Marine Bacteria Display Different Escape Mechanisms When Facing Their Protozoan Predators

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.

Differing Escape Responses of the Marine Bacterium Marinobacter adhaerens in the Presence of Planktonic vs. Surface-Associated Protist Grazers

“…While the molecular basis of the observed bacterial responses and the involved cues are unknown, our results suggest that habitat shifts are key to escape high predation pressure and allow for bacterial persistence. Our results are in line with laboratory experiments by [ 24 ] revealing similar escape behavior of four different surface-attached bacteria in the presence of a surface predator—the amoeba A. castellanii . They found that even the liquid supernatant of the A. castellanii culture induced the bacterial escape response, pointing to the fact that chemical signaling may trigger the bacterial behavior.…”

Trade-offs of lipid remodeling in a marine predator–prey interaction in response to phosphorus limitation

“…We should add, though, that such a dilemma (between escape from ingestion and resistance to digestion) is not the only trade-off that microbes face due to grazing pressure. For example, it is well known that bacteria can form biofilms or clumping ( 39 , 40 ) to avoid capture by protists at the expense of growth, with such morphological changes affecting cell-surface area-to-volume ratios, which negatively impact on nutrient uptake for optimum growth. Similarly, unicellular algae with larger cell sizes are poorly grazed, but have low ammonium uptake rates and grow poorly ( 41 ).…”

“…Afterward, the samples were directly fixed by adding formaldehyde at 4% (vol/vol) for 20 min, and a low-speed centrifugation (15 min at 900 × g ) was used to initiate and increase cell adhesion to the coverslips. Finally, cells were stained with DAPI (5 μg/mL) and ConA TRITC conjugate (20 μg/mL; Thermo Fisher Scientific) ( 39 ) and mounted with a drop of Mowiol antifade before observation using a confocal laser scanning microscope (CLSM; Zeiss LSM 880). Bacterial biovolumes and the biovolume of ConA-stained glycoconjugates in CLSM images were determined by using five images for each of the three replicates using COMSTAT software developed in MATLAB R2015a (MathWorks), as described ( 64 ).…”

“…MED193 or the plcP mutant) were inoculated at an MOI of 10, 100, or 500 (equivalent to 10, 100, or 500 prey per protist cell, respectively). After 3 h, 10 h, 24 h, and 48 h of contact time at 22.5 °C, ciliates were treated with Triton X-100 (0.05%) for 30 min on ice and mechanically lysed by mixing using a syringe and 25-g needle (BD), as described ( 39 ). Bacterial prey numbers were quantified by serial dilution on MB agar plates.…”

Pseudomonas aeruginosa Mobbing-Like Behavior against Acanthamoeba castellanii Bacterivore and Its Rapid Control by Quorum Sensing and Environmental Cues

“…Predation by protozoa is a major selective force acting on bacterial populations, producing a variety of predation avoidance mechanisms ( 1 , 2 ); assembling into bigger and harder to swallow multicellular structures ( 3 ), swimming away from a predator ( 4 ), producing antipredator toxins ( 5 ), interfering with predator phagocytosis and digestion processes, and modulating cell surfaces to evade predator recognition ( 6 ).…”