Assessment of immunofluorescence measurements of individual bacteria in direct and indirect assays for Bacillus anthracis and Bacillus cereus spores

A microfluorometer constructed by the addition of fibre optics to a standard fluorescence microscope has been used in immunofluorescence assays for spores of Bacillus anthracis and B. cereus. Variation in the fluorescence measurements of individual anthrax spores increased as the amount of conjugate bound increased. Proportionality of photodetector voltage and amount of bound conjugate is suggested by results with a dual fluorescein/tritium‐labelled antibody. The maximum fluorescence signal which could be achieved in indirect assays was virtually independent of the purity of the antispore antibody. Evidence is presented that the molecular loading of conjugate on the spore surface is similar in the direct and indirect assays.