The person is not immune to antibiotics, but the bacteria he is infected with.

The first step would be to give the person an alternate antibiotic. Those are reserved for cases where the normal ones dont work. Often these do the trick. But some strains out there have created a broad spectrum immunity to them, so even the "spare" antibiotics wont work.

For example it is forbidden (in my country) to use these spares on animals, since they are needed to treat humans.

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If even those backups dont work, you will be out of luck. You can now only be treated for the symptoms and hope that they wont kill you.

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Here is an example of an immunity experiment:

https://www.youtube.com/watch?v=w4sLAQvEH-M&t=2s&ab_channel=Veritasium

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Edit:

Here is a short wiki about it:

https://en.wikipedia.org/wiki/Multiple_drug_resistance

First of all, people do not become resistant to antibiotics. Bacteria become resistant to antibiotics.

Basically, exposure to antibiotics can cause bacteria to evolve, as the bacteria that are not resistant are killed, and those that are survive. (Some bacteria have a way to share genes through a DNA structure called a plasmid, and antibiotic resistance can be passed along that way as well).

So the skin and the gastrointestinal tract are covered / filled with bacteria. If your bacterial microbiome has resistant bacteria in it, and you develop an infection from one of these bacteria, then the standard antibiotics might not work.

What does the doctor do?

Often, when treating a bacterial infection, the physician will obtain a test called a "culture and sensitivity". They take infected material (such as urine, pus, or sputum) and try to grow the bacteria that is causing the infection. After the lab grows it, then it can be identified through a series of tests (gram stain, for example). Once the bacteria is grown and identified, then they check sensitivities to antibiotics.

Sensitivity checking involves growing the bacteria again under new circumstances. Basically, a series of small paper discs impregnated with various antibiotics are placed on the Agar plate where they are growing the bacteria. By measuring the distance at which the bacteria can grown close to each disc, they can tell if the bacterium is likely to be sensitive or resistant to a particular antibiotic. Then the physician selects an appropriate antibiotic.

people don't become resistant to antibiotics, bacteria do. So, the immune system reacts to an infection the way it would without treatment. Antibiotics can wipe out good bacteria, for example in the gut, and give bad bacteria unchecked ability to spread which can make things harder for the immune system.

English is not my first language so forgive some errors.

His body mostly likely doesn't have anything to do with it. What we call "resistence" , at least for antibiotics, is purely linked to the pathogen.

When someone uses antibiotics with no advice from the doctors or pharmacists on how much, how to and when to take it, the worst case scenario is this pathogen developing some kind of mechanism that can preserve it's life when the same medicine is being used. The most common case can be observed with amoxicilin: if someone has already used it to treat some kind of infection in the past but didn't follow strictly the doctors orders, it's very possible that on a future infection with the same kind of bacteria it will have a survival mechanism strong enough to keep it alive against the same amoxicilin. In this case, the solution is to switch to another type of antibiotics, or a stronger one from the same family.

The kind of "resistence" that we can obeserve in humans is, in most cases, with medications that mimicry some neurotrasmitters. In this case, the "resistence" is basically your body becoming less sensible for one or more of those molecules (i.e dopamine). Example: You are using too much of some medication, so your body increases the number of receptors in your cells. By doing so, you need to increase even more the medication to "fully occupy" the same receptors and get the same feeling as before. There are other cases that can seem like "resistence" from the patient, like what we call "ultrarapid metabolizers": their enzyme, naturally, just process some kind of medicines faster and, because of it, they need to use more medicine to have the same effect.

Doctor here. Cultures are taken from blood, urine, CSF, sputum or wherever else. Whatever grows will have something called a MIC which is the minimal inhibitory concentration, which is the minimum amount of the certain antibiotic that is susceptible to the bacteria, etc. hope this helps!

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