What term is used to describe dark staining granules in bacteria?

The term "metachromatic" refers specifically to a type of granule in bacteria that exhibits a distinct staining property, particularly when using certain dyes such as methylene blue or toluidine blue. These granules can be visualized under a microscope as dark-staining entities, primarily due to their ability to bind the dye and appear different from the surrounding cytoplasm. Metachromatic granules are often associated with polyphosphate deposits and are particularly characteristic of certain bacteria, like Corynebacterium diphtheriae.

The other terms refer to different structures or characteristics in bacteria. Spores are a form of dormant bacterial cell that can withstand extreme conditions, capsules are layers that encapsulate bacteria providing protection and aiding in virulence, and cysts are generally associated with certain protozoa and not a characteristic of bacterial structures. Thus, the term metachromatic is distinctly associated with dark-staining granules found within specific bacterial cells.