Antimicrobial Resistance: Causes, Types & Examples

Instructor: Amanda Robb

Amanda holds a Masters in Science from Tufts Medical School in Cellular and Molecular Physiology. She has taught high school Biology and Physics for 8 years.

In this lesson, we'll be exploring the important topic of antimicrobial resistance. We'll cover what is causing this global problem and explore types and examples of antimicrobial resistance.

What Is Antimicrobial Resistance?

Prior to the 1930s, a simple bacteria infection could be deadly. Soldiers with battle wounds from World War I were at the mercy of bacteria. Wounds festered and infection spread. This all changed in 1928 when Dr. Alexander Fleming discovered penicillin, a type of antibiotic. This chemical was naturally produced by fungi and killed bacteria. Today, bacterial infections are treated with a range of antibiotics and are easily cleared up by your physician.

Similarly, medications have been developed to combat other microbes or microscopic organisms, such as viruses, parasites and fungi. But as we wage our war on microbes, microbes fight back. Most microbes reproduce very quickly due to their small size. To understand how this strategy helps them evade our antimicrobial drugs, or drugs that kill microbes, let's go back and review cell reproduction.

Every time a cell needs to divide it must copy its DNA. When doing this, sometimes mutations occur that change the DNA. These mutations can sometimes be beneficial and confer resistance to the antimicrobials. When the microbes are exposed to antimicrobials all of the microbes die, except the ones with genes for antimicrobial resistance. These microbes survive and reproduce and soon the entire population is antimicrobial resistant.

Types of Antimicrobials


Bacteria are killed by a specific type of antimicrobial called antibiotics. Antibiotic resistance is a becoming major medical crisis in the world. In addition to random mutations, bacteria have another way to acquire antibiotic resistance, called plasmids. Plasmids are small circular pieces of DNA outside the genome. Often plasmids contain genes for antibiotic resistance and toxins. Bacteria trade these plasmids like collectors cards, swapping genes that increase illness. One bacteria with a plasmid for antibiotic resistance can quickly spread it to the rest of a population.

The emergence of antibiotic resistance is a product of human behavior. The more antibiotics are used, the more bacteria become resistant to them. For example, antibiotics are often prescribed for viral infections, in which they do not work. Livestock are treated with antibiotics to prevent infections. Even hand soaps contain antibiotics! The more we use antibiotics, the more resistance will emerge.

Example: MRSA

One of the most famous examples of antibiotic resistance is methicillin-resistant Staphylococcus aureus (MRSA). This so called super-bug is resistant to many antibiotics and has become a major issue in health care.

MRSA is resistant to many types of antibiotics

MRSA causes staph infections, which usually infect the skin but can spread to other parts of the body as well, causing sepsis and eventually death. It can be very difficult to treat due to antibiotic resistance. MRSA is a rampant cause of secondary infections in hospitals, and can also be found in communities where people share towels, mats, equipment or have other skin to skin contact.


Viruses are non-living infectious particles that cause disease. These pathogens require a host cell to reproduce. Once inside, they infect host cells and hijack the host cell machinery, forcing the cell to make more viruses. Viruses reproduce incredibly quickly, much faster than cells. The faster the virus reproduces, the greater the chances of it acquiring a mutation for antiviral resistance. Thus, viruses acquire resistance much faster than bacteria.


Human immunodeficiency virus (HIV) infects immune cells called T-cells. Left untreated it can destroy the immune system, making the patient susceptible to other infections. HIV patients often die of secondary infections like pneumonia. HIV is tricky because it accumulates mutations faster than other types of viruses, meaning it can quickly evolve antiviral resistance.

HIV shown in yellow infects a T-cell shown in blue

As the virus evolves, the originally prescribed antiviral drugs can no longer control the infection, and HIV starts to take hold again. Antiviral resistance sometimes allows HIV to become resistant to all antivirals in a particular class, even if the person has never taken them. Strict adherence to medication is one way to help prevent antiviral resistance.


Unlike the tiny HIV particles, parasites are much larger. They can range from a single cell to macroscopic multicellular organisms. Parasites are organisms that depend on a host for nutrients and are harmful to the host.


Malaria is the most common parasitic infection in the world and is responsible for hundreds of thousands of death each year, particularly in sub-Saharan Africa. Malaria is caused by four members of Plasmodium species. These parasites are transmitted through mosquito bites.

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