Biology of Microorganisms

Biology of Microorganisms Ch. 4 Study Questions I. Microscopy and Cell Morphology Ch. 4.1 Light Microscopy: 1. What are the two types of microscopes used to study cells? Two types of electron microscopy—transmission and scanning—are widely used to study cells. In principle, transmission electron microscopy is similar to the observation of stained cells with the bright-field light microscope. 2. What do each of these microscope types generally view? Scientists use three types of microscopes, each with their own purposes: optical, electron and scanning probe. 3. Define resolution: Resolution: In genetics, the degree of molecular detail on a physical map of DNA. Resolution may range from low to high. 4. What are the limits of resolution of a light microscope? The resolution of the light microscope cannot be small than the half of the wavelength of the visible light, which is 0.4-0.7 µm. When we can see green light (0.5 µm), the objects which are, at most, about 0.2 µm. Below this point, light microscope is not useful, as wavelength smaller than 400 nm is needed. 5. How is the total magnification of a microscope calculated? The total magnification for each objective is calculated by multiplying the magnification of the ocular and objective lens on your microscope. ... To get the total magnification take the power of the objective (4X, 10X, 40x) and multiply by the power of the eyepiece, usually 10X. 6. List and describe 3 commonly used organic dyes that are used to stain dead bacteria to observe their morphology or shape. (p.58 bottom) You may choose from methylene blue, Gram safranin, and Gram crystal violet. Basic stains, such as methylene blue, Gram safranin, or Gram crystal violet are useful for staining most bacteria. 7. What property of these dyes, and also the cell surfaces, allow them to ‘stick’ to the surfaces of cells so the cells can be seen more clearly under the microscope? The positive charge of the edge will adhere to the negative charged cell surface structures 8. Explain what a differential stain is. Differential Staining is a staining process which uses more than one chemical stain. Using multiple stains can better differentiate between different microorganisms or structures/cellular components of a single organism. ... This test is useful because many diseases alter the proportion of certain white blood cells. The Gram stain is the most important staining procedure in microbiology. It is used to differentiate between gram positive organisms and gram negative organisms. Hence, it is a differential stain. Gram negative and gram positive organisms are distinguished from each other by differences in their cell walls. 9. Describe the most common differential stain and why it is the most useful staining procedure in microbiology. 10. Describe the four major steps in the Gram stain. (Fig. 4.4 p. 59) a) b) Flood heat-fixed emulsion with Crystal Violet, let stand for 1 minute, rinse with water Add iodine solution, let stand for 1 minute, rinse with water Decolorize with alcohol solution holding slide on slight angle, add until run off is clear (20-30 sec), rinse with water c) d) Counter-stain with Safranin, let stand for 1 minute, rinse with water Ch. 4.3 – Electron Microscopy 11. What do transmission electron microscopes (TEM) do that light microscopes cannot do? Give an example. Electrons have much a shorter wavelength than visible light, and this allows electron microscopes to produce higher-resolution images than standard light microscopes. Electron microscopes can be used to examine not just whole cells, but also the subcellular structures and compartments within them. In the TEM, what is used instead of light to illuminate the specimens? Electron microscopes What is used instead of lenses to focus the illumination in a TEM? Electron microscopes use magnets to focus electron beams similarly to the way that light microscopes use lenses to focus light. For electrons to pass through the specimen in a TEM, the specimen must be extremely thin (20–100 nm thick). Can electron beams penetrate or pass through a cell? Electron beam irradiation can break down DNA in living organisms, resulting in microbial sterilization or death. Electrons penetrate through the cell wall and cytoplasmic membrane, causing a molecular rearrangement of the microorganism's DNA, which prevents it from reproducing. How do we get any images from electron beams if they cannot pass through a cell? Electron microscopes differ from light microscopes in that they produce an image of a specimen by using a beam of electrons rather than a beam of light. Electrons have much a shorter wavelength than visible light, and this allows electron microscopes to produce higherresolution images than standard light microscopes. 12. What are scanning electron microscopes (SEM) used for? A scanning electron microscope (SEM) scans a focused electron beam over a surface to create an image. The electrons in the beam interact with the sample, producing various signals that can be used to obtain information about the surface topography and composition.