Drug resistance facts*

*Drug resistance facts Medically Edited by: Charles P. Davis, MD, PhD

  • Drug resistance means any drug classified as an antimicrobial that has been compromised or has reduced or no activity when used to treat certain microbes (viruses, bacteria, fungi and parasites).
  • Antibiotics are medicines designed to kill or stop or slow growth of bacteria (and some fungi) while an antibacterial substance is designed to kill or slow bacterial growth.
  • MRSA and VRE are terms that describe specific types of antibacterial resistance; MRSA describes Methicillin-Resistant Staphylococcus aureus bacteria while VRE describes Vancomycin-Resistant Enterococi.
  • Drug resistance occurs when microbes survive and grow in the presence of a drug that normally kills or inhibits the microbe's growth.
  • The history of drug resistance began with the development of antimicrobial drugs, and the subsequent ability of microbes to adapt and develop ways to survive in the presence of antimicrobials.
  • There are many causes of antimicrobial drug resistance including selective pressure, mutation, gene transfer, societal pressures, inappropriate drug use, inadequate diagnostics, hospital use and agricultural use of drugs.
  • Diagnosis of antimicrobial drug resistance is performed by lab tests that challenge the isolated microbes to grow and survive in the presence of the drug.
  • Treatment of antimicrobial drug resistance depends on the type of infection and what the patient and their doctor decide.
  • Prevention of antimicrobial drug resistance is aided by preventing the overuse and misuse of antimicrobials; infections can be reduced by a healthy lifestyle, hand washing, and other good hygiene methods
  • Antimicrobial resistance is a growing health issue because more resistant microbes are being detected and societal pressures often result in overuse.
  • Current problems with antimicrobial resistance are predominantly being detected in the following organisms and diseases: MRSA, VRE, E. coli, Salmonella, Campylobacter and in gonorrhea, pneumonia, tuberculosis, influenza, HIV, malaria and others
Antibiotic resistance is a growing health care problem. Are you at risk?

Antibiotics 101

Medical Author: Melissa Conrad St?ppler, MD
Medical Editor: William C. Shiel, Jr, MD, FACP, FACR

Mary thinks she may have a bladder infection.She makes the call to her doctor and is able get an appointment to come in and give a urine sample. Sure enough, bladder infection. Her doctor prescribes an antibiotic for the infection. Mary goes to the pharmacy, fills the prescription and as she is driving home, begins to think of questions that she should have asked her doctor about the antibiotic.

This scenario has run through many patients' minds. What are the side effectsof the antibiotic? How soon should I begin to feel better? What if I don't feel better after 4 or 5 days? When should I call my doctor? Should I be concerned about a rashor other side effects that develop while taking the antibiotic?

Antibiotics 101

Antibiotics are a class of drugs that treat bacterial infections by stopping growth of bacteria or killing the bacteria directly. It's important to remember that antibiotics are ineffective in treating infections causes by viruses, which include the majority of colds, sore throats(with the exception of streptococcus-induced, or so-called "strep throat"), coughs, and flu-like illnesses.


Antimicrobial is a general term given to substances including medicines that kill or slow the growth of microbes.

Microbe is a collective name given to bacteria (e.g., Staphylococcus aureus, which causes some staph infections), viruses (e.g., influenza, which causes the flu), fungi (e.g., Candida albicans, which causes some yeast infections), and parasites (e.g., Plasmodium falciparum, which causes malaria)

Examples of antimicrobial agents:


An antibiotic is a medicine designed to kill or slow the growth of bacteria and some fungi. Antibiotics are commonly used to fight bacterial infections, but cannot fight against infections caused by viruses.

Example of an antibiotic:

  • Azithromycin or Zithromax (Z-Pak)®
  • Vancomycin is the last line of defense for certain methicillin-resistant Staphylococcus aureus (MRSA) infections.


Antibiotic Resistance (Drug Resistance, Antimicrobial Resistance) See pictures of Bacterial Skin Conditions See Images


Antibacterial is the term given to substances that kill or slow the growth of bacteria when treating human and environmental surfaces. These include substances that aid in proper hygiene.

Examples of antibacterial-containing commercial products are:

  • Hand soaps, gels, foams
  • Dishwashing detergents
  • Mattresses


Examples of antimicrobial (drug) resistance:

  • Methicillin-Resistant Staphylococcus aureus (MRSA)
  • Vancomycin-Resistant Enterococci (VRE)

What is drug resistance?

Antimicrobial resistance is the ability of microbes, such as bacteria, viruses, parasites, or fungi, to grow in the presence of a chemical (drug) that would normally kill it or limit its growth.

Picture of antibiotic resistant loop

Diagram showing the difference between non-resistant bacteria and drug resistant bacteria. Non-resistant bacteria multiply, and upon drug treatment, the bacteria die. Drug resistant bacteria multiply as well, but upon drug treatment, the bacteria continue to spread.
Credit NIAD

The History of Antimicrobial (Drug) Resistance


Microbes are living organisms that reproduce, thrive, and spread quickly and efficiently. Microbes include bacteria (e.g., Staphylococcus aureus, which causes some staph infections), viruses (e.g., influenza, which causes the flu), fungi (e.g., Candida albicans, which causes some yeast infections), and parasites (e.g., Plasmodium falciparum, which causes malaria).

Antimicrobial is a general term given to medicines that kill or slow the growth of microbes.

Antimicrobial drug resistance is the ability of a microbe to grow in the presence of a substance that would normally kill it or limit its growth.


In 1928 while working with Staphylococcus bacteria, Scottish scientist Alexander Fleming noticed that a type of mold growing by accident on a laboratory plate was protected from, and even repelled, the bacteria. The active substance, which Fleming called penicillin, was literally an antibiotic-it killed living organisms.

Thus began the age of using natural and, later, synthetic drugs to treat people with bacterial infections. Though not widely popular until the 1940s, antibiotics and other antimicrobials have saved countless lives and blunted serious complications of many feared diseases and infections. The success of antimicrobials against disease-causing microbes is among modern medicine's great achievements.

The Problem

After more than 70 years of widespread use, evolution of disease-causing microbes also has resulted in many antimicrobials losing their effectiveness.

As microbes evolve, they adapt to their environments. If something stops them from growing and spreading-such as an antimicrobial-they evolve new mechanisms to resist the antimicrobials by changing their genetic structure. Changing the genetic structure ensures that the offspring of the resistant microbes also are resistant.

Antimicrobial resistance makes it harder to eliminate infections from the body. As a result of a microbe's ability to survive in the presence of antimicrobials, some infectious diseases are now more difficult to treat than they were just a few decades ago. In fact, antimicrobials have helped people so effectively that humans are hurting the protective value of medicines through overuse and misuse.

More prudent use of antimicrobials will help to slow the development of resistance.

Causes of antimicrobial drug resistance

Microbes, such as bacteria, viruses, fungi, and parasites, are living organisms that evolve over time. Their primary function is to reproduce, thrive, and spread quickly and efficiently. Therefore, microbes adapt to their environments and change in ways that ensure their survival. If something stops their ability to grow, such as an antimicrobial, genetic changes can occur that enable the microbe to survive. There are several ways this happens.

Natural (Biological) Causes

Selective Pressure

In the presence of an antimicrobial, microbes are either killed or, if they carry resistance genes, survive. These survivors will replicate, and their progeny will quickly become the dominant type throughout the microbial population.


Most microbes reproduce by dividing every few hours, allowing them to evolve rapidly and adapt quickly to new environmental conditions. During replication, mutations arise and some of these mutations may help an individual microbe survive exposure to an antimicrobial.

Picture of Mutation Causes of Drug Resistance
Picture of Mutation Causes of Drug Resistance

Gene Transfer

Microbes also may get genes from each other, including genes that make the microbe drug resistant.

Picture of Gene Transfer Facilitates Drug Resistance
Picture of Gene Transfer Facilitates Drug Resistance

Picture of Gene Transfer Facilitates Drug Resistance

Societal Pressures

The use of antimicrobials, even when used appropriately, creates a selective pressure for resistant organisms. However, there are additional societal pressures that act to accelerate the increase of antimicrobial resistance.

Inappropriate Use

Selection of resistant microorganisms is exacerbated by inappropriate use of antimicrobials. Sometimes healthcare providers will prescribe antimicrobials inappropriately, wishing to placate an insistent patient who has a viral infection or an as-yet undiagnosed condition.

Inadequate Diagnostics

More often, healthcare providers must use incomplete or imperfect information to diagnose an infection and thus prescribe an antimicrobial just-in-case or prescribe a broad-spectrum antimicrobial when a specific antibiotic might be better. These situations contribute to selective pressure and accelerate antimicrobial resistance.

Hospital Use

Critically ill patients are more susceptible to infections and, thus, often require the aid of antimicrobials. However, the heavier use of antimicrobials in these patients can worsen the problem by selecting for antimicrobial-resistant microorganisms. The extensive use of antimicrobials and close contact among sick patients creates a fertile environment for the spread of antimicrobial-resistant germs.

Agricultural Use

Scientists also believe that the practice of adding antibiotics to agricultural feed promotes drug resistance. More than half of the antibiotics produced in the United States are used for agricultural purposes. However, there is still much debate about whether drug-resistant microbes in animals pose a significant public health burden.


MRSA Infection: Causes, Symptoms, and Treatment See Slideshow

Diagnosis of antimicrobial drug resistance

Diagnostic tests are designed to determine which microbe is causing infection and to which antimicrobials the microbe might be resistant. This information would be used by a healthcare provider to choose an appropriate antimicrobial treatment. However, current diagnostic tests often take a few days or weeks to give results. This is because many of today's tests require the microbe to grow over a period of time before it can be identified.

Oftentimes, healthcare providers need to make treatment decisions before the results are known. While waiting for test results, healthcare providers may prescribe a broad-spectrum antimicrobial when a more specific treatment might be better. The common practice of treating unknown infections with broad-spectrum antimicrobials can accelerate the emergence of antimicrobial resistance.

Treatment of antimicrobial drug resistance

If you think you have an infection of any type-bacterial, viral, or fungal-talk with your healthcare provider. Some infections will go away without medical intervention. Others will not and can become extremely serious. Ear infections are a good example: Some middle ear infections are caused by a virus and will get better without treatment. However, other middle ear infections caused by bacteria can cause perforated eardrums, or worse, if left untreated.

The decision to use antimicrobials should be left to your healthcare provider. In some cases, antimicrobials will not shorten the course of the disease, but they might reduce your chance of transmitting it to others, as is the case with pertussis (whooping cough).

Antibiotics are designed to kill or slow the growth of bacteria and some fungi. Antibiotics are commonly used to fight bacterial infections, but cannot fight against infections caused by viruses.

Antibiotics are appropriate to use when

  1. There is a known bacterial infection
  2. The cause of the infection is unknown and bacteria are suspected. In that case, the consequences of not treating a condition could be devastating (e.g., in early meningitis).

Of note, the color of your sputum (saliva) does not indicate whether you need antibiotics. For example, most cases of bronchitis are caused by viruses. Therefore, a change in sputum color does not indicate a bacterial infection.

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Prevention of antimicrobial drug resistance

To prevent antimicrobial resistance, you and your healthcare provider should discuss the appropriate medicine for your illness. Strictly follow prescription medicine directions, and never share or take medicine that was prescribed for someone else. Talk with your healthcare provider so that he or she has a clear understanding of your symptoms and can decide whether an antimicrobial drug, such as an antibiotic, is appropriate.

Do not save your antibiotic for the next time you get sick. Take the medicine exactly as directed by your healthcare provider. If your healthcare provider has prescribed more than the required dose, appropriately discard leftover medicines once you have completed the prescribed course of treatment.

Healthy lifestyle habits, including proper diet, exercise, and sleeping patterns as well as good hygiene, such as frequent hand washing, can help prevent illness, therefore also preventing the overuse or misuse of medications.

Antimicrobial resistance: A growing health issue

The emergence of drug-resistant microbes is not new or unexpected. Both natural causes and societal pressures drive bacteria, viruses, parasites, and other microbes to continually change in an effort to evade the drugs developed to kill them.

Natural causes

Like all organisms, microbes undergo random genetic mutations, and these changes can enhance drug resistance. Resistance to a drug arising by chance in just a few organisms can quickly spread through rapid reproduction to entire populations of a microbe.

Societal pressures

Antimicrobial resistance is fostered by the overuse and misuse of antimicrobial drugs in people as well as animals; a lack of diagnostic tests to rapidly identify infectious agents; and poor hand hygiene and infection control in healthcare and community settings.

Together, these forces contribute to the problem of drug-resistant infections that are increasingly difficult and costly to treat.

Methicillin-resistant staphylococcus aureus (MRSA)

Methicillin-resistant Staphylococcus aureus (MRSA) bacteria, increasingly seen not only in hospitals and healthcare settings (hospital acquired or HA-MRSA) but also in the wider community, especially among people in close contact such as athletes (community associated or CA-MRSA).

Vancomycin-resistant enterococci (VRE)

Vancomycin-resistant Enterococci (VRE) bacteria are resistant to vancomycin, an antibiotic regarded as a drug of last resort.

Microbes increasingly resistant to drugs

  • Food-borne bacteria such as E. coli, Salmonella, and Campylobacter that can cause diarrhea and gastroenteritis
  • Sexually transmitted bacteria that cause gonorrhea
  • Penicillin-resistant Streptococci responsible for pneumonia
  • Tuberculosis
  • Influenza
  • HIV
  • Malaria

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SOURCE: National Institute of Allergy and Infectious Diseases, National Institutes of Health. Antimicrobial (Drug) Resistance.