Structure of Methylosinus trichosporium exospores.

Methylosinus trichosporium exospores did not display a well-defined cortex or an exosporium. A thick, electron-dense exospore wall was characteristic of the exospores. Located on the exterior of the exospore wall was a cell wall to which a well-defined capsule was attached. An extensive lamellar intracytoplasmic membrane system characteristic of the kind in vegetative cells of this bacterium was present along the interior periphery of the exospore wall. Upon germination of M. trichosporium exospores, the thick exospore wall gradually disappeared and a germ tube formed. The intracytoplasmic membranes of the exospores extended into the germ tube which did not possess the extensive fibrillar capsule observed on the dormant exospore. Cup-shaped exospores which have an ultrastructure similar to that of mature exospores except that they are invaginated also germinated upon exposure to methane.