microscopy for microbiology: a primer
DESCRIPTION
Overview of microscopy in microbiology including differential and special stains Overview of common bacterial cell shapesTRANSCRIPT
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An Overview of MicroscopyBy BugLady
Unless otherwise stated, all micrographs and diagrams are the author’s own work.
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Microscopy
Resolution and magnification
Different types of light microscopy: principle and use
Electron microscope: principle
Stains
Recommended activity:oMicroscope tutorial
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Principles of Light Microscopy
Light passes through specimen and through a series of magnifying lenses
Important factors in light microscopy includeMagnificationResolutionContrast
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Principles of Light Microscopy: Magnification
Compound Microscope : microscope has two magnifying lenses
Lenses include ocular lens and objective lens
Lenses combine to enlarge objects Magnification is equal to the product of the ocular lens x
the objective lens: 10x X 100x = 1,000x
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Principles of Light Microscopy: Resolution
Resolving power is defined as the minimum distance existing between two points where they still appear as separate
Resolving power determines how much detail can be seen
Naked eye ≈ 0.1mm
Light microscope ≈ 0.2μm
Electron microscope ≈ 2.5nm
Apr 9, 2023 5Courtesy of the CDC
Electron micrograph of Bacillus anthracis: vegetative cell (A) and endospore (B)
Light micrograph of Bacillus anthracis
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Principles of Light Microscopy: ResolutionResolutiono Resolution depends on the quality of lenses
and wavelength of illuminating light How much light is released from the lens
oMaximum resolving power of most brightfield microscopes is 0.2 μm (1x10-6) This is sufficient to see most bacterial structures Too low to see viruses
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Principles of Light Microscopy Contrast
Reflects the number of visible shades in a specimen
Higher contrast achieved for microscopy through specimen staining
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Microscopy: The InstrumentsTwo lenses in the compound microscopeo Ocular lens (x10)
o Objective lens (x4, x10, x40, x100)
Resolution and contrast are controlled by the condenser lens and iris diaphragmo Close iris increases contrast (low mag)
o Open iris increases resolution (high mag)
Bacteria are transparent and must be stained for bright-field microscopy.
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Label the MicroscopeArm
Base
Clamp
Condenser
Coarse adjustment
Diaphragm
Fine adjustment
Light control
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Light source
Nose piece
Objective lens
Ocular lens
Power switch
Slide positioning
Stage
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Microscopy: The InstrumentsRefractive index is the light-bending ability of a medium.
The light may bend in air so much that it misses the small high-magnification lens.
The refractive indexes of oil and glass are similar.
Immersion oil is used to keep light from bending.
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Principles of Light Microscopy
Dark-Field Microscopeo Reverse image
Specimen appears bright on a dark background
Like a photographic negative
o Achieves image through a modified condenser
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Dark field microscopy of Treponema pallidum, agent of syphilisCourtesy of CDC
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Electron Microscopy
Resolution is a function of wavelength: the shorter the wavelength the higher the resolution.
Uses electrons instead of visible light.
The shorter wavelength of electron beam gives greater resolution.
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Principles of Electron Microscopy
Uses electromagnetic lenses, electrons and fluorescent screen to produce imageResolution increased 1,000 fold over brightfield microscopeo To about 0.3 nm (1x10-9)
Magnification increased to 100,000xTwo types of electron microscopeso Transmissiono Scanning
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Disadvantages of Electron Microscope
No true color
Artifacts
Large depth of field
Destroys sample
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Staining
Live or unstained cells have little contrast with the surrounding medium.
Cells are stained with dyes to make them visible.
Unstained specimens are used to observe cell behavior: motility.
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Preparation of Specimens for Light Microscopy
A smear is a thin film of a microbial liquid suspension on a slide.
A smear is usually fixed by passing the slide over a flame to attach the sample to the slide.
Most organisms are killed by heat fixing.
Slides must be treated as potential biohazards.
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Preparing Smears for Staining
Stains consist of a positive ion and negative ion.
In a basic dye, the chromophore is a cation (Chr+).
In an acidic dye, the chromophore is an anion (Chr -).
Staining the background instead of the cell is called negative staining.
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Simple StainsBacteria cell surfaces are slightly negatively charged and basic dyes are used as stains.
Use of a single basic dye is called a simple stain.
Common basic dyes includeo Methylene blue
o Crystal violet
o Safranin
o Malachite green
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Differential Stains: Gram StainThe Gram stain classifies bacteria into Gram-positive and Gram-negative bacteria.
The Gram stain is the most frequently used procedure to stain bacteria.
There are as many variations on the Gram stain procedure as there are labs.
Mostly the timing of each step and the decolorizer composition differ.
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Gram positive cocci and Gram negative rods, x1,000
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Differential Stains: Gram Stain
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Color of Gram + cells
Color ofGram – cells
Primary stain: Crystal violet Purple Purple
Mordant: Iodine Purple Purple
Decolorizing agent:Alcohol-acetone
Purple Colorless
Counterstain:Safranin/Carboxyl fuchsin
Purple Pink or Red
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Gram Stain Procedure Animation
This animation shows all the steps involved in the Gram stain.http://www.medschool.lsuhsc.edu/microbiology/Flash/gstainN.htm
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Differential Stain: Acid-fast Stain
Used to stain organisms that resist conventional staining
Used to stain Mycobacterium tuberculosiso High lipid concentration in cell
wall prevents uptake of dye
o Once stained difficult to decolorize
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Mycobacterium smegmatis
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Special StainsStain specific structures in the bacterial cell
Endospore staino Staining enhances endospore
o Uses heat to facilitate staining
Capsule staino Allows capsule to stand out around organism
Flagella staino Staining increases diameter of flagella to make
it visible
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Klebsiella pneumoniae, x1,000
Bacillus subtilis, x1,000
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Morphology of Prokaryotic Cells
Prokaryotes exhibit a variety of shapeso Most common
CoccusSpherical
BacillusRod or cylinder shapedCell shape not to be
confused with Bacillus genus
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Neisseria sicca
Bacillus megaterium
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Morphology of Prokaryotic Cells
Prokaryotes exhibit a variety of shapeso Coccobacillus: Short round rod
o Vibrio: Curved rod
o Spirillum: Spiral shaped
o Spirochete: Helical shape
o Pleomorphic: Various shapes Clubs, Chinese letters, palisade
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Treponema pallidum, a spirocheteCourtesy of CDC
Corynebacterium diphtheriaeCourtesy of the CDC
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Morphology of Prokaryotic CellsDivision along a single plane may result in pairs or chains of cells
Pairs = diplococcio Example: Neisseria gonorrhoeae
Chains = streptococcio Example: species of Streptococcus
Apr 9, 2023 26Streptococcus salivarius
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Morphology of Prokaryotic Cells
Division along two or three perpendicular planes form cubical packets
Example: Sarcina genus
Division along several random planes form clusters Example: species of Staphylococcus
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Micrococcus luteus
Staphylococcus epidermidis