Abstract
Free-living amoeba are members of microbial communities such as biofilms in terrestrial, fresh, and marine habitats. Although they are known to live in close association with bacteria in many ecosystems such as biofilms, they are considered to be major bacterial predators in many ecosystems. Little is known on the relationship between protozoa and marine bacteria in microbial communities, more precisely on how bacteria are able survive in environmental niches where these bacterial grazers also live. The objective of this work is to study the interaction between the axenized ubiquitous amoeba Acanthamoeba castellanii and four marine bacteria isolated from immersed biofilm, in order to evaluate if they would be all grazed upon by amoeba or if they would be able to survive in the presence of their predator. At a low bacteria-to-amoeba ratio, we show that each bacterium is phagocytized and follows a singular intracellular path within this host cell, which appears to delay or to prevent bacterial digestion. In particular, one of the bacteria was found in the amoeba nucleolar compartment whereas another strain was expelled from the amoeba in vesicles. We then looked at the fate of the bacteria grown in a higher bacteria-to-amoeba ratio, as a preformed mono- or multi-species biofilm in the presence of A. castellanii. We show that all biofilms were subjected to detachment from the surface in the presence of the amoeba or its supernatant. Overall, these results show that bacteria, when facing the same predator, exhibit a variety of escape mechanisms at the cellular and population level, when we could have expected a simple bacterial grazing. Therefore, this study unravels new insights into the survival of environmental bacteria when facing predators that they could encounter in the same microbial communities.
Highlights
Protozoa such as free-living amoeba are important members of microbial communities in all terrestrial, fresh, and marine environments
When we looked at the interactions at a low bacteria-to-amoeba ratio, we showed that, first, all the bacteria were phagocytized within the protozoan cells and that bacterial uptake occurred at very different efficiencies; second, none of them were totally eliminated by A. castellanii during the 48 h incubation, even when inoculated at low concentration; third and most importantly, each bacteria followed a singular intracellular fate
When we studied the interaction between preformed monospecies and multispecies biofilms and A. castellanii, which correspond to a higher bacteria-to-amoeba ratio, we showed that, first, all biofilms detached from the surface during the 24 h interaction with amoeba; second, the amoeba supernatant triggered the detachment of two of the bacterial biofilms from the surface; and third, Polaribacter sp
Summary
Protozoa such as free-living amoeba are important members of microbial communities in all terrestrial, fresh, and marine environments. The close association of bacteria and protozoa in biofilm and their long co-evolutionary history are thought to give rise to a series of bacterial adaptations ensuring survival and coexistence [3,4,5]. This aspect has been well studied when protozoa interact with pathogenic bacteria, which is best illustrated with the protozoa-Legionella pneumophila relationship. Studies on the coexistence of bacteria and protozoa including amoeba in the environment are emerging but little is known on the establishment of this type of relationship in particular when marine microorganisms are considered. Very few studies have focused on answering the following conundrum: If protozoa are bacterial grazers and part of the biofilm microbial communities, how do bacteria survive in the presence of their predators in biofilms?
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