Abstract
Intracellular growth of pathogenic Legionella in free-living amoebae (FLA) results in the critical concentrations that are problematic in engineered water systems (EWS). However, being amoeba-resistant bacteria (ARB), how Legionella spp. becomes internalized within FLA is still poorly understood. Using fluorescent microscopy, we investigated in real-time the preferential feeding behavior of three water-related FLA species, Willaertia magna, Acanthamoeba polyphaga, and Vermamoeba vermiformis regarding Legionella pneumophila and two Escherichia coli strains. Although all the studied FLA species supported intracellular growth of L. pneumophila, they avoided this bacterium to a certain degree in the presence of E. coli and mostly fed on it when the preferred bacterial food-sources were limited. Moreover, once L. pneumophila were intracellular, it inhibited digestion of co-occurring E. coli within the same trophozoites. Altogether, based on FLA–bacteria interactions and the shifts in microbial population dynamics, we propose that FLA’s feeding preference leads to an initial growth of FLA and depletion of prey bacteria, thus increases the relative abundance of Legionella and creates a “forced-feeding” condition facilitating the internalization of Legionella into FLA to initiate the cycles of intracellular multiplication. These findings imply that monitoring of FLA levels in EWS could be useful in predicting possible imminent high occurrence of Legionella.
Highlights
Legionella pneumophila, a Gram-negative bacterium, indigenous to natural and engineered water systems (EWS) [1,2], has become the number one cause of drinking waterrelated disease outbreaks in developed countries [3,4,5]
These findings imply that monitoring of free-living amoebae (FLA) levels in EWS could be useful in predicting possible imminent high occurrence of Legionella
Legionella coexists in natural and engineered aquatic environments with other bacteria and microscopic eukaryotes like free-living amoebae (FLA), ciliates, and nematodes [8,9,10,11,12], as bacteria serve as a major source of food for FLA and other microeukaryotes [13,14]
Summary
Legionella pneumophila, a Gram-negative bacterium, indigenous to natural and engineered water systems (EWS) [1,2], has become the number one cause of drinking waterrelated disease outbreaks in developed countries [3,4,5]. EWS including building water systems and cooling towers, are often reported as the source of exposure to pathogenic. Legionella coexists in natural and engineered aquatic environments with other bacteria and microscopic eukaryotes like free-living amoebae (FLA), ciliates, and nematodes [8,9,10,11,12], as bacteria serve as a major source of food for FLA and other microeukaryotes [13,14]. Legionella in EWS is considered to occur predominantly via intracellular growth within the susceptible FLA hosts [17,18,19], to very high concentrations considered necessary for causing infections through aerosol exposures [20]. Various bacteria that have developed mechanisms against protozoan predation and digestion [21,22,23,24] and even to replicate within predatory host cells [25,26,27]
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