Skip directly to search Skip directly to A to Z list Skip directly to navigation Skip directly to page options Skip directly to site content

How Antibiotic Resistance Happens

Antibiotics save lives but any time antibiotics are used, they can cause side effects and lead to antibiotic resistance.

Since the 1940s, antibiotics have greatly reduced illness and death from infectious diseases. However, as we use the drugs, germs develop defense strategies against them. This makes the drugs less effective.

Antimicrobials Treat Infections Caused by Microbes

Microbes are very small living organisms, like bacteria. Most microbes are harmless and even helpful to humans, but some can cause infections and disease. Drugs used to treat these infections are called antimicrobials. The most commonly known antimicrobial is antibiotics, which kill or stop the growth of bacteria.

Two Types of Microbes

  • Bacteria cause illnesses such as strep throat and food poisoning. Bacterial infections are treated with drugs called antibiotics (such as penicillin).
  • Fungi cause illnesses like athlete’s foot and yeast infections. Fungal infections are treated with drugs called antifungals.

How Germs Become Resistant

Icon shows lots of germs, a few are drug resistant.


You get an infection. There are a lot of bacteria making you sick. Some of those bacteria are resistant to antibiotics.

Icon shows that antibiotics kill bacteria causing the illness, as well as good bacteria protecting the body from infection.


Antibiotics kill the bacteria making you sick, but the resistant bacteria are not killed. Antibiotics also kill good bacteria that protect the body from infection.

Icon shows the drug-resistant bacteria are now allowed to grow and take over.


Resistant bacteria have defense strategies that protect them from antibiotics. They multiply and continue to make you sick.

Icon shows that some baveria give their drug-resistance to other bacteria, causing more problems.


Resistant bacteria can give their drug-resistance to other bacteria. Antibiotics cannot treat your sickness, and people can spread these resistant germs to others.

 Top of Page

Germ Defense Strategies

To survive, germs are constantly finding new defense strategies, called “resistance mechanisms,” to avoid the effects of antibiotics. Bacteria develop resistance mechanisms by using instructions provided by their DNA. Often, resistance genes are found within plasmids, small pieces of DNA that carry genetic instructions from one germ to another. This means that some bacteria can share their DNA and make other germs become resistant.

Examples of Defense Strategies for Germs

Germs can use defense strategies to resist the effects of antibiotics. Here are a few examples.

Resistance Mechanisms (Defense Strategies)
Resistance Mechanisms
(Defense Strategies)
Restrict access of the antibiotic By limiting the number or changing the size of the openings in the cell wall, resistant bacteria can keep antibiotic drugs from entering the cell altogether.

Example: Gram-negative bacteria have an outer layer (membrane) that protects them from their environment. These bacteria can use this membrane to selectively keep antibiotic drugs from entering.

Get rid of the antibiotic Resistant bacteria can use pumps in their cell walls to remove antibiotic drugs that enter the cell.

Example: Some Pseudomonas aeruginosa bacteria can produce pumps to get rid of several different important antibiotic drugs, including fluoroquinolones, beta-lactams, chloramphenicol, and trimethoprim.

Destroy the antibiotic Some resistant bacteria use enzymes to break down the antibiotic drug and make it ineffective.

Example: Klebsiella pneumoniae bacteria produce enzymes called carbapenemases, which break down carbapenem drugs and most other beta-lactam drugs

Change the antibiotic Other resistant bacteria use enzymes to alter the antibiotic drug so that it loses its effectiveness.

Example: Staphylococcus aureus bacteria add compounds to aminoglycoside drugs to change its function.

 Bypass the effects of the antibiotic Some antibiotic drugs are designed to disrupt important processes critical to a bacteria’s survival, like the process of making nutrients. If successful, the antibiotic drug will keep the bacterium from performing all the steps needed in the process. Some resistant bacteria, however, have developed different and new processes to get around these drug disruptions. The new process may be slower but they can still bypass the effects of the drug.

Example: Some Staphylococcus aureus bacteria can bypass the drug effects of trimethoprim.

 Change the targets for the antibiotic Many antibiotic drugs are designed to single out and destroy specific parts (or targets) of a bacterium. Resistant bacteria can change the look of their targets so that the antibiotic does not recognize and destroy them, allowing the bacteria to survive.

Example: E. coli bacteria with the mcr-1 gene can add a compound to the outside of the cell wall so that the drug colistin cannot latch onto it.

 Top of Page