The medical world can be a confusing place. Patients and their families might feel overwhelmed by the large vocabularies and complicated explanations they get from their health care providers. Students entering health care also struggle to grasp the complexity of health sciences, and are forced to memorize huge amounts of information. We hope to make understanding the medical world a bit easier. Look around! These videos do not provide medical advice and are for informational purposes only. The videos are not intended to be a substitute for professional medical advice, diagnosis or treatment. Always seek the advice of a qualified health provider with any questions you may have regarding a medical condition. Never disregard professional medical advice or delay seeking it because of something you have read or seen in any video.
Antibiotic resistance describes a bacteria’s ability to survive while being exposed to antibiotics.
When a bacteria is exposed to antibiotics there are three possible outcomes - they will die, they will stagnate (not multiply), or they will multiply. Three main factors will predict which is more likely to happen; antibiotic concentration, bacterial mutation, and bacterial genetic exchange.
Generally, more antibiotic getting to a bacteria will cause it to stagnate/die, and less antibiotic will allow it to multiply. Some bacteria live within a “biofilm”, which is a jelly-like substance where thousands of bacterial cells are suspended inside (think raspberry seeds in raspberry jelly). It’s sort of like a big, thick energy shield. The antibiotic has to move (diffuse) through the biofilm to reach all of the bacterial cells. Some cells that are buried deep within the biofilm are exposed to only a fraction of the antibiotic that reaches the surface.
When bacterial cells replicate, there is a small chance the new bacterial cell will not be exactly the same as the original bacterial cell. We call these errors in the copied cell a mutation. In one bacterial cell, the cell wall could be slightly different, in another an enzyme works poorly, and so on. Mutations are key to the idea of evolution, and all of the diversity you can see in nature came from a series of many mutations over hundreds of thousands of years. In animals, it can take centuries or millennia for a species to adopt a mutation which helps it survive (and sometimes these mutations create entirely new species). It takes this long in animals because it takes years for most animals to grow up and reproduce.
Bacteria on the other hand can multiply within hours, allowing for more mutations to occur over a shorter period of time. These mutations (such as a change to the bacteria’s cell wall) can make it difficult for the antibiotics to enter the bacteria or stick to it, making the antibiotic less effective at hurting or killing the bacteria.
There are four common mutations bacteria undergo to become resistant to antibiotics:
These little mutant bacteria may thrive where the non-mutant bacteria die, and new antibiotics (or more of the same antibiotic, if the mutants are only slightly resistant) must be used to kill them.
Humans continue to search for new antibiotics to help the immune system, and bacteria continue to have mutant members in their colonies that can potentially resist antibiotics!
Bacterial genetic exchange
A curious habit of bacteria is that they love to share information when they meet, like two old friends at the park. This happens even between two different bacterial species. As a result, once a single bacterial species has managed to resist antibiotics with a gene(s), that gene(s) can get copied and passed around to other bacteria. It’s like passing around a juicy bit of gossip - as more meetings occur, more and more bacteria learn how to resist an antibiotic!
In order to pick the best antibiotic for treating the infection, its useful to know how effective the antibiotic would be at preventing a bacteria from growing or simply killing the bacteria. You can do an experiment to figure it out! You can even see how resistant bacteria is to antibiotics by running the same experiment multiple times using a variety of antibiotics.
Place a tiny but equal amount of bacteria into a series of test tubes full of clear, nutritious bacterial broth (chicken soup for the bacteria!). Next, put increasing amounts of antibiotic into the test tubes (doubling the antibiotic concentration as you go). Now wait 24 hours.
1. Some of the tubes have turned cloudy! The concentration of the antibiotic in these tubes are too low to prevent the bacteria from multiplying.
2. Some of the tubes are still clear! The concentration of the antibiotic in these tubes are high enough to prevent the bacteria from multiplying. The lowest concentration of an antibiotic needed to stop bacteria from multiplying is called the Minimum Inhibitory Concentration (MIC). In the diagram above, the MIC is the first clear test tube. But wait! Are the concentrations of the antibiotic in these clear tubes enough to kill the bacteria or just stop them from multiplying? We can find out!
Take a small sample of fluid from each of the clear test tubes in step 1 and put each sample into a new test tube filled with broth. Do not put antibiotics in these new test tubes. Once again wait 24 hours. Note: There will be a bit of antibiotic carried in the sample from step 1, however not enough to affect the results in step 2.
3. Some of the test tubes have turned cloudy! The bacteria is growing again! This means the concentration of the antibiotic in step 1 didn't kill the bacteria, just stopped it from multiplying.
4. Some of the test test tubes are still clear! This means the concentration of the antibiotic in step 1 killed the bacteria. The lowest concentration of an antibiotic needed to kill the bacteria is called the Minimum Bactericidal Concentration (MBC). In the diagram above, the first clear test tube in step 2 is the MBC.
Once you know the concentration of an antibiotic needed to stop a bacteria from growing (MIC) or living (MBC), you need to know whether that concentration can be safely given to a person. If so, then we would say that a bacteria is “susceptible” to an antibiotic, and if not, then we would say that a bacteria is “resistant” to an antibiotic. The goal is to pick an antibiotic that will be effective against the bacteria causing an infection, but won’t hurt a patient or destroy their healthy ecosystem of bacteria.
Over the years some bacteria have become more resistant to antibiotics than others. Here’s a quick glance at some of the most common and/or concerning resistant bacteria:
To limit antibiotic resistance, it’s important to limit the exposure that bacteria all over the planet (inside of us, within animals, and living in the environment) have to antibiotics. two ways you can help make sure bacteria are not getting overexposure to antibiotics are: