Uses for stains:
1. The basic idea behind stains is that they facilitate viewing and examination by providing contrast.
2. Stains also have other uses, such as to distinguish organisms amongst each other. You could also do viability stains which is typically an oxymoron because when you stain it, you kill the organism. Viability stains differentiate between live and dead bacterial cells in a sample (before they all die).
3. Stains can also provide structural details and composition and in respect to viability, some status on the organism such as metabolism.
4. Stains are critical because they provide data on morphology so you could tell the shape, size, and identification but of course there’s a limit. On a gram stain you can’t specifically get the ID but you could do a presumptive identification.
Types:
Simple stains are single dyes used to stain the organism and it has limited clinical application. The dye is negative and the bacterium is positively charged and they will get stained due to the interaction of the opposite charges. It doesn’t provide a lot of detail on structure though.
Differential: They are called differential stains because they differentiate organisms. Gram stain and acid fasting are the two differential stains.
By far, the gram stain is the most widely used stain clinically. The gram stain differential between – and + and gives you clues on empiric therapy. So if you get a report that says 4+ Gram positive cocci, it usually refers to staphylococcus and gives insight to morphology, possible antibiotic treatment through empiric therapy, and also presumptive identification. You can’t tell the genus and species from a gram stain but you usually don’t need to, to choose an antibiotic. The gram stain can narrow the choices.
Acid fasting is useful because it differentiates between an acid fast bacteria vs non acid fast bacteria. Remember in the lab to do this we stain with carbolfuchsin (red), steam, decolorize and then counter-stain with methylene blue. The decolorization works on everything except acid-fast bacteria because they have a wax-like substance in their cell membrane that resists destaining. By the way, the bacterium that causes tuberculosis is an acid-fast bacteria. (All mycobacteria and some actinomycetes are acid fast bacteria and tuberculosis is caused by mycobacterium tuberculosis).
More info on gram stain procedure and role of that and AF stain in C&S in this post
Negative: Cells not stained are repelled by like charges. This has very little clinical use but it is relevant because you could use a negative stain to detect fungal meningitis. So you take a CSF sample, do a negative stain, and dependent on what’s colored, you could tell what it is. It’s a negative stain because the bacteria are negative and so is the dye so what happens when two negative charges mix? They will be repelled by like charges. As a consequence, the cells are not stained but the background will be stained. In the lab we can do a combination of a simple and negative stain using Nigrosin or India ink to reveal capsules.
Structural/Special stains: Spore, Flagella: We won’t do flagella stains because it’s tedious but we’ll certainly do spore stains which we’ll talk about in the future. That’s it for that.
Viability: Alive or dead? Beer, wine, bread, cake! More info on what that means on the viability stain post.
