporins, aminoglycosides, chloramphenicols, and streptogramins. Because
these classes of antibiotics are similar, then bacteria resistant to antibiotics
used in animals will also be resistant to antibiotics used in humans.
How do resistant bacteria spread from animals to
Resistant bacteria may be transferred to humans through the food supply or
direct contact with animals. For example, Campylobacter lives in the intestines
of chickens. People get Campylobacter diarrhea primarily from eating undercooked
chicken. In 1989, none of the Campylobacter strains from ill persons that CDC
tested were resistant to fluoroquinolone antibiotics. In 1995, the FDA approved
the use of fluoroquinolones in poultry. Soon afterwards, doctors found
Campylobacter strains from ill persons that were resistant to fluoroquinolone
What is the human health consequence of increasing antibiotic
resistance in foodborne bacteria?
When an ill person is treated with an antibiotic to which the bacteria is
resistant, the antibiotic will not help and may even make the illness worse.
Also, increasing antibiotic resistance in the bacteria harbored by animals makes
it more likely for humans who do get infected to have a resistant strain. The
illness may last longer, be more serious, or more expensive to treat.
What can be done to slow antibiotic resistance?
Decreasing unnecessary or imprudent antibiotic use will decrease the
pressure on organisms which are exposed to them to become resistant. Ongoing
efforts in human and veterinary medicine are needed to decrease the misuse and
overuse of antibiotics, so that the efficacy of antibiotics is preserved for as
long as possible. For example, medical and veterinary professional organizations
have issued recommendations to promote appropriate therapeutic use of
antibiotics by physicians and veterinarians. A Task Force of 11 government
agencies issued a Public Health Action Plan to Combat Antimicrobial Resistance
in 2001. The Public Health Action Plan and annual progress reports on
implementation of the plan are available at www.cdc.gov/drugresistance
Additional information concerning food safety issues related to
antimicrobial resistance can be found at the FDA's Center for Veterinary