Cyst and bacterial ultrastructure.

<p>Ultra-thin sections of gill tissue examined by transmission electron microscopy. The micrograph depicts cyst morphology in fish gill tissues negative for the presence of ‘<i>Ca.</i> P. salmonis’ by qPCR. (<b>A</b>) The cysts were densely packed with polymorphic bacteria of round, coccoid or short to long rod-shaped morphologies, with or without nucleoids (electron dense material in the centre of the bacterium, arrowhead). Scale bar represents 2 µm. (<b>B</b>) Cysts were limited by a membrane formed by the host cell (arrow). Plasma membrane of the host cell (arrowhead). Electron dense nucleoids were observed in many bacterial cells. Scale bar represents 500 nm. (<b>C</b>) Nucleoids were apparently absent in a low number of bacteria, resembling morphotypes described as chlamydial reticulate bodies in previous studies (arrow). Scale bar represents 500 nm. (<b>D</b>) Other cysts contained more bacterial cells with distinct nucleoids resembling morphotypes previously described as chlamydial intermediate bodies. Scale bar represents 2 µm. (<b>E</b>) A high number of the cyst associated bacteria contained large nucleoids (arrowhead) and vesicles (arrow), which may have artefactually expanded during tissue processing, compressing the surrounding structures. Scale bar represents 500 mn. (<b>F</b>) All bacteria were limited by a double membrane of which the inner (arrowhead) was clearly trilaminar while the outer (arrows) was less distinct. Scale bar represents 100 nm.