What is antibiotic resistance?

Antibiotic resistance is when bacteria are able to survive and grow in the presence of one or more antibiotics. When this occurs, the resistant bacteria continue to cause infection. Bacterial antibiotic resistance is a specific type of antimicrobial drug resistance. Other microbes, like viruses and fungi, can also become resistant to antimicrobial drugs used to treat infections with these microbes, but this article focuses on bacteria that are resistant to antibiotics.

The development of resistance commonly occurs in nature. However, because of the routine use of antibiotics, bacterial exposure to antibiotics is more frequent and resistance develops at a faster rate. Without effective antibiotics, common infections such as bacterial pneumonia, would become life-threatening once again. Complex procedures, such as open-heart surgery, would become much more dangerous and deaths from infection more common.

What are antibiotics?

Antibiotics are drugs used to treat bacterial infections. They are among the most commonly prescribed drugs for people. They target bacteria by inhibiting or stopping their growth, or by killing the bacteria.

Penicillin was the first antibiotic discovered. Alexander Fleming discovered it in 1928. It was used widely during World War II. However, the sulfonamides were the first antibiotic class used clinically (in the 1930’s). The discovery of antibiotics revolutionized medicine by making once-deadly infections treatable. There are now hundreds of antibiotics, classified into several categories. These antibiotic classes include:

  • Penicillins
  • Cephalosporins
  • Carbapenems
  • Aminoglycosides
  • Tetracyclines
  • Macrolides
  • Fluoroquinolones
  • Sulfonamides

Antibiotics work in different ways. For example, penicillin works by indirectly causing the bacterium’s cell wall to weaken and burst, so it dies. Tetracyclines, on the other hand, do not kill bacteria but inhibit their growth by stopping the bacteria from making proteins.

Some antibiotics can be used to treat a broad range of infections, while others are used to treat infections caused by specific types of bacteria. Most antibiotics can cause some side effects (e.g., stomach upset, diarrhea), though some have a higher risk of causing serious side effects (e.g., hearing damage, kidney damage).

Healthcare practitioners typically choose an antibiotic to treat an infection based on the type of infection, a person’s medical history (such as allergies to antibiotics), and often laboratory tests that can determine the bacterium causing disease and which antibiotic will work best. It is important for patients to follow instructions when taking antibiotics so that their infections are treated effectively and to help prevent the development of antibiotic-resistant bacteria.

How do bacteria become resistant to antibiotics?

There are several ways for bacteria to become antibiotic-resistant. The main one is through selective pressure. Selective pressure happens when not all the bacteria are susceptible to the antibiotic used to treat the infection, and the surviving bacteria can continue to multiply. This creates a bacterial population that is resistant to the antibiotic to which the bacteria was exposed. Selective pressure is a natural process that can be slowed but not stopped. Antibiotic overuse helps speed up selection for resistant bacteria.

How antibiotic resistance happens
Source: Melissa Brower, CDC


Bacteria can also acquire resistance when they pass genetic material back and forth from one bacteria to another. One way they can do this is through plasmids. Plasmids are pieces of bacterial DNA that can be transferred between bacteria. Some plasmids enable the bacteria to produce an enzyme that can make antibiotics useless. When the plasmid is inserted into other bacteria, antibiotic resistance can spread easily and quickly among bacteria.

Additionally, when a bacterium’s genetic material spontaneously changes, or mutates, those genetic changes can create resistance. Over time, bacteria can acquire more than one type of resistance through different mechanisms. This can lead to so-called “superbugs” that are resistant to multiple antibiotic classes. Antibiotic resistant bacteria can spread from one person to another (e.g., through touching contaminated surfaces, coughing or sneezing), resulting in the spread of hard-to-treat or untreatable infections.

Antibiotic Resistance: The Problem

Why is antibiotic resistance in bacteria a big problem?

Antibiotic resistance in bacteria is a serious global public health threat. According to the Centers for Disease Control and Prevention (CDC), 2 million people in the U.S. develop antibiotic-resistant infections each year, and at least 23,000 people die from those infections.

Resistant infections also place a burden on individuals and the medical system. Research shows that resistant infections can lead to longer hospital stays, more doctor visits, longer recovery times, and higher medical expenses. Antibiotic resistance has been shown to place a $20 billion burden on the healthcare system annually.

When alternative drugs are available to treat resistant infections, they can be less effective, produce more side-effects (more toxic), and can be more expensive.

Without effective ways to treat infections, medical procedures such as organ transplants, chemotherapy, and major surgeries become higher risk.

Antibiotic resistant bacteria can cause infections that are hard-to-treat or untreatable. Examples include:

  • Urinary tract infection
  • Blood infection (septicemia)
  • Pneumonia
  • Wound and skin infection
  • Sexually transmitted disease, particularly gonorrhea
  • Tuberculosis

Who is most at risk?

Resistant bacterial infections can affect anyone, but some groups are at higher risk than others. These include people:

  • Undergoing chemotherapy
  • Undergoing complex surgery
  • Receiving dialysis for end-stage renal disease
  • Receiving therapy that suppresses the immune system (immunosuppressive)
  • With organ or stem cell transplants
  • Who are very young or elderly
  • Who are hospitalized

How do resistant bacteria spread?

Resistant bacteria spread throughout the environment in a number of ways:

  • Person-to-person—infected people can spread resistant bacteria to others whether they have symptoms or not. This can happen through direct contact, indirect contact like coughing, or when someone leaves germs behind on a surface, like a keyboard or doorknob. This is why good hand washing practices are important to prevent the spread of resistant bacteria.
  • Animal-to-person—humans and animals can also pass resistant bacteria back and forth. Resistant bacteria are common in the guts and feces of livestock that receive antibiotics. Those bacteria can be transferred to people who are in close contact with those animals, such as farmers or veterinarians.
  • Food contamination—resistant bacteria can also end up in food as a result of agricultural uses of antibiotics. Eating contaminated food may or may not cause symptoms, but the bacteria can spread to other people either way.
  • In health care facilities—the spread of resistant bacteria in healthcare settings is of special concern. When many sick people are close together and antibiotic use is high, it creates an environment ripe for developing resistant bacteria. Improper sanitation and crowding increase the risk of health care-associated infections. Resistant bacteria also spread when infected people are transferred within or between healthcare facilities.Common healthcare-associated infections include catheter-related urinary tract infections, ventilator-associated pneumonia, and post-operative wound infections.
  • International travel—when international travelers visit regions where resistance is high, they can come into contact with resistant bacteria and move them to new locations. People hospitalized while traveling are also at risk for spreading resistant bacteria.
How antibiotic resistance in bacteria spreads
Source: Melissa Brower, CDC


How are resistant bacteria detected?

To manage resistant bacteria, first they need to be identified in individuals, healthcare facilities, and the food supply. There are a variety of laboratory tests used for identifying resistant bacteria. These include:

  • Antimicrobial susceptibility testing—Bacteria are cultured from the site of infection, identified, then exposed to antibiotics to learn which are most effective. Test results are used to choose the best drug for treatment and to monitor how resistance may change over time.
  • Molecular testing and whole genome sequencing of the bacterial DNA—These techniques rapidly identify specific bacterial genes that can confer resistance or the entire bacterial genome. The tests are used to diagnose infections and guide treatment. Whole bacterial genome sequencing is used to see how infections are related and identify outbreaks.
  • Carbapenemase production test—This test uses a culture to determine if microbes produce an enzyme called a carbapenemase that can destroy carbapenem antibiotics. If the bacteria produce the enzyme, then carbapenem antibiotics will be ineffective.
  • Colonization screening—This process is used in health care facilities to identify people who may be carrying resistant bacteria but not showing symptoms. Once carriers are identified, further screening may lead to additional actions to prevent further spread and infection.

Why are antibiotic-resistant infections increasing?

The development of antibiotic resistance in bacteria is a natural process that can’t be stopped. However, it can be slowed. Resistance is currently developing at an alarming rate because of inappropriate and unnecessary antibiotic use.

Inappropriate use in healthcare settings includes using antibiotics when they are not needed for treatment, prescribing the wrong type of antibiotic for treatment, and prescribing antibiotics for an inappropriate duration.

According to the CDC, an antibiotic prescription is inappropriate half the time. For instance, antibiotics do not resolve viral infections such as the common cold, influenza (flu), most bronchitis, most sore throats, and the majority of sinus infections. However, unnecessary antibiotic use for these viral infections is still widespread.

In food animals, antibiotics are sometimes added to livestock food and water to promote growth and prevent disease. More than half of antibiotics currently made are used to enhance livestock growth. This contributes to bacteria becoming resistant to drugs important for human health.

Examples of Antibiotic Resistant Bacteria

What are some examples of antibiotic resistant bacteria?

In 2013, the CDC identified the top 18 antibiotic resistance threats in the United States. They classified resistance threats as urgent, serious, and concerning. Urgent and serious threats require more aggressive monitoring and prevention, while concerning threats require monitoring and response to occasional outbreaks. Concerning threats have a lower risk of occurring, or there are more therapies remaining for those infections. Some key examples from each threat level category follow below.


  • Carbapenem-resistant Enterobacteriaceae (CRE): Carbapenem-resistant Escherichia coli (E. coli) and Klesbsiella infections are increasing in medical facilities and they are resistant to all or nearly all antibiotics available today. They are responsible for 600 deaths and 9,000 resistant infections in the U.S. each year.
  • Drug-resistant gonorrhea: This sexually transmitted infection (STI) is increasingly resistant to cephalosporins, the best option to treat such STIs. When cephalosporins are no longer a therapy choice, gonorrhea treatment becomes much more complex at best and untreatable at worst. Of the 820,000 estimated cases in the U.S. each year, 246,000 are resistant to all currently available antibiotics.


  • Drug-resistant tuberculosis (TB): TB is among the world’s most common infectious diseases and a frequent cause of death globally. When TB is resistant to first-line drugs, treatment becomes complex, challenging, and less effective. Of the 9,272 TB cases reported in the U.S. in 2016, 674 were drug-resistant.
  • Extended spectrum beta-lactamase-producing Enterobacteriaceae: These bacteria have an enzyme that lets them destroy a variety of penicillins and related strong antibiotics such as cephalosporins. Beta-lactamase-producing Enterobacteriaceae cause 1,700 deaths in the U.S. each year.
  • Drug-resistant non-typhoidal Salmonella: These resistant infections usually cause diarrhea, fever, and cramps. Those infections that spread to the blood can be life-threatening. There are 100,000 resistant Salmonella infections in the U.S. annually.
  • Methicillin-resistant Staphylococcus aureus (MRSA): S. aureus is one of the most common causes of bacterial infections in the industrialized world. MRSA was one of the first described antibiotic-resistant bacteria. It is resistant to penicillin-like beta lactam antibiotics, though there are other antibiotics that are still effective against it. MRSA is responsible for more than 11,000 deaths in the U.S. each year, more than HIV/AIDS, emphysema, and homicide combined.


  • Vancomycin-resistant Staphylococcus aureus (VRSA): Staphylococcus aureus is a common type of bacteria on the skin. It can enter the body through catheters or surgical procedures and cause infection. Vancomycin-resistant S. aureus is very rare, with only 14 cases confirmed in U.S. and 1 in Brazil. Vancomycin is a powerful antibiotic used to treat serious infections. There are few treatment options once bacteria become vancomycin-resistant.

Antibiotic Resistance: The Fight

What can be done to slow the development of antibiotic resistance in bacteria?

Aggressive action is necessary to slow the development of new resistance in bacteria and to prevent existing resistance from spreading. These actions can occur at multiple sources, from individuals seeking treatment and their healthcare practitioners, to health departments, health care facilities, regional laboratories, and government agencies. The following sections explain what is being done to combat antibiotic resistance and how you can help.

Preventing Infection

Preventing infections reduces the need to use antibiotics and the chances that resistance will develop. Infections can be prevented through immunization, safe food handling, frequent and thorough hand washing, good disinfection practices in healthcare settings, and using antibiotics as prescribed to prevent reinfection.


Surveillance for emerging and existing antibiotic resistant bacteria is an important step in developing strategies to combat such bacteria. The CDC sponsors a number of surveillance programs to track resistant bacteria. For example, the National Healthcare Safety Network allows health care facilities to electronically report infections, antibiotic use, and resistance.

The CDC’s Antibiotic Resistance Lab Network provides support for rapid detection and confirmation of emerging antimicrobial resistance threats through laboratory infrastructure throughout the U.S. Clinical microbiology laboratories, commercial laboratories, and state public health laboratories are all involved in surveillance and detection efforts. The CDC, FDA, and USDA also collaborate on a program to track resistances often transmitted through food.

These networks support local healthcare facilities, laboratories, and health departments so they can detect outbreaks of drug-resistant bacteria faster and respond to them before they can spread more widely.

Antibiotic Stewardship

Changing how antibiotics are used may be the single most important action to combat resistance, according to the CDC. Using antibiotics only when necessary and appropriate in people and animals is known as antibiotic stewardship.

Many health care facilities have stewardship programs to guide best practices for antibiotic use. These stewardship practices include only prescribing antibiotics when needed, choosing proper drugs, and choosing appropriate doses and durations. Stewardship programs have been shown to improve care, shorten hospital stays, and reduce pharmacy costs at healthcare facilities.

Improving Diagnostic Tools

When a patient is seriously ill and healthcare practitioners don’t have a diagnosis for an infection, they may administer multiple antibiotics until they find the best one for treatment, or simply prescribe a broad-spectrum antibiotic(s). This may harm the individual’s good bacteria (normal flora) and creates selective pressure that contributes to resistance.

Accurate diagnosis is an important step towards appropriate antibiotic use. Typically, matching an unknown infection to an antibiotic is done by culturing bacteria to identify them, then exposing the microbes to different antibiotics to learn which therapy works best. Results from these susceptibility tests usually take 24-48 hours, though some can take weeks. These wait times can hinder appropriate antibiotic use. Fortunately, new techniques that can detect bacteria independent of culture (known as “culture independent diagnostic tests” or CIDTs), such as real time molecular testing, are removing some of those barriers to appropriate antibiotic use.

Developing New Antibiotics

Since antibiotic resistance is a natural process that can be slowed but not stopped, new antibiotics will always be necessary. Growing concern about antibiotic resistance has spurred new drug development.

Between 2000 and 2010, the FDA approved five new antibiotics for clinical use. Between 2010 and 2015, the agency approved eight new therapeutic agents. The FDA is trying to make antibiotic development and approval more efficient so new treatment options are available for infections.

Antibiotic Resistance: How to Help

Everyone can help:

While bacterial resistance is an alarming global problem, there are many solutions for slowing its spread and development. Everyone has a role to play in this fight, even by taking simple actions such as hand washing and avoiding unnecessary treatment with antibiotics.

  • Prevent infections by staying current on immunizations, making a habit of frequent and thorough hand washing, and practicing safe food handling.
  • Learn when antibiotics are needed for infections. For example, they will not work for viral infections, such as the flu or the common cold.
  • Avoid pressuring health care practitioners to prescribe unnecessary antibiotics.
  • Take antibiotics exactly as your health care practitioner prescribes.
  • Avoid saving antibiotics for later or using someone else’s prescription. This could lead to taking the wrong antibiotic for an infection, allowing bacteria to multiply. If you have left over antibiotics, talk to your health care practitioner or pharmacist about the proper way to dispose of them.
  • Take steps to prevent infection while in health care facilities such as hospitals. These steps include:
    • Finding out what the facility is doing to protect against antibiotic resistance
    • Asking that anyone touching you wash their hands first
    • Washing your own hands often
    • Letting your health care practitioner know if you’ve been transferred from another facility

These resources will help you learn more about how you can help combat antibiotic resistance in bacteria:

Health care professionals can:

  • Adopt and promote antibiotic stewardship programs.
  • Strive to be precise in dosage and antibiotic selection when writing prescriptions.
  • Know what infections are present in their facility.
  • Request immediate alerts when the lab identifies resistant infections.
  • Remove temporary medical devices (e.g., catheters) as soon as they are unnecessary.

Health care facility leaders can:

  • Enforce CDC antibiotic resistance guidance.
  • Make sure their facility can identify infections and alert staff.
  • Know infection and resistance trends in nearby facilities.
  • Promote antibiotic stewardship.

For more resources on how healthcare professionals can help, visit the CDC’s page on Making Healthcare Safer.


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