Because cells typically have negatively charged cell walls, the positive chromophores in basic dyes tend to stick to the cell walls, making them positive stains. Thus, commonly used basic dyes such as basic fuchsin, crystal violet, malachite green, methylene blue, and safranin typically serve as positive stains. On the other hand, the negatively charged chromophores in acidic dyes are repelled by negatively charged cell walls, making them negative stains.
Commonly used acidic dyes include acid fuchsin, eosin, and rose bengal. Some staining techniques involve the application of only one dye to the sample; others require more than one dye. In simple staining, a single dye is used to emphasize particular structures in the specimen. A simple stain will generally make all of the organisms in a sample appear to be the same color, even if the sample contains more than one type of organism.
In contrast, differential stainingdistinguishes organisms based on their interactions with multiple stains. In other words, two organisms in a differentially stained sample may appear to be different colors. Differential staining techniques commonly used in clinical settings include Gram staining, acid-fast staining, endospore staining, flagella staining, and capsule staining. The Gram stain procedure is a differential staining procedure that involves multiple steps.
It was developed by Danish microbiologist Hans Christian Gram in as an effective method to distinguish between bacteria with different types of cell walls, and even today it remains one of the most frequently used staining techniques. However, there are several important considerations in interpreting the results of a Gram stain. First, older bacterial cells may have damage to their cell walls that causes them to appear gram-negative even if the species is gram-positive.
Thus, it is best to use fresh bacterial cultures for Gram staining. Second, errors such as leaving on decolorizer too long can affect the results. This suggests damage to the individual cells or that decolorizer was left on for too long; the cells should still be classified as gram-positive if they are all the same species rather than a mixed culture.
Besides their differing interactions with dyes and decolorizing agents, the chemical differences between gram-positive and gram-negative cells have other implications with clinical relevance. For example, Gram staining can help clinicians classify bacterial pathogens in a sample into categories associated with specific properties.
Gram-negative bacteria tend to be more resistant to certain antibiotics than gram-positive bacteria. We will discuss this and other applications of Gram staining in more detail in later chapters. Gram-negative Escherichia coli, the most common Gram stain quality-control bacterium, is decolorized, and is only visible after the addition of the pink counterstain safranin.
However, more information is needed to make a conclusive diagnosis. The technician decides to make a Gram stain of the specimen. This technique is commonly used as an early step in identifying pathogenic bacteria. Acid-fast staining is another commonly used, differential staining technique that can be an important diagnostic tool. An acid-fast stain is able to differentiate two types of gram-positive cells: those that have waxy mycolic acids in their cell walls, and those that do not.
Two different methods for acid-fast staining are the Ziehl-Neelsen technique and the Kinyoun technique. Both use carbolfuchsin as the primary stain. The waxy, acid-fast cells retain the carbolfuchsin even after a decolorizing agent an acid-alcohol solution is applied.
A secondary counterstain, methylene blue, is then applied, which renders non—acid-fast cells blue. The fundamental difference between the two carbolfuchsin-based methods is whether heat is used during the primary staining process. The Ziehl-Neelsen method uses heat to infuse the carbolfuchsin into the acid-fast cells, whereas the Kinyoun method does not use heat. Both techniques are important diagnostic tools because a number of specific diseases are caused by acid-fast bacteria AFB. Why are acid-fast stains useful?
Certain bacteria and yeasts have a protective outer structure called a capsule. The Questions and Answers of Why do we stain specimens before viewing them under a microscope? If the answer is not available please wait for a while and a community member will probably answer this soon. You can study other questions, MCQs, videos and tests for Class 8 on EduRev and even discuss your questions like Why do we stain specimens before viewing them under a microscope?
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For Your Perfect Score in Class 8. After staining cells and preparing slides, they may be stored in the dark and possibly refrigerated to preserve the stained slide, and then observed with a microscope. Next Page ». Your Account. Microscopy Created by Monica Z. Bruckner, Montana State University, Bozeman. Ethidium Bromide - this stain colors unhealthy cells in the final stages of apoptosis, or deliberate cell death, fluorescent red-orange. Fuchsin - this stain is used to stain collagen, smooth muscle or mitochondria.
Hematoxylin - a nuclear stain that, with a mordant, stains nuclei blue-violet or brown. Hoechst Stains - two types of fluorescent stains, and are used to stain DNA in living cells.
Iodine - used as a starch indicator. When in a solution, starch and iodine turn a dark blue in color. Malachite Green - a blue-green counterstain to safranin in Gimenez staining for bacteria.
This stain is often used to stain spores. Methylene Blue - stains animal cells to make nuclei more visible.
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