Ultrathin Protein Films Create Living Catalytic Interfaces on Microbial Cells.

Spatial organization underpins efficient multistep catalysis in nature, yet recreating such organization on living cell surfaces using mild, enzyme-compatible and catalytically functional materials remains challenging. Here we report an l-cysteine-triggered silk fibroin/lysozyme ultrathin film (SLF) that assembles directly on living microbial cells as a catalytic surface layer. SLF forms continuous nanoscale coatings on Saccharomyces cerevisiae, Gluconobacter oxydans and Escherichia coli Nissle 1917. Spectroscopic and structural analyses show that film formation proceeds through electrostatic complexation, restricted conformational rearrangement, increased hydrophobic exposure, β-sheet-rich ordering and network formation, rather than simple deposition. The coating maintains cell viability, enhances stress tolerance and provides size-selective permeability for small-molecule transport. SLF also incorporates exogenous enzymes efficiently during assembly. As a proof of concept, ω-transaminase-integrated SLF on G. oxydans couples host glycerol oxidation with surface transamination, enabling one-pot conversion of glycerol to serinol with a titer of 2.7 g L-1 within 12 h, up to 37-fold higher than a stepwise process. This work establishes ultrathin protein films as living catalytic interfaces for single-cell biocatalyst design and cascade biomanufacturing.