![]() Gram-positive bacteria have the following characteristics: This is due to their high resistance to antibiotics.īelow are the specific characteristics of Gram-positive and Gram-negative bacteria. Gram-negative bacteria are among the most significant public health problems in the world. Instead, they appear red or pink in color. This means they do not hold the blue dye used in Gram testing and do not appear blue. However, they have a thinner peptidoglycan cell wall. Gram-negative bacteria have an outer membrane. Gram-positive bacteria can be cocci- (spheres) or bacilli- (rod) shaped, or have branching filaments. This means that they retain the dye in gram testing, causing them to have a blue color when a person views them under a microscope. Gram-positiveĪ 2022 article notes that Gram-positive bacteria have a thick peptidoglycan cell wall. It involves staining an organism with crystal violet or methylene blue dye and then observing the organism under a microscope. The term “Gram” refers to a specimen staining method developed by Hans Christian Gram in 1884. One method that scientists use to classify bacteria is Gram staining, which is the most commonly performed laboratory procedure in microbiology. A magnification of 500x to 1000x may be needed to distinguish cell shape and arrangement.Gram-positive and Gram-negative bacteria are two different types of bacteria. View the slide using a compound microscope.The gram-negative cells should be stained red or pink, while the gram-positive cells will still appear purple or blue. Gently rinse with water no longer than 5 seconds. Apply the secondary stain, safranin, and allow it to sit for 1 minute.However, if the decolorizer is left on too long, all cells will lose color! The gram-negative cells will lose color, while the gram-positive cells will remain violet or blue. Rinse the slide with alcohol or acetone about 3 seconds, followed immediately with a gentle rinse using water.Use a dropper to apply Gram's iodine to the slide to fix the crystal violet to the cell wall.Rinsing too long can remove too much color, while not rinsing long enough may allow too much stain to remain on gram-negative cells. Gently rinse the slide with water no longer than 5 seconds to remove excess stain. Use a dropper to apply the primary stain (crystal violet) to the slide and allow it to sit for 1 minute.If too little heat is applied, the bacteria will wash off the slide during staining. Applying too much heat or for too long can melt the bacteria cell walls, distorting their shape and leading to an inaccurate result. Heat fix the bacteria to the slide by passing it through the flame of a Bunsen burner three times. Place a small drop of bacterial sample on a slide.If the staining procedure is performed correctly, gram-positive bacteria will be purple, while gram-negative bacteria will be pink. Both gram-positive and gram-negative bacteria pick up the pink stain, but it is not visible over the darker purple of the gram-positive bacteria. After the decolorizing step, a counterstain is applied (usually safranin, but sometimes fuchsine) to color the bacteria pink.The thick cell wall of gram-positive cells is dehydrated by the decolorizing step, causing them to shrink and trapping the stain-iodine complex inside. Gram-negative bacteria have much less peptidoglycan in their cell walls, so this step essentially renders them colorless, while only some of the color is removed from gram-positive cells, which have more peptidoglycan (60-90% of the cell wall). Alcohol or acetone is used to decolorize the cells.Gram-positive cells form a crystal violet-iodine complex. ![]() Gram's iodine ( iodine and potassium iodide) is applied as a mordant or fixative.Both gram-positive and gram-negative cells have peptidoglycan in their cell walls, so initially, all bacteria stain violet. The primary stain ( crystal violet) binds to peptidoglycan, coloring cells purple.
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