What staining method is applied when microorganisms do not take on the stain itself and dyes the background instead?

The negative stain method is used specifically when microorganisms do not absorb the stain, allowing the background to be stained instead. This results in a clear silhouette of the organisms against a colored backdrop. The key feature of negative staining is that it highlights the shape and arrangement of the microorganisms, making it especially useful for visualizing bacteria that may be difficult to stain with traditional methods.

In this technique, dyes like India ink or nigrosin are used, which do not penetrate the cells but stain the medium surrounding them. This approach helps preserve the morphology of delicate cells, such as certain types of bacteria, that might otherwise be distorted by heat fixation or other staining processes.

In contrast, the other staining methods focus on directly staining the organisms themselves. A simple stain uniformly colors all cells, a special stain is used for identifying specific structures or types of cells, and a differential stain helps distinguish between different types of microorganisms based on their characteristics. Hence, negative staining stands out as the method where the background is dyed rather than the microorganism itself.