1). Five out of nine protozoan strains displayed similar growth rates on these strains (Table 1). Three strains, however, had significantly lower growth rates on ATCC43928 than on DSM50090T, and one had a higher growth rate. All Pseudomonas strains producing secondary metabolites affected protozoan growth negatively (Table 1, Figs 1–3). Only C. longicauda displayed similar growth rates on all bacterial strains. Likewise,

C. longicauda was the only one of the nine tested protozoa that did not display inhibited growth on MA342 and DSS73 as compared with the bacterial strains without known production of secondary metabolites. Pseudomonas fluorescens CHA0 was the least suited food bacterium of the tested strains (Fig. 1). It supported growth of none of the tested protozoa, but C. longicauda and H. vermiformis (Table 1). Secondary-metabolite-producing selleck chemicals bacteria supported protozoan growth poorly as compared with nonproducers (Fig. 1). Thus, eight of the nine tested protozoa displayed lower growth rates when grown on secondary metabolite producers than on the nonproducers (Fig. 2, Table 1). This clearly indicates that the metabolites protect bacteria against grazing. This inhibition of protozoan growth was also observed in experiments using other protozoa and GKT137831 ic50 in a set-up investigating potential negative effects of antagonistic bacteria in soil (Schlimme et al., 1999; Johansen et al., 2005; Jousset et al., 2006; Pedersen

et al., 2010). Further, growth of different protozoa increased considerably when grown on mutants where synthesis of secondary metabolites was blocked completely compared with wild-type bacteria, which produce the secondary metabolites (Jousset et al., 2006). To examine further as to how differences in the mode of action of Pseudomonas secondary metabolites relate to their effect on protozoa, Pedersen et al. (2010) incubated the protozoan C. longicauda

in batch cultures with three different P. fluorescens strains that we also used in the experiments reported here. These three P. fluorescens strains have contrasting secondary metabolite properties. Thus, the type strain DSM50090T produces no known secondary metabolites, DR54 produces a membrane-bound cyclic lipopeptide, and CHA0 produces various extracellular PAK5 metabolites. For all three Pseudomonas strains, Pedersen et al. (2010) set up batch cultures with washed bacterial suspensions, presumed to be devoid of extracellular metabolites, as well as unwashed cultures retaining potential extracellular metabolites. In accordance with their assumptions, Pedersen et al. (2010) found that when offered washed CHA0, C. longicauda was able to multiply, whereas for the two other Pseudomonas strains washed and unwashed bacteria affected C. longicauda similarly. Likewise, Andersen & Winding (2004) found that cell extract from P. fluorescens DR54 inhibited a mixed community of soil protozoa.