Acanthamoeba castellanii (CCAP 1534/3) was found to bind avidly the common soil bacterium Pseudomonas fluorescens. This adhesion was mediated not by pili nor by the general bacterial surface but by the polar flagella. Because of the nature of the flagellar rotary motor, the cell bodies of the attached bacteria could be seen rotating clearly. While initially bacterial binding occurred uniformly over the cell membrane of Acanthamoeba. the bacteria were soon swept posteriorly to form a cap and either endocytosed or sloughed off, still agglutinated by their flagella. Such capped amoebae would not bind Pseudomonas if challenged immediately, indicating a depletion of flagella-binding sites. The bacteria could not bind to amoebae pre-treated with concanavalin A (Con A) even after the lectin had been capped to the uroid. However, capping of flagella-binding sites did not co-cap all the Con A-binding sites on the surface of the amoeba. The flagella-binding sites were not affected by pre-treatment with Pronase (1 mg ml−1) or anti-Acanthamoeba surface antibody. Proteus mirabilis also bound avidly by its flagella to Acanthamoeba and, furthermore, competition experiments suggested that Proteus and Pseudomonas adhere to a common surface site on the amoeba. The presence of sites on the cell membrane of A. castellanii that are specific for flagellin would enhance strongly the adsorption of motile bacteria prior to endocytosis. This would represent an excellent feeding strategy for a soil-dwelling phagotroph.
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