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Counting Legionella cells within single amoeba host cells
BUSE, H. Y. AND N. ASHBOLT. Counting Legionella cells within single amoeba host cells. APPLIED AND ENVIRONMENTAL MICROBIOLOGY. American Society for Microbiology, Washington, DC, 78(6):2070-2072, (2012).
Legionellosis is a major cause of health burden associated with drinking water, with Legionella pneumophila accounting for some 29% of all drinking water outbreaks since being a reported disease in the US (6). Direct exposure to aerosols containing Legionella spp. generated from cooling towers, humidifiers and shower heads are implicated as the vehicle for bacterial transmission (1, 4). While L. pneumophila may be packaged and expelled in vesicles of freshwater ciliates (3), free-living protozoa within the genera Acanthamoeba and Naegleria, are often cited as potential hosts and ecological reservoirs for L. pneumophila in drinking water systems (8). Therefore, understanding Legionella-amoeba interactions is considered fundamental to aid in developing control strategies and in supporting quantitative microbial risk assessment models for pathogenic legionellae in drinking water (9). Furthermore, if human-infectious legionellae are largely from amoebae hosts (8), it is critical to understand how many legionellae may be released per host cell within drinking water biofilms to enable back calculations of host densities that may result in significant pathogen release events (9). There appears to be no previous data reported on cell counts of intracellular L. pneumophila within single amoeba host cells. Hence, here we present the first attempt to provide intracellular pathogen ranges in Acanthamoeba and Naegleria spp. that may colonize drinking water biofilms.
Here we present the first attempt to quantify L. pneumophila cell numbers within individual amoebae hosts that may be released into engineered water systems. The maximum numbers of culturable L. pneumophila cells grown within Acanthamoeba polyphaga and Naegleria fowleri were 1348 (mean 329) and 385 (mean 44) CFU trophozoite-1, respectively.