not involve staining the cells directly. Instead, the background is stained by a negatively-charged stain that is repelled by the negatively-charged cytoplasm within the cells. As only the background is stained, negative staining may also be used to observe capsules, view microorganisms that do not stain well, and accurately determine cell size. Like the simple stain and the Gram stain, negative staining may be used to observe cell shape and arrangement as well.
3.
Escherichia coli
is a gram-negative bacillus and Staphylococcus aureus is a gram-
positive coccus. Did your gram stains produce the correct results? If not, what do you think went wrong? Our Gram stains produced the correct results. Our gram-negative bacillus, E. coli,
appeared pink/red and our gram-positive coccus, S. aureus,
appeared purple. Although our sample produced the correct results, incorrect results could happen because of errors during the staining process, contamination due to improper aseptic technique, or incorrect sample preparation.
4.
What is the function of the iodine in the gram stain procedure?
In the Gram stain procedure, iodine functions as a mordant. Iodine forms an insoluble crystal violet-iodine complex, which helps adhere the crystal violet dye to the thick peptidoglycan layer of gram-positive bacteria. If iodine is not used, then the decolorizer will remove the crystal violet stain from both gram-positive and gram-negative bacteria, and both types of bacteria will then take up the counterstain.
5.
If a smear of gram-negative bacterial cells on a slide is accidentally under-decolorized during the gram stain procedure, what color will the cells have when viewed with a microscope? Why?
When under-decolorization occurs, the decolorizer is not left on long enough to wash out the crystal violet from the gram-negative cells. This causes the gram-negative cells to appear purple (gram positive) instead of pink/red (gram negative).
6.
Why is the gram stain so important in clinical microbiology (hospital/microbiology labs)?
The gram stain is useful in a clinical lab because it distinguishes between gram-positive and gram-negative cells, which aids in the identification of bacterial species or the narrowing down of the possible bacterial species. The Gram stain also allows us to determine the cell’s arrangement, size, shape, and morphology. Although the Gram stain alone cannot be used to identify bacteria, it has proven to be a quick and effective way to help identify bacteria, and it helps predict which antibiotics are most likely to effectively kill said bacteria. Because of this, the Gram stain is typically the first differential test run on a specimen brought into the laboratory
for identification. Once Gram staining is used to successfully identify a bacterium, healthcare professionals can implement proper infection control measures, provide treatment monitoring, and conduct epidemiological surveillance.
7.
What is a gram-variable bacterium? How can incorrect gram stain results with gram-
variable bacteria be avoided?
