|Metachromatic granules are stored phosphate regions that occur in
some species of bacteria. When stained with methylene blue, like the 1000
X images shown here, metachromatic granules take up the dye more intensely
than the rest of the rod-shaped bacterium. Thus a row of irregularly shaped
circles (metachromatic granules) appear close together because they are
contained within one bacterium. The cell wall (outer border) of the bacterium
is not readily seen.
The diagnostic significance of metachromatic granules arises from the fact that the causative agent of diptheria, Corynebacterium diptheriae , characteristically produces metachromatic granules. Nonpathogens that resemble Corynebacterium diptheriae in appearance are termed "diptheroids". The arrangement of the bacteria provide a clue to distinguish the pathogen Corynebacterium diptheriae from the nonpathogenic diptheroids. C. diptheriae bacteria form 90 degree angles to one another, forming "X's", "Y's", or "Chinese letters". In contrast, diptheroids are often in parallel arrangements called "palisades". Although not every bacteria in the field of view will be in one of these arrangements, many can be seen.
Images 1 thru 5 are Corynebacterium xerosis, a nonpathogenic diptheroid that occurs in palisades. The first three images contain a magnified view of bacteria in a palisade, with several of the bacteria outlined for clarity. Images 6 thru 10 are Corynebacterium diptheriae, the pathogen that causes diptheria and produces "Chinese letter" arrangements.