Top-down effects of a lytic bacteriophage and protozoa on bacteria in aqueous and biofilm phases

摘要

Lytic bacteriophages and protozoan predators are the major causes of bacterialmortality in natural microbial communities, which also makes them potentialcandidates for biological control of bacterial pathogens. However, little isknown about the relative impact of bacteriophages and protozoa on the dynamicsof bacterial biomass in aqueous and biofilm phases. Here, we studied thetemporal and spatial dynamics of bacterial biomass in a microcosm experimentwhere opportunistic pathogenic bacteria Serratia marcescens was exposed toparticle-feeding ciliates, surface-feeding amoebas, and lytic bacteriophages for8 weeks, ca. 1300 generations. We found that ciliates were the most efficientenemy type in reducing bacterial biomass in the open water, but least efficientin reducing the biofilm biomass. Biofilm was rather resistant against bacterivores,but amoebae had a significant long-term negative effect on bacterial biomassboth in the open-water phase and biofilm. Bacteriophages had only aminor long-term effect on bacterial biomass in open-water and biofilm phases.However, separate short-term experiments with the ancestral bacteriophagesand bacteria revealed that bacteriophages crash the bacterial biomass dramaticallyin the open-water phase within the first 24 h. Thereafter, the bacteriaevolve phage-resistance that largely prevents top-down effects. The combinationof all three enemy types was most effective in reducing biofilm biomass,whereas in the open-water phase the ciliates dominated the trophic effects. Ourresults highlight the importance of enemy feeding mode on determining thespatial distribution and abundance of bacterial biomass. Moreover, the enemytype can be crucially important predictor of whether the rapid defense evolutioncan significantly affect top-down regulation of bacteria.