Impact of direct physical association and motility on fitness of a synthetic interkingdom microbial community

Enhanced metabolic entanglement emerges during the evolution of an interkingdom microbial community

While different stages of mutualism can be observed in natural communities, the dynamics and mechanisms underlying the gradual erosion of independence of the initially autonomous organisms are not yet fully understood. In this study, by conducting the laboratory evolution on an engineered microbial community, we reproduce and molecularly track the stepwise progression towards enhanced partner entanglement. We observe that the evolution of the community both strengthens the existing metabolic interactions and leads to the emergence of de novo interdependence between partners for nitrogen metabolism, which is a common feature of natural symbiotic interactions. Selection for enhanced metabolic entanglement during the community evolution repeatedly occurred indirectly, via pleiotropies and trade-offs within cellular regulatory networks, and with no evidence of group selection. The indirect positive selection of metabolic dependencies between microbial community members, which results from the direct selection of other coupled traits in the same regulatory network, may therefore be a common but underappreciated driving force guiding the evolution of natural mutualistic communities.

Tuning interdomain conjugation to enable in situ population modification in yeasts

The ability to modify and control natural and engineered microbiomes is essential for biotechnology and biomedicine. Fungi are critical members of most microbiomes, yet technology for modifying the fungal members of a microbiome has lagged far behind that for bacteria. Interdomain conjugation (IDC) is a promising approach, as DNA transfer from bacterial cells to yeast enables in situ modification. While such genetic transfers have been known to naturally occur in a wide range of eukaryotes and are thought to contribute to their evolution, IDC has been understudied as a technique to control fungal or fungal-bacterial consortia. One major obstacle to the widespread use of IDC is its limited efficiency. In this work, we manipulated metabolic and physical interactions between genetically tractable Escherichia coli and Saccharomyces cerevisiae to control the incidence of IDC. We test the landscape of population interactions between the bacterial donors and yeast recipients to find that bacterial commensalism leads to maximized IDC, both in culture and in mixed colonies. We demonstrate the capacity of cell-to-cell binding via mannoproteins to assist both IDC incidence and bacterial commensalism in culture and model how these tunable controls can predictably yield a range of IDC outcomes. Furthermore, we demonstrate that these controls can be utilized to irreversibly alter a recipient yeast population, by both "rescuing" a poor-growing recipient population and collapsing a stable population via a novel IDC-mediated CRISPR/Cas9 system.IMPORTANCEFungi are important but often unaddressed members of most natural and synthetic microbial communities. This work highlights opportunities for modifying yeast microbiome populations through bacterial conjugation. While conjugation has been recognized for its capacity to deliver engineerable DNA to a range of cells, its dependence on cell contact has limited its efficiency. Here, we find "knobs" to control DNA transfer, by engineering the metabolic dependence between bacterial donors and yeast recipients and by changing their ability to physically adhere to each other. Importantly, we functionally validate these "knobs" by irreversibly altering yeast populations. We use these controls to "rescue" a failing yeast population, demonstrate the capacity of conjugated CRISPR/Cas9 to depress or collapse populations, and show that conjugation can be easily interrupted by disrupting cell-to-cell binding. These results offer building blocks toward in situ mycobiome editing, with significant implications for clinical treatments of fungal pathogens and other fungal system engineering.

Creating new-to-nature carbon fixation: A guide

Synthetic biology aims at designing new biological functions from first principles. These new designs allow to expand the natural solution space and overcome the limitations of naturally evolved systems. One example is synthetic CO2-fixation pathways that promise to provide more efficient ways for the capture and conversion of CO2 than natural pathways, such as the Calvin Benson Bassham (CBB) cycle of photosynthesis. In this review, we provide a practical guideline for the design and realization of such new-to-nature CO2-fixation pathways. We introduce the concept of "synthetic CO2-fixation", and give a general overview over the enzymology and topology of synthetic pathways, before we derive general principles for their design from their eight naturally evolved analogs. We provide a comprehensive summary of synthetic carbon-assimilation pathways and derive a step-by-step, practical guide from the theoretical design to their practical implementation, before ending with an outlook on new developments in the field.

Metabolite Cross-Feeding Promoting NADH Production and Electron Transfer during Efficient SMX Biodegradation by a Denitrifier and S. oneidensis MR-1 in the Presence of Nitrate

Antibiotics often coexist with other pollutants (e.g., nitrate) in an aquatic environment, and their simultaneous biological removal has attracted widespread interest. We have found that sulfamethoxazole (SMX) and nitrate can be efficiently removed by the coculture of a model denitrifier (Paracoccus denitrificans, Pd) and Shewanella oneidensis MR-1 (So), and SMX degradation is affected by NADH production and electron transfer. In this paper, the mechanism of a coculture promoting NADH production and electron transfer was investigated by proteomic analysis and intermediate experiments. The results showed that glutamine and lactate produced by Pd were captured by So to synthesize thiamine and heme, and the released thiamine was taken up by Pd as a cofactor of pyruvate and ketoglutarate dehydrogenase, which were related to NADH generation. Additionally, Pd acquired heme, which facilitated electron transfer as heme, was the important composition of complex III and cytochrome c and the iron source of iron sulfur clusters, the key component of complex I in the electron transfer chain. Further investigation revealed that lactate and glutamine generated by Pd prompted So chemotactic moving toward Pd, which helped the two bacteria effectively obtain their required substances. Obviously, metabolite cross-feeding promoted NADH production and electron transfer, resulting in efficient SMX biodegradation by Pd and So in the presence of nitrate. Its feasibility was finally verified by the coculture of an activated sludge denitrifier and So.

Cross-Feeding between Filamentous Cyanobacteria and Symbiotic Bacteria Favors Rapid Photogranulation

Photogranules are dense algal-bacterial aggregates used in aeration-free and carbon-negative wastewater treatment, wherein filamentous cyanobacteria (FC) are essential components. However, little is known about the functional role of symbiotic bacteria in photogranulation. Herein, we combined cyanobacterial isolation, reactor operation, and multiomics analysis to investigate the cyanobacterial-bacterial interaction during photogranulation. The addition of FC to the inoculated sludge achieved a 1.4-fold higher granule size than the control, and the aggregation capacity of FC-dominant photogranules was closely related to the extracellular polysaccharide (PS) concentration (R = 0.86). Importantly, we found that cross-feeding between FC and symbiotic bacteria for macromolecular PS synthesis is at the heart of photogranulation and substantially enhanced the granular stability. Chloroflexi-affiliated bacteria intertwined with FC throughout the photogranules and promoted PS biosynthesis using the partial nucleotide sugars produced by FC. Proteobacteria-affiliated bacteria were spatially close to FC, and highly expressed genes for vitamin B1 and B12 synthesis, contributing the necessary cofactors to promote FC proliferation. In addition, Bacteroidetes-affiliated bacteria degraded FC-derived carbohydrates and influenced granules development. Our metabolic characterization identified the functional role of symbiotic bacteria of FC during photogranulation and shed light on the critical cyanobacterial-bacterial interactions in photogranules from the viewpoint of cross-feeding.

Tuning Interdomain Conjugation Toward in situ Population Modification in Yeast

The ability to modify and control natural and engineered microbiomes is essential for biotechnology and biomedicine. Fungi are critical members of most microbiomes, yet technology for modifying the fungal members of a microbiome has lagged far behind that for bacteria. Interdomain conjugation (IDC) is a promising approach, as DNA transfer from bacterial cells to yeast enables in situ modification. While such genetic transfers have been known to naturally occur in a wide range of eukaryotes, and are thought to contribute to their evolution, IDC has been understudied as a technique to control fungal or fungal-bacterial consortia. One major obstacle to widespread use of IDC is its limited efficiency. In this work, we utilize interactions between genetically tractable Escherichia coli and Saccharomyces cerevisiae to control the incidence of IDC. We test the landscape of population interactions between the bacterial donors and yeast recipients to find that bacterial commensalism leads to maximized IDC, both in culture and in mixed colonies. We demonstrate the capacity of cell-to-cell binding via mannoproteins to assist both IDC incidence and bacterial commensalism in culture, and model how these tunable controls can predictably yield a range of IDC outcomes. Further, we demonstrate that these lessons can be utilized to lastingly alter a recipient yeast population, by both "rescuing" a poor-growing recipient population and collapsing a stable population via a novel IDC-mediated CRISPR/Cas9 system.

Quorum sensing responses of activated sludge to free nitrous acid: Zoogloea deformation, AHL redistribution, and microbiota acclimatization

Free nitrous acid (FNA) has been widely employed for improvement of wastewater management by altering sludge characteristic and function based on its polymer lysing and biocidal capacity. Sludge characteristic and function are commonly considered as the joint consequence of microbial individual behaviors and quorum sensing (QS) involved collective behaviours, but the role of the latter in FNA treatment was still as-yet-unidentified and addressed in this research. The results of sludge morphology and component characterized FNA-induced zoogloea deformation, including inner cell exposure, half of extracellular polymeric substances (EPS) reduction and adsorption site depletion. During zoogloea deformation, four acyl-homoserine lactones (AHLs), including C4-HSL, C8-HSL, C10-HSL and C12-HSL, transferred inward of microbiota, and their total contents reduced by 66% because of depressed signal production, augmented decomposer and recognition. Transcriptome analysis revealed that differentially expressed QS driven by AHL redistribution facilitated microbiota acclimatization including cellular motility and hydrolase synthesis for EPS consumption. Boosted motility may favor escaping from stress spot and moderating intercellular acidity based on cell motility test. Feasible EPS consumption provided nutrition for heterotrophic metabolisms testified by pure culture with EPS as sole nutrition. Our work thus comprehensively revealed QS behaviours responding to FNA and deepened the understanding to FNA treatment performance in wastewater management.