Role of Microcolony Formation in the Protistan Grazing Defense of the Aquatic Bacterium Pseudomonas sp. MWH1 M.W. Hahn, E.R.B. Moore, M.G. Ho ¨fle GBF—National Research Center of Biotechnology, Microbial Ecology Group, D-38124 Braunschweig, Germany Received: 1 December 1999; Accepted: 16 February 2000; Online Publication: 5 May 2000 A B S T R A C T 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 absence and presence of a potential competitor (Vibrio sp. CB5), as well as under starvation conditions and in a situation of unlimited growth. In the competition experiment the two bacterial strains were distinguished by immunofluorescence microscopy. When the Pseudomonas strain was cultured in the absence of the predator Ochromonas sp. DS, only mobile single cells were detectable. Grazing by this bacterivorous flagellate resulted in all experiments in the occurrence of a Pseudomonas subpopulation, which grew as floclike, suspended microcolonies. These microcolonies consisted of up to approximately 1,000 cells and were, because of their large size, protected against flagellate grazing. The microcolony subpopulation dominated the total Pseu- domonas population in situations of high grazing pressure at a wide range of bacterial growth conditions. Thus, the formation of the microcolonies is interpreted as a successful grazing-defense strategy, which is effective under several growth conditions, allowing for the survival of the strain even when substrate depletion is combined with strong grazing pressure. Batch culture experiments demonstrated that the change in morphology of Pseudomonas sp. MWH1 is not controlled by growth rate, although no formation of microcolonies was observed after the addition of 0.2-μm- filtered flagellate cultures to Pseudomonas cultures, indicating that a chemical trigger released by the flagellate is not involved in the control of this defense mechanism. Introduction Free-living bacteria and their major predators, bacterivorous protists, have coexisted in several types of habitats over long geological periods. Thus, one should expect that bacteria have evolved a wide range of different grazing defense mechanisms. The size of bacterial cells is an important fea- ture in the predator–prey relationship between bacteria and protistan predators because grazing by predators is size- selective [e.g., 5, 23]. In general, this results in reduced graz- ing mortality for small bacterial cells and full protection of Correspondence to: M.W. Hahn; Fax: 43-6232-3578; E-mail: martin.hahn@oeaw.ac.at. MICROBIAL ECOLOGY Microb Ecol (2000) 39:175–185 DOI: 10.1007/s002480000026 © 2000 Springer-Verlag New York Inc.