When intracellular bacteria are present in a body fluid cytospin, they classically appear within neutrophils (see arrows). When the bacterial burden is high, intracellular bacteria may also be found in monomacrophages.
It is important to note that monomacrophages are less effective bacterial phagocytes than neutrophils; they will never contain intracellular bacteria, if the neutrophils do not contain intracellular bacteria as well. If you think you are seeing bacteria in a monomacrophage and the neutrophils do not contain intracellular bacteria, you may actually be observing granules of hemosiderin. Hemosiderin usually stains more green-blue or black than bacteria on a Wright-stained smear, and the granules are less regular in size than ingested bacteria. It would be helpful to compare the Wright-stained smear to a Gram stain to determine if bacteria are present. It would also be helpful to perform an iron stain to determine if the intracellular elements in the monomacrophages are hemosiderin.
