The Microbial World - Improving Contrast in Light Microscopy

4 important questions on The Microbial World - Improving Contrast in Light Microscopy

What is the Gram stain?

The Gram stain is a differential stain. Differential stains separate bacteria into groups. The Gram stain is widely used in microbiology. Bacteria can be divided into two major groups:
  • Gram-positive
  • Gram-negative

Gram-positive bacteria appear purple, and gram-negative bacteria appear red after staining.

What is phase-contrast microscopy?

Phase-contrast microscopy was invented in 1936 by the Dutch physicist Frits Zernike. Phase ring amplifies differences in the refractive index of cell and surroundings. Improves the contrast of a sample (without using a stain). It allows for the visualization of live samples. Resulting image is dark cells on a light background.

What is dark-field microscopy?

In dark-field microscopy the light reaches the specimen from the sides. Light reaching the lens has been scattered by specimen and the image appears light on a dark background (the background is probably water). This method is excellent for observing motility.
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What is fluorescence microscopy?

Fluorescence microscopy is used to visualize specimens that fluoresce. They emit light of one color when illuminated with another color of light. Cells fluoresce naturally (autofluorescence) or after they have been stained with fluorescent dyes. This method is widely used in microbial ecology for enumerating bacteria in natural samples.

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