The Study of Microbial Structure: Microscopy and Specimen Preparation Contd...

Types  of light microscopes(contd....)

3. Phase Contrast Microscope
     It is useful for examining intra-cellular component of living cells at relatively high resolution. It is used to observe unstained living microorganisms.  It is based on principle that cells slow the speed of light passing  via them thus differ in the refractive index from their surroundings. This result in a difference in phase  between cells & its's surroundings.This subtle difference is a  amplified by a special ring called phase plate in the objective lens which leads in the formation of dark image on light background.  


 Examples of Dark-Field and Phase-Contrast
Microscopy.  Treponema pallidum, the spirochete that causes
syphilis; dark-field microscopy ( 500).  Volvox and Spirogyra;
dark-field microscopy ( 175). Note daughter colonies within the
mature Volvox colony (center) and the spiral chloroplasts of Spirogyra
(left and right).  Spirillum volutans, a very large bacterium with
flagellar bundles; phase-contrast microscopy ( 210).  Clostridium
botulinum, the bacterium responsible for botulism, with subterminal
oval endospores; phase-contrast microscopy ( 600). Paramecium
stained to show a large central macronucleus with a small spherical
micronucleus at its side; phase-contrast microscopy ( 100).


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