Polymorphic bacterial symbionts in the anaerobic ciliated protozoon Metopus

Abstract

In the anaerobic ciliate Metopus palaeformis, all endosymbionts are electron dense, rod-shaped bacteria. In Metopus contortus, rod-shaped symbionts co-exist with a variety of other morphological forms of the symbiont. The latter appear to represent stages in a changing morphology of the rods, a process which entails progressive stripping of the bacterial cell wall, as a prelude to attachment of the transformed symbiont to one or more hydrogensomes. In M. palaeformis, the morphology of the symbionts does not change and they do not become attached to the hydrogenosomes. All symbionts in both ciliate species show F420 autofluorescence and inhibition by bromoethanesulfonic acid, so they are all, probably, methanogens.