Chp. 3   Microscopy and Staining


I.                   Principles


Microscopy is the technology of making very small things visible to the human eye.  Most microbes are so small that they are measured in micrometers or nanometers.  A typical bacterial cell is about 1 um while a virus is more in the range of 10-100 nm.

Remember, Leeuwenhoek was probably the 1st to see microorganisms in the 1600’s with his invention of his simple microscopes.


Resolution is the ability to see two objects as separate, discreet entities….kind of like the ability to see railroad tracks as being separate tracks….GOOD resolution is being able to distinguish the two tracks as separate…..once the two tracks merge into one, the resolution is poor!!!


Refraction is the bending of light as it passes from one medium to another of different density.  Immersion oil, which has the same index of refraction as glass, is used to replace air and to prevent refraction at a glass-air interface.  An example would be when one looks at objects just below the surface of water in a pond or other body of water…..the objects become refracted or “distorted” from the true image.


The total magnification  of a light microscope is calculated by multiplying the magnifying power of the objective lens by the magnifying power of the ocular lens.  Increased magnification is of no value unless good resolution can also be maintained.


            Scanning (3X) x (10X) = 30X total

            Low power (10X) x (10X) = 100X total

            High “dry” (40X) x (10X) = 400X total

            Oil immersion (100X) x (10X) = 1000X total


Most microscopes are designed so that when the microscopist increases or decreases the magnification by changing from one objective lens to another, the specimen will remain very nearly in focus.  Such microscopes are said to be parfocal (par means equal).


Types of microscopes:


            Compound Light***  (this is what we will use…know the parts and functions….we will spend more time on this in the lab.









            Scanning Electron-


II.                Techniques of Light Microscopy


A.                 Preparation of specimens


Wet mounts are used to view living organisms.  The hanging drop technique is a special type of wet mount, often used to determine whether organisms are motile.


Smears of appropriate thickness are allowed to air-dry completely and are then passed through an open flame.  This process, called heat fixation, kills the organisms, causing them to adhere to the slide and more readily accept stains.


B.                 Principles of Staining


A stain, or dye, is a molecule that can bind to a structure and give it color.


Most microbial stains are cationic (positively charged), or basic dyes, such as methylene blue, crystal violet, or safrannin.  Some are anionic dyes (negatively charged), or acidic dyes, such as nigrosin or India ink.


MOST bacterial surfaces are negatively charged so they will attract the basic dyes.


Simple stains use one dye and reveal basic cell shapes and arrangements.  Differential stains use two or more dyes and distinguish various properties or organisms.  The Gram stain, spore stain, and acid-fast stain are examples.


Negative stains color the background around cells and their parts, which resist taking up the stain. (acidic dyes will “stick” to the glass slide since glass has a + charge).


Imagine a magnet when thinking of basic and acidic dyes…..basic dyes (+) will attract to bacteria due to their (-) parts but will be repelled by the glass because of its (+) charge!!  Acidic dyes, on the other hand, will attract to the (+) glass but be repelled by the (-) bacterial parts!!


We will also be covering these in great detail in the laboratory.


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