Production of respirable vesicles containing live Legionella pneumophila cells by two Acanthamoeba spp

Two Acanthamoeba species, fed at three temperatures, expelled vesicles containing living Legionella pneumophila cells. Vesicles ranged from 2.1 to 6.4 microns in diameter and theoretically could contain several hundred bacteria. Viable L. pneumophila cells were observed within vesicles which had been exposed to two cooling tower biocides for 24 h. Clusters of bacteria in vesicles were not dispersed by freeze-thawing and sonication. Such vesicles may be agents for the transmission of legionellosis associated with cooling towers, and the risk may be underestimated by plate count methods.