For the last 70 years, infectious diseases caused by microbes such as bacteria, viruses and parasites have been treated by drugs known as antimicrobial agents. However, antimicrobial agents have now been used so widely that some of the microbes targeted by the drugs have adapted and become resistant to these drugs.1,2
According to the Centers for Disease Control and Prevention (CDC), at least 2 million people become infected with antimicrobial-resistant bacteria in the US every year. Around 23,000 people die as a result.1
According to the latest update (2014) from the National Institute of Allergy and Infectious Diseases, 5-10% of all hospital patients develop an infection, leading to about 90,000 deaths each year, up from 13,300 patient deaths in 1992.3
The CDC also estimate that resistance to antibiotics in the US costs around $20 billion a year in excess health care costs, $35 million in societal costs and over 8 million days of labor that people spend hospitalized.3
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You will also see introductions at the end of some sections to any recent developments that have been covered by MNT's news stories. Also look out for links to information about related conditions.
Here are some key points about antimicrobial resistance. More detail and supporting information is in the main article.
- Antimicrobial resistance can develop in bacteria, fungi, parasites and viruses.
- Antimicrobial resistance can make treating infections a costly, difficult and prolonged process.
- Antimicrobial resistance can develop due to both biological and societal causes.
- Prominent forms of antimicrobial-resistant infections include MRSA, tuberculosis, HIV and malaria.
- Antimicrobial drugs must be used exactly as they are prescribed, or else they can contribute toward antimicrobial resistance.
- Not every infection needs to be treated with antimicrobial drugs.
- A prescribed course of antimicrobial drugs should always be completed, even if the symptoms go away before treatment is finished.
- If antimicrobial drugs do not fully kill all of the microbes, they can develop resistance to subsequent treatment.
- The World Health Organization believe that a post-antibiotic era is approaching due to the prevalence of antimicrobial resistance.
- No major new types of antibiotics have been discovered in the past 30 years.
What is antimicrobial resistance?
Antimicrobial resistance (AMR) can also be referred to as drug resistance. AMR develops when micro-organisms (bacteria, fungus, parasites or viruses) no longer respond to a drug to which they were previously susceptible. As a result, standard forms of treatment to eradicate the micro-organisms no longer work.4,5
Drug-resistant forms of tuberculosis can take years to treat properly.
There are multiple consequences for AMR, including infections that are harder to control and which last much longer within the body. Hospital stays can be prolonged, increasing the economic and social costs of the disease. The risk of disease spreading is also heightened, and the risk of death is, in some cases, doubled.
A major concern of AMR is that it could lead to a post-antibiotic era; a time where antibiotics no longer work, leading to a situation where common infections and minor injuries once again have the capacity to kill.
It is important to make a distinction between antibiotic resistance and antimicrobial resistance. While antibiotic resistance refers specifically to the ability of bacteria to resist antibiotics, antimicrobial resistance is a more general term that covers resistance from any microbes to the drugs used to kill them.
As such, AMR can develop not only in bacteria but also in fungi (e.g. Candida), parasites (e.g. malaria) and viruses (e.g. HIV).
Causes of antimicrobial resistance
There are many reasons why microbes become resistant to drugs. These can be divided into biological causes and societal causes.6-8
- Mutation: when microbes replicate themselves, genetic mutations can occur. Sometimes, these mutations can lead to the creation of a microbe with genes that aid it in surviving exposure to antimicrobial agents.
- Selective pressure: microbes that carry resistance genes survive to replicate themselves. The progeny of these resistant microbes will eventually become the dominant type.
- Gene transfer: microbes can also acquire genes from other microbes. Genes that have drug-resistant qualities are easily transferred between microbes easily.
- Inadequate diagnostics: sometimes, a doctor will diagnose an infection without all the necessary information. This can lead to antimicrobials being prescribed "just in case," or broad-spectrum antimicrobials being prescribed when a specific drug would be more apt. Both occasions accelerate AMR.
- Inappropriate use: if a course of antimicrobial drugs is not fully completed then some microbes may survive and develop resistance to the drug. If drugs are used for conditions they cannot treat then microbes can also develop resistance.
- Agricultural use: it is thought that more antibiotics are used in animal agriculture than for human diseases. The use of antibiotics in farm animals can promote drug resistance, and drug-resistant bacteria may be found on meat and in food crops exposed to fertilizer or water that is contaminated with animal feces, providing a route for animal to human transmission.
- Hospital use: critically ill patients are often given much higher doses of antimicrobials. The combination of more frequent utilization and close contact among sick patients creates an environment that is ideal for the spread of AMR germs.
According to the CDC, the prescription of antibiotics in outpatient settings could be reduced by over 30% without any adverse effects to patients' health.
A study by the CDC published in 2014 in the journal Pediatrics, found that 71% of cases of C. difficile identified in children aged 1-17 were community-associated, i.e. their infection was not associated with a hospital stay.21 What is more, 73% of these children had received a prescription for antibiotics in the 12 weeks prior to developing C. difficile, with most prescriptions made out in a doctor's office.
As mentioned above, several studies have found that at least 50% of antibiotics prescribed in doctor's offices for children are for respiratory infections, most of which do not require antibiotics.21
In the US each year, at least 2 million people become infected with drug-resistant bacteria and around 23,000 people die as a result. But now, researchers have looked into horizontal gene transfer - also known as bacterial sex - to unveil why it can lead to the spread of traits including antibiotic resistance.
A new gene that makes bacteria resistant to polymyxins - the last-resort antibiotics for treating infections - is widespread in a large family of bacteria sampled from pigs and people in South China, says a new study published in The Lancet Infectious Diseases.
Antimicrobial resistance examples
Antimicrobial resistance can be found in bacteria, viruses, fungi and parasites. For each of these groups, there are a number of high-profile examples of AMR. Here are just a few examples of antimicrobial resistance:
The parasites responsible for causing malaria are transmitted by the bite of infected mosquitoes.
- Tuberculosis: Tuberculosis (TB) is a severe airborne disease caused by bacterial infection. It commonly affects the lungs and two forms of drug-resistant TB have been identified. These forms of TB are immune to standard antibiotic treatment and require multidrug treatment that can last for up to 2 years.9
- MRSA: Methicillin-Resistant Staphylococcus aureus (MRSA) is a bacterial infection that is most commonly acquired in hospitals. Formerly a well-controlled infection, over time, MRSA has become a major public health concern.10
- Gonorrhea: Gonorrhea is a sexually transmitted disease and the second most commonly reported infectious disease in the US. Recently, drug-resistant forms of this bacterial disease have been reported.11
- E. coli: Escherichia coli, commonly referred to as E. coli, is a bacterium and common cause of food-borne disease and urinary tract infection.12
- HIV: Human immunodeficiency virus (HIV) is the virus that causes AIDS. Treatment often requires different types of antiretroviral medicines, which can have unpleasant side effects. Not fully adhering to drug regimens can create new strains of HIV that are drug resistant.13
- Candida: Candida albicans is a fungus that can cause vaginal yeast infection. According to the CDC, nearly 75% of women experience a yeast infection at some point in their lives. Candida can become resistant to antifungal agents, with resistance often observed in patients who also have HIV.
- Malaria: Malaria is a disease caused by a parasite and kills around 1 million people every year worldwide. In many parts of the world, the evolution of drug-resistant parasites has led to certain antimalarial drugs becoming ineffective.16,17
On the next page, we look at treatment options, ways of preventing antimicrobial resistance and the implications of a post-antibiotic era.