No abstract

Pseudomonas aeruginosa was shown previously to attack amoebae and other predators by adhering to them and injecting them with virulent substances. In this work, we show that an active, coordinated group behavior is enacted by the bacteria to utilize these molecular components, responding to both predator and bacterial population density.

The mere presence of predators causes prey organisms to enact predation-avoidance strategies. This presence is often revealed through predator-released kairomones. It was previously suggested that in many cases, the predator microbiota composition plays an essential role in releasing these kairomones, however, this mechanism is still poorly understood. Ovipositing mosquito females of several species are repelled by kairomones released from larvivorous fish. In this study, we looked into the effects of the microbiota originated by Gambusia affinis (Baird and Girard) on the oviposition behavior of gravid mosquito females in an outdoor mesocosm experiment. We show that interference with the fish microbiota significantly reduces its repellant effect. We further show that the bacterium Pantoea pleuroti isolated from the skin of the fish repels oviposition of Culex laticinctus (Edwards) and Culiseta longiareolata Macquart mosquitoes similarly to the effect of live fish. These results highlight the importance of bacteria in the interspecies interactions of their hosts and the potential conflict of interest in this system, where bacteria may benefit from the absence of the bacterivore mosquito larvae, but the fish lose access to prey.

Mobbing, a group attack of prey on predator, is a strategy enacted by many animal species. Here we report bacterial mobbing carried out by the bacterium Pseudomonas aeruginosa towards Acanthamoeba castellanii, a common bacterivore. This behavior consists of bacterial taxis towards the amoebae, adhesion en masse to amoebae cells, and eventual killing of the amoebae. Mobbing behavior transpires in second’s timescale and responds to predator population density. A mutant defective in the production of a specific quorum sensing signal displays reduced adhesion to amoeba cells. This deficiency ameliorated by external addition of the missing signal molecule. The same quorum sensing mutant also expresses long term deficiency in its ability to cause amoeba death and shows higher susceptibility to predation, highlighting the importance of group coordination to mobbing and predation avoidance. These findings portray bacterial mobbing as a regulated and dynamic group behavior.

Many plants produce fleshy fruits, attracting fruit-eating animals that disperse the seeds in their droppings. Such seed dispersal results in a conflict between the plant and the animal, as digestion of seeds can be highly beneficial to the animal but reduces plant fitness. The plant Ochradenus baccatus uses the myrosinase-glucosinolates system to protect its seeds. We show that hydrolysis of the O. baccatus fruit glucosinolates by the myrosinase enzyme inhibited digestive enzymes and hampered digestion in naïve individuals of the bird Pycnonotus xanthopygos. However, digestion in birds regularly feeding on O. baccatus fruits was unaffected. We find that Pantoea bacteria, dominating the gut of these experienced birds as well as the fruits, thrive on glucosinolates hydrolysis products in culture. Augmentation of Pantoea protects both naïve birds and plant seedlings from the effects of glucosinolates hydrolysis products. Our findings demonstrate a tripartite interaction, where the plant-bird mutually beneficial interactions are mediated by a communal bacterial tenant.