The sample is now checked for typical signs of disease. Sometimes, the larval remains are dif-ficult to locate because of the condition of the comb. Some discretion must be used when using this tech-nique as both honey and pollen will also fluoresce. In most cases, American foulbrood disease can be diagnosed by observing B. larvae by light microscopy. Suspect material is mixed with water, and a drop of this suspension is placed on a cover-slip, dried and fixed by heat, and stained with carbol fuchsin or a suitable spore stain for 30 seconds.
Any excess stain is washed off with water.
While the preparation is still wet, the cover-slip is inverted with a very thin layer of immersion oil. The slide is gently blotted dry and examined by high-power microscopy. By examining fields where pockets of water have formed in the oil, the spores of B. larvae will be seen exhibiting Brownian movement. This is a valuable diagnostic tech-nique as the spores formed by other members of the genus Bacillus associated with known bee diseases remain fixed. This, taken together with the characteristic morphology of the spores, permits a diagnosis.