Probably the most important feature made obvious when you stain bacterial cells is their cellular morphology (not to be confused with colonial morphology, which is the appearance of bacterial colonies on an agar plate). Negative stain of Cyptococcus neoformans, an encapsulated yeastįigure 2. See Figures 1 and 2 for examples of both. Anionic dyes are repelled by the cells, and therefore the cells are bright against the stained background. Cationic dyes bind the bacterial cells which can be easily observed against the bright background. In a simple stain, dyes that are either attracted by charge (a cationic dye such as methylene blue or crystal violet) or repelled by charge (an anionic dye such as eosin or India ink) are added to the smear. Heat fixed smears are ready for staining. Heat fixing is an easy and efficient method, and is accomplished by passing the slide briefly through the flame of a Bunsen burner, which causes the biological material to become more or less permanently affixed to the glass surface.
To prevent the bacteria from washing away during the staining steps, the smear may be chemically or physically “fixed” to the surface of the slide. Before you can begin the staining procedure, the cells have to be mounted (smeared) and fixed onto a glass slide.Ī bacterial smear is simply that-a small amount of culture spread in a very thin film on the surface of the slide. Some involve a single stain and just a few steps, while others use multiple stains and a more complicated procedure. There are many different stains and staining procedures used in microbiology. Contrast, however, can be improved by either using a different type of optical system, such as phase contrast or a differential interference contrast microscope, or by staining the cells (or the background) with a chromogenic dye that not only adds contrast, but gives them a color as well. Resolution is a limitation that we can’t do much about, since most bacterial cells are already near the resolution limit of most light microscopes. Two of the most important concerns are resolution and contrast. The microscope is a very important tool in microbiology, but there are limitations when it comes to using one to observe cells in general and bacterial cells in particular. Differential Staining Techniques Viewing Bacterial Cells
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