How does finishing the prescription ensure that all the bacteria is killed and that there are no surviving or remaining bacteria? Have always been told that finishing the prescription does this but never how
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The course is purposefully longer than it needs to be to ensure it’s completely effective, a lot of people will stop when they start to feel better which can cause the infection to flare back up. By finishing the course you make sure you’ve completely eradicated the infection.
Finishing a course finishes off stragglers who have slightly elevated immunity relative to the background level of immunity so they don’t survive to reproduce and pass on their genes, producing a new generation of slightly more resistant bacteria.
Suppose you have a population of bacteria of which 99.99% experience a certain antibiotic drug lethal at a certain dose `d` sustained for a certain amount of time `t`.
In a population with random variation, there will naturally be some that have a chance of being more resistant—meaning they can tolerate a sustained dose of `d` for `t` time and still survive. But maybe they’re not so strong and resistant that if they are exposed to a dose of `d` for `t * 1.05` time they will die. Then prescribing such a course will ensure those bugs that won the genetic lottery won’t survive to pass on their superior genes.
But if you stop short of the full course, they’ll slip through the cracks and go on to produce a new generation of bacteria with slightly stronger immunity. Repeat this enough times and you get superbugs that resist antibiotics dosages 1000x the original bacteria could survive.
To be mathematically precise. Finishing a course will not necessarily get every last bacteria. But there are really huge differences between 90%, 99%, 99.9%, 99.99%, and 99.999%. It is much better for oneself and everyone else that one gets as close to 100% as one can get by finishing the full course.
ETA: There is also a large difference in killing most but leaving enough bacteria to survive and grow and killing so many that there are too few to thrive and replicate.
Every dose will kill a certain percentage of the bacteria, so the longer you take it, the fewer there are. If you take the full course, it should be enough to kill a large enough percentage of the bacteria that your body (immune system) can easily finish the job.
A few of the bacteria may develop resistance to the drug, but not many and your body will take care of them. However, if you don’t knock down the infection enough, you risk that the antibiotic resistant ones multiply. Then you end up with an infection that won’t be knocked down by antibiotics, and if your body can’t keep up with it and kill it, it might kill you.
Antibiotics can be “bacteriostatic” or “bactericidal”: “bacteriostatic” means that it prevents the growth of bacteria. bactericidal means it kills the bacteria.
In human cyanide would be human-cidal and withholding water would be human-static. If you wanted to get rid of that pesky human infection, you’d need to withhold water for the entire prescribed treatment schedule. If you stopped the treatment and gave water, the survivors would be just fine.
Bacteria infections are like a bus of highschool children on a school trip wreaking havoc in your body. Like the schoolchildren, the bacteria are somewhat diverse. Every class has smaller children, and every class has 1 or 2 giants compared to the other children.
These giants take much more food(antibiotics) before feeling full (die)
I realised in hindsight that school kids might not be the best euphemism, but it’s still eli5.
Infectious diseases specialist here. This is largely a myth. In almost every occasion researchers have tested in a randomised trial shorter vs longer courses of antibiotics, the shorter course is non inferior (ie no worse than longer) Eg https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6736742/
Humans are mostly comprised of bacteria – those living on us and in us outnumber human cells. When you have a bacterial infection there are pathogenic (bad) bacteria which are causing disease (symptoms like cough and fever) which have found and exploited our weakness.
The aim of using antibiotics or antimicrobials is designed to get the blood concentration level of the antibiotic (and then the tissue or site where there is infection) above the inhibitory level of the bacteria where they cannot replicate or where it kills them. You will not kill all the (pathogenic ie bad) bacteria. You are tilting the human vs bad bacteria in favour of the human. The immune system does the rest.
The mention of the need to complete the course is because if you take too short a course or have really poor adherence to the dosing schedule you will not have tilted the fight sufficiently in favour of the human and there will be too many pathogenic bacteria remaining.
The full course is pretty much overkill. The problem happens when people feel better after a few days and stop taking them. Then they gets sick again from the surviving bacteria that may be more resistant to the antibiotics or they go out drinking and spread it.
If they spread it, then you have an antibiotic resistant strain of bacteria on the loose. People don’t want to take seven days of antibiotics when they feel fine in three or four, you can’t drink on antibiotics and they wanna go out partying on the weekend. Taking antibiotics after you feel better (finishing the course) is just making damn sure you finish the buggers off.
Think of bacteria like a fire and antibiotics as water…everydose makes the fire smaller, but doesn’t extinguish it. If you stop mid way, the fire builds up again…only scary part is(global problem here) the fire learns to eat the water and enjoy it.
The way antibiotics generally work is that they rarely out right kill “all of the bacteria.” The drugs just limit the ability the bacteria to reproduce in some way and this gives your immune system the advantage and it moves in and does it’s job the finish the infection off.
The first doses of antibiotics allows your body to quickly do in the weakest of their populations. This is why if an antibiotic is going to work at all they do so quickly. This sets people up to mistakenly think they are cured and need not finish off every last dose. As soon as the antibiotics are withdrawn the remaining population of slightly more resistant bacteria go back to doing their thing and you have a super infection with a slightly more antibiotic resistant strain.
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