| Summary: | Intracellular growth of pathogenic <i>Legionella</i> 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 <i>Legionella</i> 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, <i>Willaertia magna, Acanthamoeba polyphaga</i>, and <i>Vermamoeba vermiformis</i> regarding <i>Legionella pneumophila</i> and two <i>Escherichia coli</i> strains. Although all the studied FLA species supported intracellular growth of <i>L. pneumophila</i>, they avoided this bacterium to a certain degree in the presence of <i>E. coli</i> and mostly fed on it when the preferred bacterial food-sources were limited. Moreover, once <i>L. pneumophila</i> were intracellular, it inhibited digestion of co-occurring <i>E. coli</i> 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 <i>Legionella</i> and creates a “forced-feeding” condition facilitating the internalization of <i>Legionella</i> 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 <i>Legionella</i>.
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