Cercomonads eating bacteria

Bin7 Movie G Cropped.mov

Response of beneficial soil bacteria to predation by protozoa.
Protist grazing is known to control the abundance of bacteria in soil rhizospheres and provide benefits for plants. One way they do this is by facilitating nutrient exchange, whereby plants trade carbon for other essential nutrients, like nitrogen. This idea was set forth by Marianne Clarholm in her classic "microbial loop" hypothesis. In its simplest form, this hypothesis suggests that plants send a large amount of carbon into the rhizosphere to fuel the growth of bacteria which are otherwise carbon-limited. These bacteria must extract other nutrients from soil in order to grow. The bacteria are eaten by protists that then secrete excess nutrients into the rhizosphere where they are consumed by plants Evidence that such a loop includes studies where exclusion of protists from roots resulted in lower plant yields.

Many bacteria are able to detect predatory protozoa and respond in ways that render them less susceptible to predation. In the case of biocontrol Pseudomonas fluorescens, cells detect predatory amoebae through water-soluble signals and then induce protective genes. Resistance of bacteria to predation can also come about by increasing cell size, by increasing growth rate so as to outgrow predators, by formation of biofilms, or by predator avoidance behavior. In order for bacteria to protect themselves against predators they must first detect predators and then initiate appropriate responses. The physiological responses of bacteria under attack (e.g. filamentous growth, biofilm formation, chemotaxis, exotoxin gene induction) have been examined individually, and under static conditions, but not in aggregate or dynamically.

In a USDA-funded project we are using microfluidics, microscopy, biochemistry and genetics to study how the beneficial soil bacteria, Sinorhizobium meliloti and P. fluorescens detect and respond to the presence of bacterial predators. Current work is focused on characterizing biofilm formation in response to predators and resulting changes in gene expression. This work is being done in collaboration with the lab of Dr. Leslie Shor.