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Gram positive vs gram negative shape7/2/2023 The Gram-positive cell wall is composed of hundreds of layers of peptidoglycan. Gram-positive bacteria have a cell envelope composed of two layers, a cell wall and a cell membrane. Most bacteria are divided into two major groups- Gram-positive bacteria and Gram-negative bacteria based on the cell envelope composition. The Gram stain, developed by Christian Gram in 1884, is the most widely used differential stain in bacteriology. If you wish to keep heat-fixed, unstained slides for further work, you can wrap the slides in a paper towel or put them in a slide box and store them in your lab drawer. A heat-fixed slide can be stained immediately or kept for months. The staining procedures you perform may be done in stages. This is a common mistake made by students learning to make bacterial smears. If there is a dark spot of color visible, it means that the bacteria are stacked too densely for adequate evaluation. Bacterial morphology (shape) can be seen with both types of stains.Ī good stained smear should be somewhat difficult to see with the naked eye. Certain types of bacteria will stain contrasting colors depending on their properties and characteristics. This contrasts with “differential” stains in which two stains are used. In a “simple” stain, the bacteria are stained with one dye and subsequently, the bacteria will all be the same color. Since the organism is seen indirectly, this type of staining is called a negative stain. Instead, it forms a deposit around the organism, leaving the organism itself colorless. Since the color portion of the stain is on the negative ion, it will not readily combine with the negatively charged bacterial cytoplasm. An acidic stain (negatively charged), due to its chemical nature, reacts differently. Therefore, when using a basic stain (positively charged), the positively charged color portion of the stain combines with the negatively charged bacterial cytoplasm and the organism becomes directly stained. Some examples of acidic stains include nigrosin, congo red and eosin.īecause of its chemical nature, the cytoplasm of all bacterial cells has a slight negative charge when growing in a medium of near neutral pH. If the color portion of the stain is in the negative ion, it is called an acidic stain. Some examples of basic stains include methylene blue, crystal violet, and safranin. If the color portion of the stain resides in the positive ion, it is called a basic stain. Stains may be divided into two groups: basic and acidic. For example, the stain methylene blue is the salt methylene blue chloride which will dissociate in water into a positively charged methylene blue ion which is blue in color and a negatively charged chloride ion which is colorless. A salt is a compound composed of a positively charged ion and a negatively charged ion. Stains are generally salts in which one of the ions is colored. To understand how staining works, it will be helpful to know a little about the physical and chemical nature of stains. To see them with the microscope we often use chemical compounds called stains. Unstained bacteria are nearly transparent.
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