Biology subjects

Sieber, M.

Publications and source records attributed to Sieber, M..

4 recordsLinked to original sources

bioRxiv2018

The Neutral Metaorganism

Almost all animals and plants are inhabited by diverse communities of microorganisms, the microbiota. The hosts often pose highly selective environments, which only a subset of the environmentally available microbes are able to colonize. From the hosts perspective it seems useful to shape the community composition of these allowed microbes to promote a beneficial host-microbe symbiosis. In contrast to this, neutral models assume that the structure of the microbiota is entirely shaped by random population dynamics and dispersal. We here show that microbiota community structure from a wide range of host organisms, in particular including previously understudied invertebrates, is in almost all cases consistent with neutral expectations. Moreover, we demonstrate that apparent discrepancies with the neutral model can be due to transient community states rather than host selection. In conclusion, even though hosts are often assumed to control microbiota composition to ensure beneficial interactions, our broad-scale analysis highlights that following colonization, it could rather be neutral processes that determine microbial community structure.

ecology↗

bioRxiv2018

Dietary lipids modulate Notch signaling and influence adult intestinal development and metabolism in Drosophila

Tissue homeostasis is a complex balance of developmental signals and environmental cues that dictate stem cell function. However, it remains poorly understood how nutrients interface with developmental pathways. Using the Drosophila midgut as a model we found that during the first four days of adult life, dietary lipids including cholesterol, determine how many enteroendocrine (ee) cells differentiate and persist in the posterior midgut where lipids are preferentially absorbed. The nuclear hormone receptor Hr96 which functions to control sterol trafficking, storage, and utilization, is required for sterol-mediated changes in ee number. Dietary cholesterol influences new intestinal epithelial cell differentiation from stem cells by altering the level and persistance of Notch signaling. Exogenous lipids modulate signaling by changing the stability of the Delta ligand and Notch intracellular domain and their trafficking in endosomal vesicles. Lipid-modulated Notch signaling occurs in other nutrient-dependent tissues such as the ovary, suggesting that Delta trafficking in many cells is sensitive to cellular sterol levels. These diet-mediated alterations in ee number in young animals contribute to a metabolic program adapted to the prevailing nutrient environment that persists after the diet changes. A low sterol diet also slows the proliferation of enteroendocrine tumors initiated by disruptions in the Notch pathway. These studies show that a specific dietary nutrient can modify a key intercellular signaling pathway to shift stem cell differentiation and cause lasting changes in tissue structure and physiology.

developmental biology↗

bioRxiv2018

One man’s trash is another man’s treasure - the effect of bacteria on phytoplankton-zooplankton interactions in chemostat systems

Chemostat experiments are employed to study predator-prey and other trophic interactions, frequently using phytoplankton-zooplankton systems. These experiments often use population dynamics as fingerprints of ecological and evolutionary processes, assuming that the contributions of all major actors to these dynamics are known. However, bacteria are often neglected although they are frequently present. We argue that even without external carbon sources bacteria may affect the experimental outcomes depending on experimental conditions and the physiological traits of bacteria, phytoplankton and zooplankton. Using a static carbon flux model and a dynamic simulation model we predict the minimum and maximum impact of bacteria on phytoplankton-zooplankton population dynamics. Under bacteria-suppressing conditions, we find that the effect of bacteria is indeed negligible and their omission justified. Under bacteria-favouring conditions, however, bacteria may strongly affect average biomasses. Furthermore, the population dynamics may become highly complex resulting in wrong conclusions if bacteria are not considered. Our model results provide suggestions to reduce the bacterial impact experimentally. Next to optimizing experimental conditions (e.g. the dilution rate) the appropriate choice of the zooplankton predator is decisive. Counterintuitively, bacteria have a larger impact if they are not ingested by the predator as high bacterial biomasses and complex population dynamics arise via competition for nutrients with the phytoplankton. Only if the predator is at least partly bacterivorous the impact of bacteria is minimized. Our results help to improve both the design of chemostat experiments and their interpretation and thus advance the study of ecological and evolutionary processes in aquatic food webs.

ecology↗

bioRxiv2017

Temperate phages as frequency-dependent weapons in bacterial competition

Microbial communities are accompanied by a diverse array of viruses. Through infections of abundant microbes, these viruses have the potential to mediate competition within the community, effectively weakening competitive interactions and promoting coexistence. This is of particular relevance for host-associated microbial communities, since the diversity of the microbiota has been linked to host health and functioning. Here, we study the interaction between two key members of the microbiota of the freshwater metazoan Hydra vulgaris. The two commensal bacteria Curvibacter sp. and Duganella sp. protect their host from fungal infections, but only if both of them are present. Coexistence of the two bacteria is thus beneficial for Hydra. Intriguingly, Duganella sp. appears to be the superior competitor in vitro due to its higher growth rate when both bacteria are grown seperately, but in coculture the outcome of competition depends on the relative initial abundances of the two species. The presence of an inducible prophage in the Curvibacter sp. genome which is able to lytically infect Duganella sp., led us to hypothesise that the phage modulates the interaction between these two key members of the Hydra microbiota. Using a mathematical model we show that the interplay of the lysogenic life-cycle of the Curvibacter phage and the lytic life-cycle on Duganella sp. can explain the observed complex competitive interaction between the two bacteria. Our results highlight the importance of taking lysogeny into account for understanding microbe-virus interactions and show the complex role phages can play in promoting coexistence of their bacterial hosts.

microbiology↗