Final answer:
A negative stain repels bacteria and is used to create a clear contrast between the bacteria and the background, typically highlighting protective structures like capsules as clear halos around cells. It is notable for not requiring the heat-fixing process.
Step-by-step explanation:
How does a negative stain interact with bacteria, and what result is obtained? The correct answer is B) The stain repels bacteria, creating a clear background.
Negative staining techniques involve the use of anionic dyes, which are repelled by the negatively charged bacterial cells. This type of staining routine is particularly useful in cases where bacteria have a capsule - a protective layer that is not penetrable by most simple stains. Instead of coloring the cells themselves, the negative stain colors the background, leaving the cells transparent or lighter in appearance. Capsules may appear as halos around the cells when using a negative stain, because the dye does not penetrate the capsule either. In practice, this method enhances the contrast between bacteria and the surrounding medium without having to heat-fix the specimen, which can sometimes distort cell structure and size.
When performing a negative stain, India ink or nigrosin can be added to a specimen. These substances do not penetrate the bacterial cell walls or capsules, thereby highlighting the morphology of these structures against the dark background. This is different from positive staining methods like the Gram stain, where the use of dyes such as crystal violet or safranin results in the staining of bacterial cell walls.