Gram-Stain

Gram Stain is the most common and often the first test performed on unidentified bacteria in order to determine which of the many bacteria it is.  The process of Gram staining involves multiple steps.  As most of us know by now, the process begins by applying crystal violet stain to the bacteria smear.  This stain penetrates the Cell wall of the bacteria, however thick or thin it may be.  After rinsing the excess crystal violet off, apply Gram's Stain to the same bacteria sample.  This two should stain the bacteria directly, similar to the crystal violet.  Next it is important to put alcohol, (95% ethanol as we used), on the slide temporarily for a short time.  The purpose of this is to relieve the excess Gram Stain away from the sample smear.  Next apply a counter stain to the same specimen.  This will determine the ultimate results.  Our class used Safranin as the counter stain, but that is not the only available option.  If the bacteria has a thick layered cell wall, the counter stain will not penetrate the cell and therefore the cell will remain stained the purple color that it was before the Safranin is added.  However, if the cell wall is of thinner content, the counter stain may penetrate it and change the composition of the stain, mainly the color.  At this point the cell will have a pink color rather than the purple.  Depending on results, a purple conclusion means the bacteria is Gram-Positive, and pink thereby Gram-Negative.

Lactobacillus Thermophilus is a Gram-Positive Bacteria.

The test of Gram Staining originated from a Danish bacteriologist in 1882, Hans Christian Gram.

It is fact that Gram-positive bacteria have a higher peptidoglycan count, as well as a lower lipid count, than Gram-negative bacteria.

Peptidoglycans are polymers composed of sugars and amino acids.  They form a netting around the plasma membrane, almost like a mesh, of some bacteria.  Thus providing another explanation for why the thicker cell walls are Gram-positive.

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