Negative staining is used to demonstrate
**Core Concept**
Negative staining is a technique used in microscopy to visualize the morphology of microorganisms, such as bacteria, by staining the background and not the organisms themselves. This technique is particularly useful for observing delicate or fragile structures that may be damaged by traditional staining methods.
**Why the Correct Answer is Right**
Negative staining works by using a stain that does not penetrate the cell wall of the microorganism, allowing the cell's natural opacity to stand out against a stained background. The most common stain used for negative staining is nigrosin, which is a dark brown or black dye that stains the background but does not penetrate the cell wall of gram-negative bacteria. This technique is particularly useful for observing the morphology of bacteria, such as their shape, size, and arrangement.
**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because negative staining is not used to demonstrate the presence of viruses.
* **Option B:** This option is incorrect because negative staining is not used to demonstrate the presence of fungi.
* **Option C:** This option is incorrect because negative staining is not used to demonstrate the presence of parasites.
**Clinical Pearl / High-Yield Fact**
It's worth noting that negative staining is a useful technique for differentiating between gram-negative and gram-positive bacteria, as gram-negative bacteria will appear as darkly stained cells against a light background, while gram-positive bacteria will appear as light cells against a dark background.
**Correct Answer: D. Morphology of microorganisms**