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How does alcohol chemically fix specimens?

Found in: Microscopy FAQ

Alcohol can be used for fixing specimens, but is not commonly used for fixing bacteria.

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Before making a permanent mount, the specimen has to be dehydrated and fixed. Fixing kills the cells, denatures the proteins of the sample and preserves the specimens. Most specimens are naturally wet, i.e. the cells contain water. This water is removed by the fixing process by placing the specimen in subsequently higher concentrations of alcohol. Placing the specimen directly into concentrated alcohol may remove the water so quickly that the structures shrink and deform, which is undesirable.

Ethyl alcohol has several effects on the cells:

  • It removes water, it dehydrates the cells. This is important when mounting the cells in non-aqueous mounting medium.
  • It denatures proteins. This way the metabolism of the cell is stopped and the cell dies. The metabolism is dependent on enzymes, which are proteins.
  • It dissolves and removes lipids. The cell membrane(s) of the bacteria is harmed by the alcohol.

Bacteria are, however, generally fixed in a different way and not by using alcohol. The bacteria are streaked on the slide, dried and then heat-fixed. Heat-fixing sticks the bacterial cells to the glass slide so that they can not be washed away during the subsequent staining process.

Read the following for further information on heat-fixing: Heat-fixing and staining human cheek cells

This post is in response to a reader’s question.

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