When someone is sick with an infection and they go to the doctor and get antibiotics, they rarely think about how the doctor determines which antibiotic to use. That is where the culture and sensitivity test comes in. It's how doctors objectively determine what infection we have and what the best treatment for that infection is.

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What is an Infection?

An infection is when a pathogenic microorganism gets into a living thing's body and causes illness. We live with microorganisms all the time, living in and on us. We even live with ones that make us sick, but we don't get sick from them when they are kept in proper check by the 'good bugs' in our body that are helpful.

When the balance tips, for various reasons, and the pathogenic ones are able to proliferate, or we get exposed to a pathogen we can't fight off, we get sick. When we get sick from one of these pathogens, we call it an infection. Infections can be viral, fungal, bacterial, or even from parasites, but generally we'll be talking about bacterial infections just to simplify matters.

Culture Isn't Just for Countries

When we get certain infections, the doctor can take a sample of the infection and sent it off to the lab to find out what the infection is and what antibiotics will work the best (again, assuming it's a bacterial infection). Sometimes a sample of an infection is easier or harder to obtain. In the case of a wound, for instance, they can just swab the wound while a suspected liver abscess might be a lot more challenging.

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This is not the kind of culture we are talking about
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In any case, once the sample is obtained, they will do a procedure you may have done at some point in school and rub the sample all over a small disk containing a medium favorable for growth of the pathogen. In most cases, it's a gelatin made from seaweed called agar made completely sterile through heat so that only the bug you want to grow will grow. Add some nutrients to it and it becomes a bit like leaving bread out to grow mold.

Next Comes the Sensitivity

Then you wait. It will take 2-3 days to get a good growth in the dish and, once this occurs, a sample will be sent to a specialized doctor to positively identify the culprit. Next they will put a half dozen or so dots soaked in whatever treatments have been best to treat that pathogen in the past. In the case of a bacterial infection, they will put dots soaked in different antibiotics.

Then again, they wait another day and open up the agar and look to see what happened. Around each dot, the technician or doctor will be able to see whether and how much area around each dot the bacteria have been killed off.

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If the area around the dot is small or nonexistent, they will determine that the bacteria is resistant to that antibiotic. If there is a large area around the dot where the bacteria have been killed, then the bacteria will be determined sensitive to that antibiotic, and that is usually the best antibiotic for that infection.

This is Impractical, Expensive, and Takes a Long Time

Unfortunately, I've just described a 3 day process, and most people don't have 3 days to wait if they are sick. So, in most cases, doctors will put someone on an antibiotic based on the information that has already been gathered from millions upon the millions of these cultures and sensitivities that have been collected over the decades, and send you on your way without even taking a sample.

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In cases where someone has taken antibiotics multiple times and still have the infection, or they have a complex or unusual infection, like a high fever where they suspect a blood infection, they will do these tests to ensure the correct antibiotic is used.

And that is the general procedure used to determine what antibiotic you should be put on when you go to the doctor with some kind of infection. While watching pathogens grow is not the most exciting job in the world, someone has to do it, and it's a vital part of the health care system we simply couldn't get along without.