- [Voiceover] Let's talk about how we can
actually tell bacteria apart.
We're going to pretend
that we're the scientists
or pathologists looking
underneath a microscope
to see these bacteria.
So here's something that
they might be looking at,
and you can see that
there are five examples.
So let's just write down what we see.
So vertically, in the top two
at least, it looks purple,
and on the right I can
see that they're pink.
And there's this thing I
can barely see down here,
so I'm just not going to
write a color for now,
but vertically, purple and pink.
So now let's look at the rows,
and I can see that in the first row
that they kind of look
like little circles,
kind of maybe spherical.
So I'm just going to draw a circle here,
and in the second row I can see that
they're more stretched out
and kind of like a rod.
So I'm just going to draw
this kind of stretched out
oval-looking shape or a rod.
And this bottom area, I can barely see.
So since I've been
using a light microscope
for the top view, I'm
going to use the opposite.
It's called dark field microscopy,
but don't worry too much about the name.
Just basically know that
because these don't stain the same way,
they have to be viewed differently,
and the best thing I can
call this is a squiggle,
and I hope that makes sense.
Let's go over that quickly one more time,
before we get into the official
names for these things.
So first row, I can
see that they're round,
and that's a sphere,
and in the second row,
it looks more like a stretched
out rod-looking shape,
and in the third row it kind
of looks like a squiggle.
So the single version of this is coccus,
or a bunch of them are called cocci.
And a single rod is called a bacillus,
whereas a bunch of them
are called bacilli.
And a single squiggle
is called a spirochete,
and the plural version is spirilla.
So now we know about the three main shapes
that bacteria can take,
but we haven't talked too
much about the color yet,
and this color comes from a special stain
called the Gram stain,
and it basically stains the
outside of the bacteria.
And if it stains well, it stains purple,
and if it gets washed off and
stained with something else,
it turns pink.
And if it stains purple,
we call that Gram positive,
whereas if it stains pink,
the bacteria is called Gram negative.
So here I've done that
with a positive sign
and a negative sign.
So we're going to take a closer
look at this color aspect.
So this might look familiar to you,
but this is just the
general bacterial structure
that we've gone over before.
And again, because the stain is only on
the external layers of the bacteria,
pay attention to just the external layer.
So even though right here it says capsule,
cell wall, and plasma membrane,
you'll see in a minute that these layers
are really different between Gram positive
or Gram negative bacteria,
which is shown here actually.
So think of this as a zoomed in version
of this peeled off bit.
It does have the plasma membrane layer,
a really thick cell wall
layer, and a capsule layer,
but for Gram negative bacteria
they actually have the
plasma membrane again,
but then they have a cell
wall that is much thinner
compared to Gram positive bacteria,
and it's followed by
another plasma membrane
and then a special layer called
a lipopolysaccharide layer
and then a capsule.
And don't worry that you didn't
quite catch all that right now,
because we're going to now
draw them really zoomed in
so you can see it.
So here is that plasma membrane,
and remember that it's
just a lipid bilayer
with some integral membrane proteins
which are drawn here in little blocks.
And the unique bit is this
really thick peptidoglycan layer,
and I'm just going to write
this out so you can see it.
So, peptidoglycan.
So glycan just basically
means sugar, like glucose,
and these rods that you
see here are made of
these long chains of sugars.
So that's the glycan part.
And then there's peptido,
which just means peptides,
so your proteins, and those
are just the connections
between these rods that you see here.
Oh and I just realized this is
a little bit of an optical
illusion, but there you go.
So it's almost like a brick wall
with these kinds of connections.
So that's where the word
peptidolgycan layer comes from.
It's just long sugar chains
connected by proteins.
And then the final layer is
the capsule or the slime layer.
So that's Gram positive bacteria.
So now let's take a look at
the other type of structure.
You have this inner membrane,
and then you have this very
thin peptidoglycan layer.
And again, this is made of
sugars all along these rod areas,
so I'm not going to write that out again,
because it's a super-long word,
and it's going to take up space.
Then you have an outer membrane, so here.
And then next is this brown layer,
also called the lipopolysaccharide layer,
which most people abbreviate as LPS
because lipopolysaccharide, LPS,
and it's also exactly what it sounds like.
There is a lipo part,
which is these lipid areas at the bottom,
and polysaccharides, which
are these sugar chains
at the top, polysaccharides,
and then this layer is
covered by a capsule.
And then there's a little bit of space
next to the plasma membrane on both.
So above this plasma membrane here
and between these two plasma
membranes over on this side,
and this is just called
the periplasmic space.
So, I'm just going to write it over here,
because it's easier.
And again, think of it peri, next to,
and plasmic, plasma membrane,
so next to the plasma membrane space.
Between these two layers
and above this layer,
before the peptidoglycan layer here.
So now we can think about
why this would show up purple
and this one pink.
So the Gram stain, when you
first apply it to a bacteria,
it just stains the whole thing purple,
and then you'll wash it off.
And because the Gram negative bacteria
has this very thin peptidoglycan
layer in their cell wall,
it washes right off, and
later they'll restain it
with something called Safranin,
which isn't important,
but they come in and restain this
to make sure that you
can see it and it's pink.
So, that's Gram negative.
But because this Gram positive bacteria
has this super-thick peptidoglycan layer,
it will keep its purple stain,
and it will stay purple.
So this should explain why
these particular bacteria
stain purple or pink.
And now you should have
a good understanding of
one of the main ways
to tell bacteria apart,
both by their shape and color,
and you should also understand
where that color comes from,
because of their cell wall structure.
