Hello and welcome to Bacteria. My learning
goals for Bacteria include, you'll understand
the differences between gram positive and
gram negative bacteria. You will appreciate
how microbes take up substances from their
environment. You'll be familiar with components
of bacterial cells. You will have an overview
of the metabolic capabilities of bacterial
cells. And perhaps most importantly, you'll
have a newfound appreciation for all that
bacteria can do.
Let's start with a little review of what we know
about bacteria. Here's an electron micrograph
of a common bacterium from your intestine,
E. coli. These are rod shaped cells and they're
magnified about 100,000 times. Bacteria, in contrast
to the cells that make us up, eukaryotic cells,
bacteria have no nuclei and no membrane-bound
organelles like mitochondria or chloroplast.
They form spheres, rods and spirals, so they
look differently under the microscope. And
remember they're found everywhere on earth, not
just in you and on you, but in every animal
that exists, in the oceans, the soils and
even the skies. And remember the number of
bacteria on earth is an impressive number,
5×10 to the 30th; they outnumber every living
thing on the planet.
So let's take a look in more detail now at
the structure of bacteria. In the inside of
every bacteria cell is the cytosol, which
is a fluid phase medium that contains a lot
of the activity of the bacteria. The nucleic
acid, the DNA of a bacteria is present in
the structure called the nucleoid. The cytoplasm
of bacteria also contains ribosomes, these
are the places where protein synthesis occurs
and many bacteria have plasmids, these are
small pieces of DNA, they are often circular
that are apart from the main chromosome of
the bacteria. Surrounding this cytosol is
what we call a cell membrane, on top of the
cell membrane there is typically a cell wall,
another structure. And many bacteria have on
the very outside what we call a capsule. On
the very external part of the bacteria are
some structures that help movement, one is
called the fimbriae or pili and the other
is the flagellum. So that's an overview of
the bacterial cell and the various components.
Let's take a look at some of these
in some detail.
First let's look at the cell membrane. This
is a membrane that looks very much like the
membrane that you or I may have on ourselves,
an eukaryotic cell membrane. It's made up
of phospholipids. It's a phospholipid bilayer.
It has many proteins embedded in i. And it
functions by allowing the uptake of substrates,
molecules that the bacterium needs by specific
transport proteins. Now this is a very fragile
membrane, because bacteria are typically exposed
to either cellular fluids or to the environment,
the cell membrane has to have more protection,
it can't just exist on its own. And so it
has to be stabilized against detergents, osmotic
pressure and so forth. There are two solutions
in order to recognize the bacteria.
These are called gram-positive and gram-negative
after the Danish microbiologist, Gram,
who devised the stain in order to distinguish
these two kinds of bacteria.
You can see the structure of the Gram-positive
bacteria on the left side of the slide.
You can see, on the top of the cytoplasmic membrane, there
is a thick layer of peptoglycan which protects the cell.
In the Gram-negative bacteria the cytoplasmic
membrane is covered with a thin layer of peptidoglycan,
and then there is a second membrane on the outside
which is called the outer membrane.
The space between the two membranes of the Gram-negative bacteria
is called periplasmic space and contains important
enzymes such as beta-lactamases, which play an
important role in the development of antibiotic resistance.
There are a few other solutions for protecting
that cell membrane, but we won’t be going
into them in our discussion.
Now the Gram stain differentiates gram-positive
and gram-negative bacteria according to color.
Gram-positive bacteria stain purple and these
are the round bacteria that you can see in
this image, whereas gram-negative bacteria
stain red and you can see they're rod like bacteria
in this image. A few examples of different
kinds of bacteria based on the Gram stain
and the morphology will help you put these
into context. So gram-positive cocci includes
staphylococcus and streptococcus, these are both
causes of serious human infections. Gram-negative
cocci include the Neisseria, these are the
causative agents of gonorrhea, a sexually
transmitted disease. Gram-positive rods include
the clostridium and the corynebacterium.
The clostridia can cause many different diseases,
one of them is tetanus and the corynebacteria
can also cause diphtheria. And finally gram-negative
rods include, E. coli, a common inhabitant
of our gut and salmonella. Both of these bacteria
can cause gastrointestinal illness in human.