Both protozoan and bacterial strain, as well as their particular combinations, significantly influenced the outcome of their interactions (Table 1). Pseudomonas fluorescens CHA0 was especially harmful (Figs 1 and 2, Table 1). This strain efficiently restrains growth of various plant-pathogenic fungi, inhibits egg hatch and cause mortality of plant-pathogenic nematode juveniles, (Keel et al., 1992; Siddiqui et al., 2006) and inhibits several nontarget fungi (Winding et al., 2004). Jousset et al. (2006) found that only mutants STA-9090 molecular weight completely devoid of metabolite production (GacA/GacS-negative)

supported protozoan growth, which suggests that the high toxicity of CHA0 is linked to the production of a broad

range of different secondary Epacadostat cost metabolites. We observed that the strains producing extracellular metabolites, i.e. CHA0 and DSS73, were more harmful to protozoa than strains that mainly produce membrane-bound metabolites, i.e. DR54 and MA342 (Fig. 1). To analyze this matter further, we arranged our Pseudomonas strains into three groups: those without secondary metabolites, those that produce membrane-bound secondary metabolites, and a group of bacteria producing extracellular secondary metabolites. We then correlated growth rates of each of these three groups to the growth rates of E. aerogenes. We found a very high correlation between the growth rates of E. aerogenes and the supposedly harmless Pseudomonas (r2=0.85, P=0.0002); we obtained no correlation at all between 4��8C E. aerogenes and the Pseudomonas with extracellular metabolites (r2=0.02, P=0.36), whereas Pseudomonas with membrane-bound metabolites correlated better and almost significantly (r2=0.26, P=0.08). We suggest that the relatively increased ability to cope with membrane-bound toxins in organisms with higher growth rates can be attributed to egestion of harmful remnants enclosed in the food vacuole (membrane parts) whereas

extracellular metabolites are in contact with the cell surface and are difficult to avoid. This is in accordance with the mechanism discussed by Deines et al. (2009). They elegantly showed that volume-specific clearance rate correlated positively with toxin tolerance; probably because organisms with a relative higher clearance rate use their food less efficiently, and egest cell remnants that contain harmful substances. Everything else being equal, volume-specific clearance rate and intrinsic growth rate will correlate. Hence, we suggest that egestion of harmful remnants can explain the higher tolerance. The ability of protozoa to grow on specific bacteria did not correlate particularly well with low-level taxonomic group (Table 1). For example, the two strains of B. designis reacted quite differently to the presented bacteria.