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Gram-negative Solution – Bacteria

by Vincent Racaniello, PhD
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    00:02 The gram-negative solution is quite different and this is an overview of how that works.

    00:06 Again, we have a cell or an inner membrane surrounding the cytosol of the bacteria, the goal is how do we protect that, remember the gram-positive solution was to put a thick murein layer on top, gram-negative solution we have a thin murein layer and then we have a second membrane on top called the outer membrane, so let's explore its structure in some detail. So the outer membrane is the unique feature of the gram-negative cell wall, it is chemically distinct from any membrane that we know about, it is quite resistant to harm, it happens to be a lipid bilayer, very much like the inner membrane or the cell membrane, with the exception is the outer leaflet of that bilayer is made of what we call lipopolysaccharide and this is a compound only found in bacteria. Lipopolysaccharide has three parts, the first part is lipid A and that allows the whole molecule to embed itself into the outer leaflet of the membrane, so lipid A is also known as endotoxin and this compound on its own can cause fever and shock. And so when you have a bacterial infection and you're getting fever or shock, it could be caused by lipid A. LPS is also made up of a core polysaccharide which is shown here and then finally on the outermost edge of the molecule, there are repeating subunits of what we call O-antigen. These are hydrophilic carbohydrate chains, again they exclude hydrophobic compounds, the dangerous ones.

    01:50 O-antigen varies highly between different species of bacteria and you may be familiar with a strain of E. coli called O157:H7, this has been associated with outbreaks of diarrheal disease when people eat contaminated beef typically from undercooked hamburgers.

    02:14 So the moral here is make sure your hamburger meat is well done. And this strain of E. coli happens to have a very specific kind of O-antigen, this is why we call it O157. These O-antigens are also toxic in themselves and they can account for some of the virulence of some of the gram-negative bacteria.

    02:34 Now we've looked through the gram-positive and the gram-negative solution for protecting that inner cell membrane. Now let's explore how do molecules get in, because bacteria need to take up molecules from the external environment. These are typically hydrophilic, so they can pass through these barriers and those barriers exclude hydrophobic molecules which are the dangerous ones. So here is an overview of the gram negative solution, the gram-positive solution is quite simple, the murein or the thick peptidoglycan layer passes the necessary molecules because they are hydrophilic. The gram negative solution is a little more complicated. The outer membrane actually has pores in it that will allow small hydrophilic compounds to diffuse through, but bacteria do need some larger hydrophilic compounds like vitamins and larger sugars or materials complex with iron. These get moved through proteins that are embedded in the membrane that are actually transport complexes. They can grab a compound on the outside and pull it into the cell. That brings the molecules to the periplasmic spaces. Now there are two of them in the gram-negative bacteria, there is one between the murein and the inner cell membrane and one between the murein and the outer cell membrane. Those periplasmic spaces actually contain enzymes that can digest material as it comes through the outer membrane and transport it further on. The inner cell membrane has also transport mechanisms that allow any materials that have been brought in to then get into the cytosol of the bacteria.

    04:19 There are other exterior structures of the bacterial cell that we should talk about and they're shown here, the fimbriae, the capsule and the flagellum. Many gram-positive and gram-negative bacteria have an outer capsule, this is usually made of amino acids or sugars and this capsule has a number of functions, it's a determinant of the ability to colonize a specific niche, it could be an organ or it could be a particular environment outside, it also protects the cell from drying out or desiccating. The capsule is also a defense mechanism against immune attack, phagocytic cells often try to engulf bacteria and the capsule may prevent that. Bacteria need to move, whether they be in the soil or in your intestine and one of the ways they do that is by a structure called the flagellum or flagella for plural. These actually rotate very much like a fan and propel the bacteria forward. The other way bacteria can move is by these hairy structures on the outside called fimbriae or pili. They are shorter than flagella, they are distributed over the entire surface.

    05:33 Most gram negative bacteria have pili, a few gram-positives do, but not all of them. Pili can help bacteria to attach to surfaces, so it could be a mucosal surface, in the respiratory or gastrointestinal tract and the pili also help the bacteria to move. The way they do this is they will grip a surface, then they will disappear and new pili will appear behind them, the bacteria will move forward and then this cycle is repeated over and over again, so it's a bit different from the way flagella works.


    About the Lecture

    The lecture Gram-negative Solution – Bacteria by Vincent Racaniello, PhD is from the course Microbiology: Introduction.


    Included Quiz Questions

    1. Gram negative is thinner
    2. Gram negative is thicker
    3. Gram negative has fewer teichoic acids
    4. Gram positive is thinner
    5. Gram negative is impermeable
    1. Bacteria
    2. Virus
    3. Human
    4. Canine
    5. Fungi
    1. Lipid A
    2. Murine
    3. Teichoic acid
    4. Lipoteichoic acid
    5. Lipid B

    Author of lecture Gram-negative Solution – Bacteria

     Vincent Racaniello, PhD

    Vincent Racaniello, PhD


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