The Dichotomous Key...

Do the filaments contain sulfur granules in - situ or after applying the S test...?

Under phase contrast observation, sulfur granules appear as bright yellow - colored cell inclusions, either in the shape of spheres observed for Thiothrix spp., Beggiatoa spp., and Type 021N; or in the shape of squares for Type 0914. Type 0914 does not respond to the S test.
"S" Test : (1) On a microscope slide mix 1 drop of activated sludge sample and 1 drop sodium sulfide (Na₂S.9H₂O) solution (1.0 g/L, prepare weekly), (2) Allow to stand open to the air 10 - 20 min., (3) Place a coverslip on the preparation and gently press to exclude excess solution, remove expelled solution with a tissue, (4) Observe at 1,000 x using phase contrast. A positive S test is the observation of highly refractive, yellow - colored intracellular granules (S granules)