The paradigm of the 'microbial loop' has became increasingly important for understanding the structure and function of aquatic ecosystems. Most of the microbial loop studies have focused on energy flow and nutrient cycling. Much less is known, however, about the importance of grazing as a force shaping the structure and community composition of planktonic bacteria. Theoretical considerations of predator-prey interactions suggest that predator evasion mechanisms should have evolved for bactena in the same way as in other predator-prey systems (e.g. zooplankton-phytoplankton). Consistent with this hypothesis, field data show that bacteria are often the most stable component of planktonic communities. Refuges from grazing are one of the possible mechanisms buffering bacterioplankton against strong seasonal fluctuations in abundance. Substantial direct and indirect evidence exists for the occurrence of grazing-resistant bacteria (GRB) in both marine and freshwater habitats. We summarize the potenbal mechanisms for grazing resistance, including morphological, chemcal and behavioral defenses as well as growth in spatial refuges. Cell size appears to be an important factor influencing susceptibility to grazing, with a refuge at the lower and upper ends of the bacterial size range. Thus, a relative grazing resistance can be assumed for the large number of ultramicrobactena as well as for morphologically complex growth forms such as filaments and aggregates. Besides morphological features, resistance may be achieved by other mechanisms for which, however, much less information is available. We describe how GRB can be included in conceptual models of the interactions among metazooplankton, bacterivorous protozoans and bacteria. It is suggested that the relative importance of GRB increases with increasing grazing pressure exerted by protozoans, whereas it decreases with increasing top-down control of protozoans by metazooplankton. GRB may reduce the productivity of planktonic systems through decreased trophic transfer efficiencies and reduced regeneration of bacterially bound nutrients.

In continuous culture enrichments that were inoculated with activated sludge and were fed with polymeric substrates, freely dispersed single-celled bacteria belonging to theCytophaga group dominated among the initial populations, irrespective of the activated sludge source. These populations were grazed by flagellated protozoa which after several days reached high cell densities. Other morphologic bacterial groups such as spiral-shaped or filamentous bacteria then became dominant. In defined mixed culture experiments with bacterial isolates from the enrichment cultures, it was shown that a "grazing-resistant"Microcyclus strain outgrew aCytophaga strain in the presence of grazing protozoa. In contrast, theCytophaga strain competed successfully with theMicrocyclus strain and with other "grazing-resistant" strains under protozoa-free conditions. Furthermore, it was demonstrated that assumed grazing resistance factors such as floccing or filamentous growth were lost by some of the strains when they were grown for several generations in continuous culture under the same conditions, but in the absence of protozoa.

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