Experimental assessment of marine microbial interactions: from predatory protists promoting bacterial survival to bacterial lysis of the protists

Bacteria in aquatic environments are a principal food source for predatory protists. Whereas interactions between bacteria and protists are recognized to play important roles in determining 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 the outcome of 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, ten, five, and three bacterial isolates were capable of lysing the protists Acanthamoeba polyphaga, Tetrahymena pyriformis and Euglena gracilis, 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 in the presence of viable protists but not heat-killed protists, and were observed in protist vacuoles. In this respect, thus, marine bacteria are similar to several protist-dependent human pathogens, including Legionella. Analyses of growth patterns in low-nutrient media showed that co-cultivation with A polyphaga 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 surviving encounters with, and even actively killing, bacterivorous protists, indicates that diverse (and possibly novel) bacterial defense strategies and virulence mechanisms to access nutrients are widespread among marine bacteria. The diversity of interactions uncovered here has important implications for understanding ecological and evolutionary consequences of population dynamics in bacteria and protists. IMPORTANCEThe microbiome constitutes the base of food webs in marine waters. Its composition partly reflects biotic interactions, where bacteria primarily are considered as 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 the marine microbiome.