Surface bacteria of Streblomastix strix are sensory symbionts

Streblomastix strix, a protist living in the hindguts of Zootermopsis sp. (termites) is covered with pseudomonad-like rod-shaped bacteria. These bacteria were demonstrated to be sensory (chemotactic) symbionts for S. strix. S. strix moved toward a source of sodium acetate, a presumed food molecule, when its bacteria were intact. If the bacteria were removed by antibiotic (carbenecillin) treatment, S. strix was unable to orient efficiently in a sodium acetate gradient. The mechanism of this interaction was not determined. This is the first case documented of a sensory symbiosis involving a surface layer of bacteria. However, it may not be an exceptional case given the large numbers of organisms with surfaces covered with symbionts. The assays for chemotaxis used in this research may be applied in other cases in which motile organisms are covered with symbionts.