Emiliania huxleyi is a cosmopolitan phytoplankton species that forms massive blooms in the oceans. It has great ecological and biogeochemical importance by playing a central role in global carbon and sulfur cycling. E. huxleyi produces large amounts of dimethylsulfoniopropionate (DMSP), a metabolite known to mediate bacterial interactions with algae. E. huxleyi bloom demise is attributed to viral infection, however, the involvement of additional pathogens is underexplored. We isolated a naturally co-occurring bacterium, Sulfitobacter D7, which exhibited a behavioral switch when co-cultured with E. huxleyi. First, there was a coexistence phase in which both partners grew. The interaction shifted when bacterial pathogenicity towards E. huxleyi was invoked upon exposure to high concentrations of algal DMSP, when the algae reached stationary growth or when applied exogenously. To characterize the behavioral switch, we grew D7 in E. huxleyi-derived conditioned media (CM), which mimic the coexistence and pathogenicity phases of the interaction, and analyzed bacterial transcriptomes. Addition of DMSP to CM containing algae-derived metabolites induced major modulations in bacterial gene expression, which corroborated the observed behavioral switch in co-cultures. However, in defined media, lacking algal metabolites, DMSP barely affected gene expression. This demonstrates that DMSP has a signaling role that strongly affects bacterial behavior through gene expression, but an additional algal factor is required. Transporters for various compounds were upregulated in pathogenic mode, suggesting that Sulfitobacter D7 benefits from E. huxleyi-derived metabolites. We discovered that algae-produced benzoate promoted the growth of Sulfitobacter D7, and negated the behavioral switch to pathogenicity. We concluded that benzoate is important for bacteria and algae coexistence and suggested that as long as Sulfitobacter D7 benefits from E. huxleyi, by receiving growth substrates, the switch to pathogenicity will not occur. This work demonstrates how metabolic cross-talk governs the nature of algae-bacteria interactions, which greatly impact large-scale oceanic processes.

_Emiliania huxleyi_ is a cosmopolitan phytoplankton species that forms massive blooms in the oceans. It has great ecological and biogeochemical importance by playing a central role in global carbon and sulfur cycling. _E. huxleyi_ produces large amounts of dimethylsulfoniopropionate (DMSP), a metabolite known to mediate bacterial interactions with algae. _E. huxleyi_ bloom demise is attributed to viral infection, however, the involvement of additional pathogens is underexplored. We isolated a naturally co-occurring bacterium, _Sulfitobacter_ D7, which exhibited a behavioral switch when co-cultured with _E. huxleyi_. First, there was a coexistence phase in which both partners grew. The interaction shifted when bacterial pathogenicity towards _E. huxleyi_ was invoked upon exposure to high concentrations of algal DMSP, when the algae reached stationary growth or when applied exogenously. To characterize the behavioral switch, we grew D7 in _E. huxleyi_-derived conditioned media (CM), which mimic the coexistence and pathogenicity phases of the interaction, and analyzed bacterial transcriptomes. Addition of DMSP to CM containing algae-derived metabolites induced major modulations in bacterial gene expression, which corroborated the observed behavioral switch in co-cultures. However, in defined media, lacking algal metabolites, DMSP barely affected gene expression. This demonstrates that DMSP has a signaling role that strongly affects bacterial behavior through gene expression, but an additional algal factor is required. Transporters for various compounds were upregulated in pathogenic mode, suggesting that _Sulfitobacter_ D7 benefits from _E. huxleyi_-derived metabolites. We discovered that algae-produced benzoate promoted the growth of _Sulfitobacter_ D7, and negated the behavioral switch to pathogenicity. We concluded that benzoate is important for bacteria and algae coexistence and suggested that as long as _Sulfitobacter_ D7 benefits from _E. huxleyi_, by receiving growth substrates, the switch to pathogenicity will not occur. This work demonstrates how metabolic cross-talk governs the nature of algae-bacteria interactions, which greatly impact large-scale oceanic processes.

Behavioral switch in bacteria in response to algal metabolits

Exploring the intricacies of host-pathogen communication is vital in order to understand why
some microbes persist within a host, while others are effectively cleared. As such,
host-pathogen communication is an urgent area of research underpinning human health
and disease, especially given the emerging concerns of antibiotic resistance.
Understanding host-pathogen pathways requires intricate knowledge in the fields of both
microbiology and immunology.

Living in a world of specialists, where scientists sometimes invest their whole career
studying one organism, a specific pathway or a protein, multidisciplinary discussions of
host-pathogen interactions are highly needed. This international conference will provide
such a forum for scientific exchange at the highest level, between scientists from different
research areas related to host-pathogen interactions, with the aim to trigger fruitful
collaborations addressing the urgent questions in the field.

This meeting has an additional special focus of bringing researchers from Ireland, Denmark
and Israel together to learn about cutting edge biology and form new collaborations.
Thus, the conference will be a showcase for the exciting and original research emanating
from these three small but innovative countries.

Host-Pathogen Communication (HPC) 2021

Helminths battle type 2 immunity for control of the intestinal stem cell niche