Re: anti-biotic resistance theory false?
- From: "wade" <wade.hines@xxxxxxx>
- Date: 29 Sep 2005 07:11:39 -0700
nando_ronteltap@xxxxxxxxx wrote:
> Solely based on my understanding of natural selection theory, and the
> faults in the standard formulations of it, I suggest the theory about
> anti-biotic resistance is false.
>
> The current anti-biotic resistance theory goes as follows:
> You should take your medication until the program is finished,
> otherwise a mutation may occur and a resistant strain of the bateria
> infection spreads.
No. This is not the reason, or at best it is incomplete.
First it needs to be understood that the majority or antibiotics
are bacteriostatic and not bacteriocidal. This are fancy words
to say that they stop the bacteria from growning rather than
kill them. That's not 100% accurate either but is close enough.
Generally speaking, antibiotics stop bacteria from growing and
other things kill them, like your immune system. By stopping
growth processes, bacteria generally become weaker and are
more susceptable to being killed or just plain dying. In effect,
the bacteria also become sick.
Second, mutations certainly occur. They occur all the time.
Many will have occurred prior to taking the antibiotics and
thus there will be population diversity before you even get
a chance to start taking an antibiotic.
Some of these mutations, it has been documented, help the
bacteria stave off the effects of the antibiotic. Often
they are not highly effective at this but they can provide
a competative advantage to subpopulations of the bactera.
Examples of such mutations include the existence of enzymes
that are partially effective at chemically destroying the
antibiotic (typically a hydroylsis), simply binding and
sequestering it, making more of the protein it targets so
that a given dose doesn't effect that subpopulation as severely
as those with less of the enzymes that the antibiotic targets
or enzymes that has a reduced preference for binding to
the antibiotic in the first place. Understand that some
antibiotics work by entering into a biochemical pathway and
mimicing a natural substrate then not reacting the way the
natural substrate does. So an enzymes that is supposed to
be part of a busy assembly line stops doing its job. If there
is a lesser affinity for the antibiotic, less of the
assembly line gets disabled and that bacterium is less
sick than others.
Now with all this going on, the problem with only taking
part of your course of antibiotics is that you are effective
at stopping 99+% of the bacteria that are making you sick
and start to feel better. Those bacteria largely die off.
The subpopulations that have an ability to resist are less
prone to die and thus when the antibiotic is removed the
proportion of more resistent bacteria is enlarged.
Moreover, as you stop taking the antibiotic, the concentration
of antibiotic in your system slowly decreases. Bacteria
that can tolerate a modest concentration of antibiotic
can start to grow and their population will out grow the
remaining normal bacteria. During this phase, any new mutations
that make that population even better at resisting residual
antibiotic concentrations provide yet a further competetive
advantage.
That's significantly more than a magic mutation just
making a bacterium resistant to an antibiotic.
> I suggest this story should be in most cases:
> You should take your medication until the program is finished,
> otherwise the variant bacteria that are more resistant to anti-biotics
> will spread.
> That is to say there mostly are already resistant strains in the
> population, and this resistant strain mostly goes out of control to
> anti-biotics when a critical populationsize of resistant variant
> bacteria is reached, not that you have to stop the mutation from
> happening in the first place.
This is mostly true and the straw man reason you are refuting
deserves to be refuted. I expect many people have simplified
the story to sound like the one you are arguing against.
The real key though, is to understand that even the slighly
resistent strains that already exist generally have their
growth slowed and are sickened when you take antibiotics.
If you take the full course, a high proportion of these will
also die or be killed by your immune system.
If you only take your antibiotic long enough to stop growth,
you have the biggest problem when relatively large numbers
of bacteria rebound as the concentration of antibiotic in
your system decreases.
A similar problem would occur if instead of taking the recommended
dose, you took 10% of that dose. Think of the rubric, "that which
doesn't kill us only makes us stronger".
> This finding would be counter-intuitive to natural selection formulated
> as differential reproductive success, because there we are not looking
> at real numbers but relative numbers. That is to say a resistant
> bacteria may still be quite unlikely to reproduce, eventhough it is
> more likely to reproduce as a non-resistant one.
>
> Counter-intuitive and therefore ignored.
Actually, your "refutation" is far closer to the well understood
problems with not finishing your antibiotics than is the story
you seem to want to refute.
So to recap, yes, population variation will exist before you
ever start taking antibiotics and that variation is probably
more significant to resistence than any mutations that happen
during the active treatment with antibiotics. This is especially
true as even "resistent" bacteria will tend to have their growth
rate slowed in the presence of antibiotics. Yes, both partially
resistent and non resistent populations of bacteria are reduced
during treatment with antibiotics. Yes, partially resistent
populations will become a more significant fraction of the
total population of bacteria.
But what you are not considering is the effect of the horse
race for renewed growth that occurs when the dose is finished,
especially if a significant number of bacteria are still around
because the antibiotic was only bacteriostatic and not
bacteriocidal.
There are additional effects owing to bacterial sex and
the ability to share multiple half effective strategies
but regardless, finish your course of antibiotics for your
own good and for the good of others.
.
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