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Antibiotic Resistance

What is an 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 the bacteria.   

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 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 spread. 

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 risk? 

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 humans? 

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, about 25 million pounds of antibiotics and related drugs are used every year in livestock for nontherapeutic purposes. The Animal Health Institute 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 humans? 

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 antibiotics. 

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