why do you get resistant bacteria if you don't finish an antibiotics course

andieje used Ask the Experts™

It is generally stated that it is important to finish a course of antibiotics because if you don't you can create antibiotic-resistant bacteria.

The logic normally given is that bacteria reproduce very quickly and mutate/evolve which each division and that one of these divisions could yield a resistant phenotype. However my primitive logic here is: if the bacteria has evolved resistance, what would it matter if you continued taking the antibiotics or not as you can no longer kill it now by taking the rest of your remaining course of antibiotics.

Perhaps it is a matter of semantics. I understand that bacteria could evolve to be more resilient to the antibiotic as the mechanisms of action of an antibiotic often involve multiple steps. In this case you should keep taking the course of the antibiotic to 'finish them off' before they get the chance to spread and go from resilient to resistant.

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a "bacterium" does not evolve
a "bacteria strain" does

the premise is basically an example of "survival of the fittest"

if you take a partial does of antibiotics, you kill off some strains but not all.
Those strains that you killed were competing with the strains you did not kill.

With a partial dose you let the ones you did not kill stick around and grow without competition.
Thus creating a population that is no longer susceptible to the antibiotic.

But that's only half of it.
Bacterium that is damaged but survives can become resistant, similar but not the same as your own immune system does.    Chicken pox might knock you for a loop, but once you have it once, you can fight it off better the next time.  This is where the mutations apply

So, you now have a population consisting of only those bacteria that are already not affected by the antibiotics
as well as those that have become resistant.

If you take all the antibiotics you can hopefully kill all of them, or reduce the infection to such a small population that your immune system can do the rest on its own.

Populations develop immunity because some individuals die off.  Those that survive (which resisted the treatment) are the only ones left to propagate.

The sad thing is that there are many people in healthcare who give antibiotics to their own family members, firneds, and patients in this fashion.  The very people who should know better are contributing to the problem.

The same thing happens with insects.  Roaches are hard to kill because repeat exposures to spray insecticides don't kill the entire population.  Resistance is developed, and scientists have moved to study baits that are carried back to the population instead of direct application of a chemical that loses effectiveness with every generation.


so when you get a bacterial infection it typically contains multiple strains?
and some of those strains may already be resistant to the antibiotic but they are in effect kept under control by competition with other strains. But when you kill off the susceptible strains the resistant strains can grow freely

But if that is the case the resistant strains would never have been killed by the antibiotics in the first place? How do antibiotics help kill these strains? Is it by killing the other strains to reduce the total numbers to the immune system can kill the resistant strains?
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>>> so when you get a bacterial infection it typically contains multiple strains?

sometimes, or, if not distinctly identified/named strains, at least minor variations.  All bacteria aren't identical after all.

yes, if you have a resistant strain, killing off all the others is still a good idea to give you system the best chance of fighting off what's left.
Think about the way an innoculation works on you. You are given a small dose of something like polio or tetanus and your antibodies race to develop into something that can defeat that virus if it is ever seen again.
If you were given a much larger dose than a small innoculation then it could kill you before your immune system can begin to fight it.
It will be the same with antibiotics, the bacteria that are left after an incomplete dose will multiply and be more resistant to the antibiotic - the ones that are developing now are developing from the few that weren't killed instantly by the antibiotic so they already have a resistance to it. A couple more days of the medicine might be enough to get rid of them completely, but not if you give them a chance to reproduce too many times.
Note 'resistant' isn't quite equal to 'immune'.

A bacterium might be somewhat resistant and therefore hang on longer than others. This bacterium reproduces and creates a small population that already has a degree of resistance. That population will have some mutations, some of which will result in a little higher resistance in the next generation. That cycle will continue for as long as the survivors can continue. Each generation will have on average a slightly better defense.

But the initial resistance is rarely equal to immunity. When the antibiotic is administered over a reasonable time, a bacterium with some initial resistance is less likely to survive long enough.

Also, a mutation may increase resistance to the new antibiotic threat while simultaneously reducing some other defense that a bacterium has against other threats. Those other threats may have time and opportunity to kill the new generation.

Also, as non-resistant bacteria are killed, the remaining resistant ones will likely be part of a much smaller population. Most of the "weak" ones are gone, taken out of the fight by the antibiotic. A body's natural defenses can then be focused only on the smaller population. While the smaller population is being handled, the risk of new infections may be limited because of the continued presence of the antibiotic even if it is ineffective against the newly resistant bacteria that remain.

An antibiotic is not necessarily the most important defense. It's often just a single weapon inside the body.



thanks a lot

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