The defence mechanisms of 2 novel bacterial isolates against protistan grazing were investigated in experiments with batch and continuous cultures. Strains MWH55 and MWH73 were isolated from the plankton of mesotrophic lakes using culture conditions with strong flagellate grazing pressure. The analysis of their 16S rRNA genes revealed that both strains belong to the Betaproteobacteria and demonstrated a close phylogenetic relatedness to bacteria previously detected by culture-independent

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... -independent methods in river biofilms and lake snow aggregates. Both strains showed a very weak sensitivity to flagellate predation, and both formed planktonic microcolonies that exceeded the upper size limit for ingestion by flagellates, and were thus protected from predation. These microcolonies consisted of cells embedded in a common exopolymeric matrix, yet lacked direct cell-to-cell contact between neighbouring cells. The structure of the exopolymeric matrix of the microcolonies was visualized by electron microscopy, while detection of exopolysaccharides by several fluorescently labeled lectins failed. The matrix possessed a complex 3-dimensional structure with strainspecific characteristics. Currently, it is not clear whether the 2 isolates possess a planktonic life strategy or whether they switch between sessile (biofilm or lake snow aggregates) and planktonic dispersal stages. In both cases, however, a low sensitivity of the planktonic stages to protistan predation is ecologically advantageous. The ability to form an exopolymeric matrix enabling the formation of predation-protected microcolonies is assumed to play a key role in the ecology of these bacteria.