What are dark staining granules in bacteria called?

Dark staining granules in bacteria are referred to as metachromatic granules. These granules are composed of polyphosphate and are significant because they play a role in the storage of nutrients, particularly phosphates, and can be stained by specific dyes that highlight their distinctive properties, leading to the term "metachromatic." When stained, these granules can appear dark against the lighter background of the bacterial cells, which is crucial for identification and observation under a microscope.

Metachromatic granules are frequently associated with bacteria such as Corynebacterium diphtheriae, where their detection can be an important diagnostic feature. Their ability to change color when stained with certain dyes demonstrates their unique structural composition.

The other options do not accurately describe the specific type of granule in question. Polysaccharides refer to large molecules made up of sugar units and are not specifically related to granules. Granulocytes are a type of white blood cell and are not a feature of bacteria. Chromatophores are related to pigments in certain organisms, particularly in photosynthetic bacteria, but do not describe the storage granules found in bacterial cells. Therefore, the distinctive name for dark staining granules in bacteria is indeed metachromatic granules.