Gram Stain

in this video, we're going to begin our lesson on the gram stain. And so recall from our previous lesson videos that the gram stain is an example of a differential stain that can be used to differentiate groups of bacteria. And in fact the gram stain separates bacteria into two major groups based on cell wall differences. And those two major groups are the gram positive bacteria and the gram negative bacteria. Now the gram stain itself actually consists of a series of four steps that we have numbered down below in our table 123 and four. And so notice that this table is outlining the overall procedure for graham standing and in the far left that has a description of the specific steps. And this column here it has a description of the state of the bacteria and here what we have are the gram positive cells after each step and the gram negative cells after each step as well. And so in the very first step of the gram staining procedure, the sample is going to be stained with a particular die called crystal violet dye. And the crystal violet dye is going to stain all of the cells a purple color regardless if they are gram positive or gram negative, all of the cells will turn purple. And so notice that before the grandstanding procedure even begins the gram positive and gram negative cells are unstained. And so you can see that these cells are all white, which represents that they are unstained. They're not stained. But after the very first step of the grandstanding procedure, staining them with the crystal violet died. All of the cells take on that crystal violet dye. So the gram positive and gram negative cells are all going to be purple. And so in step number two, the sample is actually going to be treated with an iodine solution and the iodine serves as what is known as a mordant. Okay and the mordant is really just going to help ensure that the crystal violet dye remains a fixed to the Grand positive cells. It basically helps the Grand positive cells to hold on to that crystal violet dye more strongly. And so all of the cells are still going to remain purple. Um after uh step number two. And so really all of these cells are going to remain purple and nothing really has changed other than the fact that the grand positive cells are going to retain and uh have a stronger affinity to that crystal violet died. And so in step number three um the sample was going to be treated with a D. Kulla riser and that D colorize er is usually going to be alcohol or acetone. And so after this step the D colorize er the gram positive cells are going to remain purple whereas the gram negative cells become colorless and they actually lose that crystal violet dye. And so notice that the grand positive cells once again our remaining purple whereas the gram negative cells have lost the crystal violet dye. And so they are unstained once again as they were originally. And so in the 5th I'm sorry in the fourth and final step because these colorless gram negative cells are somewhat difficult to visualize. Usually the sample is going to be stained with a counter stain called saffron in and so saffron in is just a counter stain that comes and stains the gram negative cells this pinkish color. And so once again the grand positive cells are going to still remain purple whereas the gram negative cells have changed from being colorless to taking on this pinkish color. And so that's exactly what we see is that the grand positive cells pretty much remained purple throughout the process. Whereas the gram negative cells uh start off being purple and then they become colorless for and then they become pink with this counter stained saffron in. And so ultimately what happens at the very end of the gram stain is that all of the gram positive cells are going to be purple, whereas all of the gram negative cells are going to be pink and so you are able to visually distinguish these different groups of bacteria. And so this year concludes our introduction to the gram stain and we'll be able to get some practice applying these concepts as we move forward. So I'll see you all in our next video.