|DE BRUIJN, IRENE|
Submitted to: Applied and Environmental Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/24/2009
Publication Date: 8/28/2009
Citation: Mazzola, M., De Bruijn, I., Cohen, M.F., Raaijmakers, J.M. 2009. Protozoan-induced regulation of cycliclipopeptide biosythesis is an effective predation defense mechanism for Pseudomonas fluorescens. Appl. Environ Microbiol. 75:6804-6811.
Interpretive Summary: Environmental bacteria are exposed to a myriad of biotic interactions. The grazing activity of protozoan predators significantly impacts the dynamics, diversification and evolution of bacterial communities in soil ecosystems. To evade protozoan predation, bacteria employ various defense strategies. Here, we show that certain soil bacteria can sense the presence of protozoan predators. When confronted with the protozoan predator Naegleria americana, these bacteria are triggered at the molecular level to produce specific detergent-like compounds, called cyclic lipopeptides, which protect them from being attacked and consumed. These findings highlight the functional versatility of these natural detergents and document the existence of a previously unknown sensing-signaling mechanism in the molecular dialogue between bacteria and protozoan predators.
Technical Abstract: The grazing activity of protozoa significantly impacts the dynamics, diversification and evolution of bacterial communities in soil ecosystems. The feeding preference of protozoa is related to their inability to ingest or digest specific bacteria. Pseudomonas fluorescens strains SBW25 and SS101 used in this study produce viscosin and massetolide, respectively, two structurally related cyclic lipopeptides (CLPs) composed of an oligopeptide linked to a fatty acid. Confrontation assays on agar surfaces showed that trophozoites of the amoebae Naegleria americana exhibited significant differences in feeding preference; the amoebae showed limited or no migration along the linear growth of CLP-producers SS101 and SBW25, whereas enhanced feeding occurred on CLP-deficient mutants. In soil environments, P. fluorescens SS101 exhibited superior persistence relative to its CLP-deficient mutant when confronted with N. americana. When exposed to different concentrations of purified viscosin or massetolide, trophozoite lysis occurred rapidly. Real-time quantitative PCR analyses demonstrated that N. americana modulated expression of CLP-biosynthesis genes in P. fluorescens: bacterial cells collected at the feeding margin exhibited mass and visc expression levels that were significantly higher (up to 3.5 fold) than in cells collected at 4-cm distant. Thus, our study documents the existence of a previously unknown sensing-signaling mechanism in bacteria-protozoa interactions that results in activation of an antipredator response in prey populations and does not inevitably require intimate contact as was reported for the killing effect of biofilm-associated P. aeruginosa toward Acanthamoeba castellanii. Considering the importance of CLPs in bacterial motility and biofilm formation (5), this study highlights the functional versatility of these natural detergent-like compounds and provides novel insights into the regulation and evolution of antiprotozoal genes and compounds in environmental bacteria.