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Observing Microorganisms Through a Microscope
Chapter 3 Observing Microorganisms Through a Microscope Lectures prepared by Christine L. Case © 2013 Pearson Education, Inc. 1
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Tick Actual size Red blood cells
Figure 3.2 Microscopes and Magnification. Unaided eye ≥ 200 m Light microscope 200 nm – 10 mm Tick Actual size Scanning electron microscope 10 nm – 1 mm Red blood cells Transmission electron microscope 10 pm – 100 m E. coli bacteria T-even bacteriophages (viruses) Atomic force microscope 0.1 nm – 10nm DNA double helix
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Units of Measurement 1 µm = 10–6 m = 10–3 mm 1 nm = 10–9 m = 10–6 mm
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Microscopy: The Instruments
A simple microscope has only one lens
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Microscope replica Lens Location of specimen on pin
Figure 1.2b Anton van Leeuwenhoek’s microscopic observations. Lens Location of specimen on pin Specimen-positioning screw Focusing control Stage-positioning screw Microscope replica
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Light Microscopy The use of any kind of microscope that uses visible light to observe specimens Types of light microscopy Compound light microscopy Darkfield microscopy Phase-contrast microscopy Differential interference contrast microscopy Fluorescence microscopy Confocal microscopy
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Principal parts and functions
Figure 3.1a The compound light microscope. Ocular lens (eyepiece) Remagnifies the image formed by the objective lens Fine focusing knob Coarse focusing knob Body tube Transmits the image from the objective lens to the ocular lens Arm Objective lenses Primary lenses that magnify the specimen Stage Holds the microscope slide in position Condenser Focuses light through specimen Diaphragm Controls the amount of light entering the condenser Principal parts and functions Illuminator Light source Base
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Compound Light Microscopy
In a compound microscope, the image from the objective lens is magnified again by the ocular lens Total magnification = objective lens ocular lens
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The path of light (bottom to top)
Figure 3.1b The compound light microscope. Ocular lens Line of vision Path of light Prism Body tube Objective lenses Specimen Condenser lenses Illuminator Base with source of illumination The path of light (bottom to top)
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Compound Light Microscopy
Resolution is the ability of the lenses to distinguish two points A microscope with a resolving power of 0.4 nm can distinguish between two points ≥ 0.4 nm Shorter wavelengths of light provide greater resolution
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Compound Light Microscopy
The refractive index is a measure of the light-bending ability of a medium The light may bend in air so much that it misses the small high-magnification lens Immersion oil is used to keep light from bending
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Oil immersion objective lens
Figure 3.3 Refraction in the compound microscope using an oil immersion objective lens. Unrefracted light Oil immersion objective lens Without immersion oil most light is refracted and lost Immersion oil Air Glass slide Condenser lenses Condenser Iris diaphragm Light source
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Brightfield Illumination
Dark objects are visible against a bright background Light reflected off the specimen does not enter the objective lens ANIMATION Light Microscopy
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Figure 3.4a Brightfield, darkfield, and phase-contrast microscopy.
Eye Eye Eye Ocular lens Ocular lens Diffraction plates Only light reflected by the specimen is captured by the objective lens Undiffracted light (unaltered by specimen) Objective lens Objective lens Refracted or diffracted light (altered by specimen) Specimen Unreflected light Specimen Condenser lens Condenser lens Opaque disk Annular diaphragm Light Light Light Brightfield
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Darkfield Illumination
Light objects are visible against a dark background Light reflected off the specimen enters the objective lens
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Figure 3.4b Brightfield, darkfield, and phase-contrast microscopy.
Eye Eye Eye Ocular lens Ocular lens Diffraction plates Only light reflected by the specimen is captured by the objective lens Undiffracted light (unaltered by specimen) Objective lens Objective lens Refracted or diffracted light (altered by specimen) Specimen Unreflected light Specimen Condenser lens Condenser lens Opaque disk Annular diaphragm Light Light Light Darkfield
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Phase-Contrast Microscopy
Accentuates diffraction of the light that passes through a specimen
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Figure 3.4c Brightfield, darkfield, and phase-contrast microscopy.
Eye Eye Eye Ocular lens Ocular lens Diffraction plates Only light reflected by the specimen is captured by the objective lens Undiffracted light (unaltered by specimen) Objective lens Objective lens Refracted or diffracted light (altered by specimen) Specimen Unreflected light Specimen Condenser lens Condenser lens Opaque disk Annular diaphragm Light Light Light Phase-contrast
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Differential Interference Contrast Microscopy
Accentuates diffraction of the light that passes through a specimen; uses two beams of light
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Figure 3.5 Differential interference contrast (DIC) microscopy.
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Fluorescence Microscopy
Uses UV light Fluorescent substances absorb UV light and emit visible light Cells may be stained with fluorescent dyes (fluorochromes)
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Figure 3.6b The principle of immunofluorescence.
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Confocal Microscopy Cells are stained with fluorochrome dyes
Short-wavelength (blue) light is used to excite the dyes The light illuminates each plane in a specimen to produce a three-dimensional image Up to 100 µm deep
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Figure 3.7 Confocal microscopy.
Nucleus
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Two-Photon Microscopy
Cells are stained with fluorochrome dyes Two photons of long-wavelength (red) light are used to excite the dyes Used to study cells attached to a surface Up to 1 mm deep
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Food vacuoles Figure 3.8 Two-photon microscopy (TPM).
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Scanning Acoustic Microscopy (SAM)
Measures sound waves that are reflected back from an object Used to study cells attached to a surface Resolution 1 µm
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Figure 3.9 Scanning acoustic microscopy (SAM) of a bacterial biofilm on glass.
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Electron Microscopy Uses electrons instead of light
The shorter wavelength of electrons gives greater resolution ANIMATION Electron Microscopy
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Transmission Electron Microscopy (TEM)
Ultrathin sections of specimens Light passes through specimen, then an electromagnetic lens, to a screen or film Specimens may be stained with heavy-metal salts
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Figure 3.10a Transmission and scanning electron microscopy.
Electron gun Primary electron beam Electron beam Electromagnetic condenser lens Electromagnetic lenses Viewing screen Specimen Viewing eyepiece Electromagnetic objective lens Electron collector Electromagnetic projector lens Secondary electrons Fluorescent screen or photographic plate Specimen Amplifier Transmission
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Transmission Electron Microscopy (TEM)
10,000–100,000; resolution 2.5 nm
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Scanning Electron Microscopy (SEM)
An electron gun produces a beam of electrons that scans the surface of a whole specimen Secondary electrons emitted from the specimen produce the image
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Figure 3.10b Transmission and scanning electron microscopy.
Electron gun Primary electron beam Electron beam Electromagnetic condenser lens Electromagnetic lenses Viewing screen Specimen Viewing eyepiece Electromagnetic objective lens Electron collector Electromagnetic projector lens Secondary electrons Fluorescent screen or photographic plate Specimen Amplifier Scanning
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Scanning Electron Microscopy (SEM)
1,000–10,000; resolution 20 nm
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Scanned-Probe Microscopy
Scanning tunneling microscopy (STM) uses a metal probe to scan a specimen Resolution 1/100 of an atom
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Figure 3.11a Scanned-probe microscopy.
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Scanned-Probe Microscopy
Atomic force microscopy (AFM) uses a metal-and-diamond probe inserted into the specimen Produces three-dimensional images
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Figure 3.11b Scanned-probe microscopy.
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Preparing Smears for Staining
Staining: coloring the microbe with a dye that emphasizes certain structures Smear: a thin film of a solution of microbes on a slide A smear is usually fixed to attach the microbes to the slide and to kill the microbes
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Preparing Smears for Staining
Live or unstained cells have little contrast with the surrounding medium. Researchers do make discoveries about cell behavior by observing live specimens. ANIMATION Microscopy and Staining: Overview
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Preparing Smears for Staining
Stains consist of a positive and negative ion In a basic dye, the chromophore is a cation In an acidic dye, the chromophore is an anion Staining the background instead of the cell is called negative staining
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Simple Stains Simple stain: use of a single basic dye
A mordant may be used to hold the stain or coat the specimen to enlarge it ANIMATION Staining
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Differential Stains Used to distinguish between bacteria Gram stain
Acid-fast stain
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Gram Stain Classifies bacteria into gram-positive or gram-negative
Gram-positive bacteria tend to be killed by penicillin and detergents Gram-negative bacteria are more resistant to antibiotics
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Gram Stain Color of Gram-Positive Cells Gram-Negative Cells
Primary Stain: Crystal Violet Purple Mordant: Iodine Decolorizing Agent: Alcohol-Acetone Colorless Counterstain: Safranin Red
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Figure 3.12b Gram staining. Rod (gram-negative) Cocci (gram-positive)
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Acid-Fast Stain Stained waxy cell wall is not decolorized by acid-alcohol Mycobacterium Nocardia
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Acid-Fast Stain Color of Acid-Fast Non–Acid-Fast Primary Stain:
Carbolfuchsin Red Decolorizing Agent: Acid-alcohol Colorless Counterstain: Methylene Blue Blue
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Figure 3.13 Acid-fast bacteria.
M. bovis
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Special Stains Used to distinguish parts of cells Capsule stain
Endospore stain Flagella stain
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Negative Staining for Capsules
Cells stained Negative stain
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Figure 3.14a Special staining.
Capsules Negative staining
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Endospore Staining Primary stain: malachite green, usually with heat
Decolorize cells: water Counterstain: safranin
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Figure 3.14b Special staining.
Endospore Endospore staining
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Flagella Staining Mordant on flagella Carbolfuchsin simple stain
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Figure 3.14c Special staining.
Flagellum Flagella staining
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