Here, we describe protozoan features that affect their HSP assay ability to grow on secondary-metabolite-producing bacteria, and examine whether different bacterial secondary metabolites affect protozoa similarly. We investigated the growth of nine different soil protozoa on six different Pseudomonas strains, including the four secondary-metabolite-producing Pseudomonas fluorescens DR54 and CHA0, Pseudomonas chlororaphis MA342 and Pseudomonas sp. DSS73, as well as the two nonproducers P. fluorescens DSM50090T and P. chlororaphis ATCC43928. Secondary metabolite producers affected protozoan growth

differently. In particular, bacteria with extracellular secondary metabolites seemed more inhibiting than bacteria with membrane-bound metabolites. Interestingly, protozoan response seemed to correlate with high-level protozoan taxonomy, and amoeboid taxa tolerated a broader range of Pseudomonas strains than did the non-amoeboid

taxa. This stresses the importance of studying both protozoan and bacterial characteristics in order to understand bacterial defence mechanisms and potentially improve survival of bacteria introduced into the environment, for example for biocontrol purposes. Protozoan grazing increases bacterial turnover of organic matter and reduces bacterial biomass (Rønn et al., 2002; Bonkowski, 2004; Christensen et al., 2006). Furthermore, particular EGFR inhibitor protozoa consume different bacteria to different extents (Rønn et al., 2001, 2002; Mohapatra & Fukami, 2004; Pickup et al., 2007). Factors that presumably affect bacterial susceptibility to grazing include cell size, speed of movement, extent of biofilm production, and the composition of the bacterial envelope (Matz & Kjelleberg, 2005). Bacteria that produce secondary metabolites may likewise be less suitable as protozoan food (Rønn et al., Metalloexopeptidase 2001; Andersen & Winding, 2004; Matz et al., 2004; Jousset et al., 2006; Pedersen et al., 2009). The genus

Pseudomonas is interesting in this context as it includes strains that produce a wide range of secondary metabolites (Haas & Défago, 2005). Protozoa can discriminate between different food items (e.g. Jürgens & DeMott, 1995; Boenigk et al., 2001; Jezbera et al., 2006; Pedersen et al., 2009) and therefore only ingest some bacterial strains. Hence, protozoa graze different taxonomic groups of bacteria differently (Matz et al., 2004). Still, we know only little about how protozoan features correlate with which bacteria they can ingest and hence digest. Here, we focus on protozoan characteristics; thus, we hypothesize that protozoan taxonomic affiliation (Adl et al., 2007) can be used to predict which bacteria they can subsist on, depending upon the bacterial production of secondary metabolites. Thus, we hope to find protozoan characteristics that correlate with their ability to grow on specific bacteria.