Protozoan predation drives adaptive divergence in Pseudomonas fluorescens SBW25; ecology meets experimental evolution.

Abstract

Protozoan predators can affect the structure of bacterial communities, but investigations of how predation might influence bacterial evolution and antagonistic behaviours are scarce. Here, we performed a 20-day predator-prey evolution experiment on solid media to investigate the effect of continuous protozoan predation on bacterial traits using Pseudomonas fluorescens SBW25 as prey and Naegleria gruberi as an amoeboid predator. We observed the divergence of colony morphotypes coincident with an increase in bacterial grazing resistance and relative prey fitness in selected bacterial isolates. When subjected to these resistant prey, N. gruberi show reduced activity (increased encystment) and limited replication. An investigation of the mutations responsible for predation resistance reveals mutations in wspF and amrZ genes, affecting biofilm formation and motility. The bacterial mutants in the wspF gene successfully colonise the air-liquid interface and produce robust cellulose biofilms that prevent predation. The mutation in the amrZ mutant withstands predation but this variant produces low levels of cellulose and limited swarming motility. Our findings suggest that protozoan predation can profoundly influence the course of genetic and phenotypic evolution in a short period.