1980;25:943C948. with the flagellate. In contrast to strain CB5, PX54 responded to grazing with a strong development of cell size distribution toward large, filamentous cells. These changes in cell Rabbit polyclonal to AnnexinA10 morphology resulted in a high percentage of inedible cells in the PX54 human population but not in the strain CB5 human population, which caused the observed switch in the relative abundances of the Rosiglitazone (BRL-49653) strains. Batch tradition experiments without the flagellate demonstrated the elongation of PX54 cells was dependent on their growth rate. This indicates that the event of Rosiglitazone (BRL-49653) filamentous PX54 cells is not a direct response to chemical stimuli released from the flagellates but rather a response to increased growth rates due to flagellate grazing. Predator-prey relationships of coexisting, free-living aquatic bacteria and bacterivorous protozoa have coevolved for more than a billion years (28). This enormous time span and the short generation instances of both groups of microorganisms should have resulted in a high degree of evolutionary adaptation on both sides. Bacteria may have developed defense strategies to prevent themselves from becoming ingested (preingestional strategies) or digested (postingestional strategies) by their protozoan predators, which, expectedly, adaptated to circumvent the bacterial defense mechanisms. Information about the strategies involved in these predator-prey relationships is scarce. Recently, Jrgens and Gde (20) examined the strategies of bacteria and stressed the lack of knowledge with this field. Studies on size-selective ingestion (grazing) of bacterivorous protozoa (6, 10, 25) show that very small and large bacteria are partly or totally safeguarded from protozoan grazing (12, 20). This getting is supported by field and experimental observations showing the event and persistence of large bacterial filaments and aggregates during instances of high grazing pressure (11, 21, 29, 41). The experimental evidence for protection and the increasing quantity of reports on the presence of filamentous bacteria in freshwater ecosystems (12, 13, 19, 35, 39, 41) indicate that this bacterial morphotype exhibits an ecologically significant defense strategy against protozoan grazing. It is Rosiglitazone (BRL-49653) not known to which varieties these safeguarded forms belong. Additionally, it is unclear if the filamentous bacteria grow permanently with these, with respect to grazing, advantageous morphological properties or if they express these characteristic features only under strong grazing pressure. In a recent study, Pernthaler et al. (30) shown that a slow-growing bacterial community reacted to the addition of bacterivorous flagellates within 1 day: one group produced filamentous, Rosiglitazone (BRL-49653) grazing-resistant forms, and another group of bacteria reacted with a massive growth rate increase. Similarly, Jrgens et al. (21) observed in enclosure studies, after experimentally increasing the protozoan grazing pressure, that there was a rapid and strong switch in the morphological structure of the bacterial community. After 3 days, primarily filamentous and additional inedible bacterial cells dominated the bacterial biomass, having a prevalence of 80 to 90%. Different mechanisms are conceivable for such changes in the morphological structure of bacterial areas. First, nonfilamentous, edible strains may just become replaced after some time by inedible, permanently filamentous strains. In situations with bacterial generation times longer than 1 day and undetectably low abundances of filamentous cells (30), such an indirect selection mechanism can hardly cause visible changes in community structure within 24 h. But the probability cannot be ruled out that this mechanism is definitely of relevance in natural ecosystems. Second, medium-size, edible cells may become elongated and thus form filaments. This type of response to strong protistan grazing might be controlled by two different mechanisms: (i) elongation of the cells due to grazing-mediated changes in bacterial growth conditions (indirect induction of filament formation) or (ii) direct induction of morphological changes by chemical stimuli. Such chemical stimuli might be produced and released from the protozoan predators (predator kairomone) or produced by the prey bacteria and set free from the predators during digestion. The second type of stimuli would act as an alarm compound. It is not known if selection or one of the induction mechanisms triggers the observed reactions of bacterial areas. Pernthaler et al. (30) speculated that a chemical stimulus caused the observed changes in their experiments, since they found an immediate response upon addition of a flagellate grazer. Detailed information within the relationships of bacteria with protozoan grazers and the producing bacterial defense strategies are necessary for a comprehensive understanding of a number of important issues in microbial ecology. This includes questions on the subject of the influence of protozoa on (i) the bacterial varieties composition of natural communities,.