Final answer:
Stains adhere to bacterial cells due to interactions between the dye's charge and the cell wall's charge. Cationic dyes serve as positive stains, while anionic dyes act as negative stains. The usefulness of dyes in staining depends on their chemical properties and the specimen's characteristics.
Step-by-step explanation:
The cause for a stain adhering to bacterial cells lies in the interaction between the dye and the cell wall of the bacteria. Since bacteria typically have negatively charged cell walls, cationic dyes, which are positively charged, like methylene blue or crystal violet, tend to stick to these cell walls. This is why they are often used as positive stains in simple staining techniques, making the bacteria visible against a bright background.
On the other hand, anionic dyes contain negatively charged chromophores that are repelled by the bacterial cell wall. These dyes, such as eosin or India ink, do not stain the bacteria but the background instead, resulting in a silhouette of the organisms; this is known as negative staining. Not all colored dyes are useful for simple staining because the effectiveness of a dye depends on its chemical properties and the properties of the specimen, which determine how the dye will interact with the specimen.
Differential staining techniques, like the Gram stain, use more than one dye to differentiate structures or organisms based on their interactions with the stains employed. This can indicate characteristics such as whether bacteria are gram-positive or gram-negative, providing valuable information for classification and identification of bacteria in clinical settings.