We are not sure if type IV pili are visible in our EM figures. They are very thin (less than 1 nm) and normally visualized by negative staining. In the published EM figures of Phormidium (conventional and cryo EM), type IV pili have not been noted. The filaments that we see in our EM figures could be components of sheath materials.
As depicted in some of the electron micrographs in Figure 3, the authors describe the presence of thin filaments within the sheaths surrounding the motile filaments. Is it possible that these are in fact type IV pili? How does the structure compare to other published reports of type IV pili?