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Abstract Maximizing protein secretion is an important target in the design of engineered living systems. In this paper, we characterize a trade-off between cell growth and per-cell protein secretion in the curli biofilm secretion system of Escherichia coli Nissle 1917. Initial characterization using 24-h continuous growth and protein production monitoring confirms decreased growth rates at high induction, leading to a local maximum in total protein production at intermediate induction. Propidium iodide (PI) staining at the endpoint indicates that cellular death is a dominant cause of growth reduction. Assaying variants with combinatorial constructs of inner and outer membrane secretion tags, we find that diminished growth at high production is specific to secretory variants associated with periplasmic stress mediated by outer membrane secretion and periplasmic accumulation of protein containing the outer membrane transport tag. RNA sequencing experiments indicate upregulation of known periplasmic stress response genes in the highly secreting variant, further implicating periplasmic stress in the growth–secretion trade-off. Overall, these results motivate additional strategies for optimizing total protein production and longevity of secretory engineered living systems Graphical Abstract 

Living systems have not only the exemplary capability to fabricate materials (e.g., wood, bone) under ambient conditions but they also consist of living cells that imbue them with properties like growth and self-regeneration. Like a seed that can grow into a sturdy living wood, can living cells alone serve as the primary building block to fabricate stiff materials? Here is reported the fabrication of stiff living materials (SLMs) produced entirely from microbial cells, without the incorporation of any structural biopolymers (e.g., cellulose, chitin, collagen) or biominerals (e.g., hydroxyapatite, calcium carbonate) that are known to impart stiffness to biological materials. Remarkably, SLMs are also lightweight, strong, and resistant to organic solvents and can self- regenerate. This living materials technology can serve as a powerful biomanu- facturing platform to design and develop advanced structural and cellular materials in a sustainable manner. 

Petrochemical-based plastics have not only contaminated all parts of the globe, but are also causing potentially irreversible damage to our ecosystem because of their non-biodegradability. As bioplastics are limited in number, there is an urgent need to design and develop more biodegradable alternatives to mitigate the plastic menace. In this regard, we report aquaplastic, a new class of microbial biofilm-based biodegradable bioplastic that is water-processable, robust, templatable and coatable. Here, Escherichia coli was genetically engineered to produce protein-based hydrogels, which are cast and dried under ambient conditions to produce aquaplastic, which can withstand strong acid/base and organic solvents. In addition, aquaplastic can be healed and welded to form three-dimensional architectures using water. The combination of straightforward microbial fabrication, water processability and biodegradability makes aquaplastic a unique material worthy of further exploration for packaging and coating applications.