Role of microcolony formation in the protistan grazing defense of the aquatic bacterium Pseudomonas sp MWH1

The defense strategy of the aquatic bacterium Pseudomonas sp. MWH1 against flagellate grazing was investigated in chemostat and batch experiments. The influence of predation on the Pseudomonas population was studied in the ab sence and presence of a potential competitor (Vibrio sp. CB5), as well as u nder starvation conditions and in a situation of unlimited growth. In the c ompetition experiment the two bacterial strains were distinguished by immun ofluorescence microscopy. When the Pseudomonas strain was cultured in the a bsence of the predator Ochromonas sp. DS, only mobile single cells were det ectable. Grazing by this bacterivorous flagellate resulted in all experimen ts in the occurrence of a Pseudomonas subpopulation, which grew as floclike , suspended microcolonies. These microcolonies consisted of up to approxima tely 1,000 cells and were, because of their large size, protected against f lagellate grazing. The microcolony subpopulation dominated the total Pseudo monas population in situations of high grazing pressure at a wide range of bacterial growth conditions. Thus, the formation of the microcolonies is in terpreted as a successful grazing-defense strategy, which is effective unde r several growth conditions, allowing for the survival of the strain even w hen substrate depletion is combined with strong grazing pressure. Batch cul ture experiments demonstrated that the change in morphology of Pseudomonas sp. MWH1 is not controlled by growth rate, although no formation of microco lonies was observed after the addition of 0.2-mu m-filtered flagellate cult ures to Pseudomonas cultures, indicating that a chemical trigger released b y the flagellate is not involved in the control of this defense mechanism.