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Have you ever had strep throat? Not all sore throats are strep, but strep throat is caused
by bacteria. And if you have had it before, it typically really hurts. Chances are, if
you have had it, you have taken a course of antibiotics. Antibiotics are specifically
designed to destroy prokaryote cells---which in your body will be bacteria---instead of
your own cells which are eukaryote cells.
We have another video all about bacteria and antibiotics, but this video is going to mention
a concern we all have right now about antibiotic resistance that can happen in bacteria. It’s
also the perfect opportunity to illustrate a process that usually takes a very long time
to see----natural selection. Let’s explain a basic example of natural selection.
Consider these frogs sitting on logs in their habitat. Let’s assume these are the same
species of frog so they can breed with each other and they pass down their DNA to their
offspring. Naturally, there is variety in these frogs. Some of these frogs are darker
green---maybe almost a brown. Some of them are lighter green. There is a variety of traits
even in the same species. Just like in people.
So back to the log. You know what else is in this habitat? Predators. And the predators
are finding here that the lighter frogs are much easier to see in this habitat than the
darker frogs. So, in this particular environment, the darker frogs are having an easier time
surviving and potentially more fitness if they can breed. Fitness, in the biological
sense, is determined by not how long they live---but by how many offspring they have.
These darker frogs pass down their DNA to their offspring so that the new baby frogs
will have DNA from their parents. The lighter frogs are being selected against since they
are easier to see in this habitat. Over a long period of time, you could expect to see
a higher frequency of darker frogs. If it continues for a very long time, it could even
result in all frogs in this area being darker. Evolution----which is change over time----could
take place because natural selection has occurred. Natural selection is a mechanism of evolution.
One thing to point out…it doesn’t necessarily mean that the allele for lighter color is
gone completely. The allele could be recessive and carried within the population. And because
of processes like crossing over and mutations, there are opportunities for variety. But if
this habitat and predators do not change, darker frogs will continue to have more fitness.
Now we do want to point out that variation or mutations are not things that a frog can
“will” itself to have. Frogs can’t just think, “You know, it’s better for me to
be darker so I’m just going to have some variation to be darker.” Because if that
was true, well, let’s just say I might be a bit different. Mutations and variation are
RANDOM. It’s possible they might not have any effect on an organism’s fitness so in
that case, the genes are be passed on if that organism happens to be able to reproduce.
Or variations and mutations could be negative. If they are negative and negatively affect
the organism’s fitness----meaning no babies----well then that trait will not be passed down. But
if they have a positive effect on the fitness, then that frog may have more babies than average
because that trait is helping them survive and reproduce. More babies will receive the
passed those genes. Over time, that trait that is an advantage will be more frequent
in the population.
So what does this have to do with natural selection in bacteria that occurs from antibiotics?
Well keep in mind, there’s variation in bacteria as well. For example, some of them
might have an enhanced cell wall that makes it hard for an antibiotic to enter or they
could contain enzymes that could assist in breaking down an antibiotic. Now remember
the bacteria didn’t “will” themselves to have those traits.
Like our frog example, they are random variations. However when you take
antibiotics, the environment is being altered. The bacteria that have traits that allow them
to survive that particular antibiotic have higher fitness and can reproduce, passing
on their DNA. Bacteria that do not have traits to help them with the antibiotics do not have
very much fitness. Because they’re dying. They can’t have fitness if they’re dead.
Selection has taken place and the resistant bacteria are the survivors. Once all of their
competition is killed off, they can reproduce easily with few competition and their offspring
can inherit the gene that allows them to be resistant to that specific type of antibiotic.
Bacteria also have this amazing ability to transfer genes to other bacteria, so they
could share that resistant gene with other bacteria. It is possible that this specific
type of antibiotic may not be as effective for this person in the future. Hospitals especially
have this challenge because they are treating multiple patients with bacterial infections
in a closed environment. Healthcare workers must have good hygiene practices so that they
do not spread resistant bacterial infections from one infected person to another.
So while this is an example of natural selection that is observable in a shorter amount of
time---which can result in evolution in bacteria, what can be done about this resistance issue?
Scientists are often in the process of developing new antibiotics----because we do have many
types of antibiotics available---so they can stay one step ahead of bacteria. But there
are still bacterial strains out there that have developed a resistance to commonly used
antibiotics making it very difficult to treat. So it’s important to only take antibiotics
for bacterial infections. Antibiotics don’t work against viruses, and you don’t want
to be introducing antibiotics in your body when you don’t need them. That will just
further select for bacteria that have some form of resistance. It’s also important
to mention that many vaccines like the DTaP vaccine can protect you from serious, potentially
deadly bacterial infections such as diptheria, tetanus, and pertussis. Well that’s it for
the amoeba sisters and we remind you to stay curious!