Final answer:
The appropriate next step after observing colorless areas in a Gram-stained smear, particularly in the context of Gram-positive bacilli, would be to apply an endospore stain to highlight the presence of endospores that are resistant to the Gram stain. The correct option is B.
Step-by-step explanation:
If you observe colorless areas in cells in a Gram-stained smear, especially if these areas look like “outlines” or “ghosts” of cells, this suggests the presence of structures that are not picking up the gram stain, such as endospores or capsules.
Since the question provides the context of Gram-positive bacilli, and given that endospores are commonly associated with this group and are known to be resistant to staining, the next step would be B. an endospore stain. The endospore stain, using a technique like the Schaeffer-Fulton method, will stain the endospores green and the vegetative cells pink, clearly differentiating the two.
The capsule stain is another differential stain, but it is specific for bacteria with a protective capsule and involves a negative staining technique, resulting in a halo effect around the cells. Acid-fast stains are used for bacteria with waxy cell walls, such as Mycobacterium species, and flagella stains are used to visualize the motility appendages of bacteria.
However, based on the fact that we are dealing with Gram-positive bacilli and seeing structures that resemble endospores, an endospore stain is the most appropriate next step.