Experimental assessment of interactions between marine bacteria and model protists: from predator-prey relationships to bacterial-mediated lysis

Bacteria in aquatic environments are a principal food source for predatory protists. Whereas interactions between bacteria and protists are recognized to determine the pathogenesis and epidemiology of several human pathogens, few studies have systematically characterized the interactions between specific aquatic bacteria and protists beyond the prey-predator relation. We, therefore, surveyed individual co-cultures between 18 different genome-sequenced marine bacteria with known virulence gene repertoires and three model protist species widely used for assessing bacteria-protist interactions. Strikingly, 10, 5, and 3 bacterial isolates were capable of lysing the protists <i>Acanthamoeba polyphaga, Tetrahymena pyriformis,</i> and <i>Euglena gracilis</i>, respectively. A majority of the bacteria were able to grow and/or maintain viable populations in the presence of viable protists. Some bacteria survived longer with viable protists but not heat-killed protists and were observed in protist vacuoles. In this respect, marine bacteria are similar to several protist-dependent human pathogens, including <i>Legionella</i>. Analyses of growth patterns in low-nutrient media showed that co-cultivation with <i>A. polyphaga</i> allowed one bacterial strain to overcome nutritional stress and obtain active growth. Five isolates depended on viable amoebae to grow, notwithstanding nutrient media status. The remarkable capability of these marine bacteria to survive encounters with, and even actively kill, model predatory protists under laboratory conditions suggests that diverse bacterial defense strategies and virulence mechanisms to access nutrients may be important in shaping microbial interactions. If verified with native marine and freshwater populations, the diversity of interactions uncovered here has implications for understanding ecological and evolutionary consequences of population dynamics in bacteria and protists.IMPORTANCEThe microbiome constitutes the base of food webs in aquatic environments. Its composition partly reflects biotic interactions, where bacteria primarily are considered prey of predatory protists. However, studies that focus on one or a few species have shown that some bacteria have abilities to escape grazing and may even be capable of lysing their protist predators. In this study, we substantially extend these findings by systematically investigating interactions among multiple taxa of both bacteria and protists. Our results show that marine bacteria display a wider and more complex range of interactions with their predators than generally recognized-from growth dependency to protist lysis. Given that such interactions play key roles in the pathogenesis and epidemiology of several human pathogens, our findings imply that bacterial virulence traits can contribute to defining the structure and ecology of aquatic microbiomes.