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Bacteria and Bacterial Structure – Bacteria

by Vincent Racaniello, PhD
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    00:01 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.

    00:38 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.

    01:12 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.

    01:44 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.

    02:59 Let's take a look at some of these in some detail.

    03:05 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 that we recognize, to this problem of protecting the cell membrane of bacteria, and they include gram-positive and gram-negative. These are two ways of constructing the outside of the bacteria that are named after a Dutch microbiologist, his name was Gram and he devised the stain to distinguish these two kinds of bacteria. So we have gram-positive bacteria, we have the cell membrane on the very inside of the bacterium surrounding the cytosol, and then on top of that, is a thick layer called the peptidoglycan, and this protects the cell. Between the peptidoglycan and the cell membrane is a space, and we call this the periplasmic space. So that's the gram-positive solution. There is a gram-negative solution, where the cell 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. And so now, we have two periplasmic spaces as opposed to the one in the gram-positive bacteria.

    05:00 There are a few other solutions for protecting that cell membrane, but we won’t be going into them in our discussion.

    05:07 Now the Gram stain differentiates gram-positive and gram-negative bacteria according to color.

    05:14 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.

    06:03 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.


    About the Lecture

    The lecture Bacteria and Bacterial Structure – Bacteria by Vincent Racaniello, PhD is from the course Microbiology: Introduction.


    Included Quiz Questions

    1. Rod
    2. Spiral
    3. Round
    4. Sheet
    5. Squamose
    1. 5x10^30
    2. 5x10^10
    3. 5x10^20
    4. 5x10^40
    5. 5x10^59
    1. Nucleoid
    2. Nucleus
    3. Nucleolus
    4. Nuclei
    5. Nucleotide
    1. Staining
    2. Size
    3. Shape
    4. Solubility
    5. Mobility

    Author of lecture Bacteria and Bacterial Structure – Bacteria

     Vincent Racaniello, PhD

    Vincent Racaniello, PhD


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