Explain the preparation methods of specimens for optical microscopes.

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1. Fresh living preparation
- Live sample of microorganisms are placed in wet mounts or in hanging drop mounts in order to observe them as near to them natural state as possible
- The cells are suspended in a suitable fluid (water saline) that maintains viability and provides pace and a medium for locomotion.
A Wet Mount
- Consists of a drop or two of the culture placed on a slide and over laid with a cover glass
- It is quick and easy to prepare
Disadvantages
- The cover glass can damage larger cells
- The slide is susceptible to drying and contamination
Procedures for a wet mount slide preparation
- Add a drop of water to a slide
- Place the specimen in the water
- Place the edge of a cover slip on the slide so that it touches the edges of the water
- Slowly lower the cover slip to prevent forming and trapping air bubbles

2. A hanging drop preparing
- Made with a special concave (depression) slide
- A Vaseline adhesive is placed around the depression of the slide
- A drop of the sample is placed on a cover slip
- The slide is placed on the over slip and turned over such that the drop of sample is suspended
NB: these short term mounts provide a true assessment of size, shape, arrangement color and motility of cells.

3. Fixed stained smears
- A permanent mount for long term study can be obtained by preparing fixed stained specimens
- The techniques (developed by Robert Koch) consist of spreading a thin film made from a liquid suspension of cells on a slide and air drying it.
- The air dried smear is heated gently by a process called heat fixation that kills the specimens and secures it to the slide. (fixation also preserve various cellular components in a natural state with minimal distortion)
- Fixation of some microbial cells is performed with chemicals such as alcohol and formation.
- The process of developing a smear to create contrast and make inconspicuous features stand out requires staining techniques
- Staining is any procedure that applies colored chemicals (dyes) to specimens
- Dyes impart color to cells or cell parts by becoming affixed to them through a chemical reaction
- Examples of stains – Methylene Blue, Crystal Violet, Carbol Fuschin, Malachite Green and safranin- Basic dyes /Cationic carry positive charge – attracted to negative bacterial cell surface: stain the cell
- Eosin, Nigrosin and India ink – Acidic/Anionic dyes carry negative charge – repelled by negative bacterial cell surface – stain background and give outline of the cell.

Kavungya answered the question on March 13, 2019 at 05:58

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Explain the preparation methods of specimens for optical microscopes.

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