Chemists Give Living Bacteria New Powers With a Universal Cell-Surface Coating Technique

Bacteria already perform some of the most remarkable chemistry on Earth, from digesting pollutants to producing vitamins in the human gut. Now, a team of researchers at the Massachusetts Institute of Technology and collaborators at the University of Toronto and ETH Zurich has developed a way to give living microbial cells entirely new capabilities without touching their DNA. Writing in Molecular Systems Biology, the group describes a universal surface functionalization technique that attaches high densities of functional molecules to the outer surfaces of diverse bacteria while keeping the cells alive and growing. The method achieved roughly fifty times higher functionalization efficiency than previous approaches, and it worked across Gram-positive and Gram-negative species, aerobic and anaerobic alike.

The central idea is elegantly simple. Bacterial cell surfaces are studded with free primary amines, found at the N-termini of surface proteins and in lysine side chains, which are abundant across microbial taxa. The researchers exploited these native chemical handles using a hetero-bifunctional crosslinker called DBCO–sulfo–NHS. One end of this molecule, the sulfo-NHS ester, reacts with the surface amines under biocompatible aqueous conditions. The other end, a dibenzocyclooctyne group, remains available for a strain-promoted azide-alkyne cycloaddition, a bioorthogonal click reaction that specifically couples any molecule bearing an azide tag. This two-step design decouples the species-specific optimization of the surface reaction from the choice of the added functionality, meaning a single standardized protocol can be paired with fluorophores, enzymes, nucleic acids, or potentially any azide-containing compound.

The sulfo group on the NHS moiety proved critical. By enhancing the hydrophilicity of the crosslinker, it eliminated the need for organic solvents and reduced cellular internalization, both of which helped preserve cell viability. The team validated the approach on Escherichia coli, treating cells with a range of crosslinker concentrations from zero to 1000 milligrams per liter followed by an azide-modified Alexa Fluor 488 dye. Confocal microscopy showed bright, uniform surface fluorescence in fully treated cells, while controls lacking either the linker or the fluorophore showed no background signal, confirming that the reaction was specific rather than a product of nonspecific binding or autofluorescence.

Quantifying the trade-off between modification efficiency and cell survival was a key part of the study. Using flow cytometry alongside calibration curves built from fluorescent beads, the researchers converted fluorescence measurements into absolute numbers of molecules per cell. At the highest crosslinker concentration, roughly 210,000 molecules were tethered to each E. coli cell, corresponding to a surface density of about one molecule every 30 square nanometers, or a mean spacing of roughly five nanometers between adjacent molecules. Importantly, viability measured with the dead-cell stain SytoX Orange remained at 91 percent, and growth rates were indistinguishable from untreated controls. Only at extreme concentrations did cell death begin to rise, and even then the effect was modest.

Recognizing that membrane integrity alone does not capture the full picture of cellular health, the team also introduced a new standardized assay they call the regrowth dynamics assay. Rather than simply counting stained cells or colony-forming units, the method tracks the optical density of treated cultures over time, fits the curves to exponential or logistic growth models, and estimates the initial active population. Because cells that survive but lag in regrowth contribute less to the estimate, the assay is more stringent than conventional viability measures. Reassuringly, the regrowth-based estimates closely matched the dye-based ones, with 88 percent viability at the highest crosslinker dose, and neither method detected a significant departure from full viability at working concentrations.

To test universality, the researchers applied the protocol to five additional taxonomically diverse isolates: two aerobic marine Gram-negative strains, Vibrio splendidus and Neptunomonas sp.; the anaerobic gut bacterium Bacteroides ovatus; and two Gram-positive probiotic species from human stool, Lactococcus lactis and Lactobacillus rhamnosus. The anaerobic strains were functionalized under anaerobic conditions. Across all six organisms, the method tethered between 60,000 and 250,000 molecules per cell without significant viability losses or growth-rate changes. Gram-positive strains took up more molecules at a given crosslinker concentration but were also more sensitive to the treatment, a pattern the authors say reflects the distinct surface architectures of the two groups. Benchmarking against prior techniques made the advantages stark: metabolic labeling achieved at most a tenfold fluorescence increase compared with the 110-fold increase seen here, periodate-based oxidation dropped E. coli viability below 0.01 percent, and established biotin–sulfo–NHS protocols attached fewer than 4,000 molecules per cell versus 210,000 with the new crosslinker.

The practical power of the technique became clear in three demonstrations. First, cells coated with azide-modified beta-lactamase enzymes gained striking antibiotic resistance. In minimum inhibitory concentration assays, cells functionalized with 40 micromolar enzyme showed more than a 200-fold increase in ampicillin tolerance relative to untreated controls, with the effect scaling with enzyme dose. Even more striking, surface-bound enzymes outperformed the same enzymes free in solution, achieving equal or greater protection with tenfold fewer enzyme molecules. This spatial localization effect suggests that tethering catalytic machinery directly to the cell surface is a fundamentally efficient way to extend a microbe’s biochemical repertoire, with obvious implications for engineered probiotics that could survive antibiotic co-treatment.

Second, and perhaps most visually dramatic, the team used the method to program cell-to-cell adhesion with single-stranded DNA. Two populations of E. coli, each coated with a different 20-nucleotide azide-modified sequence, were mixed together. Complementary strands hybridized on contact, driving the cells to self-assemble into macroscopic aggregates visible within an hour. By fitting the sedimentation kinetics to a simplified aggregation model, the researchers calculated adhesion probabilities per cell encounter, which rose from a background of 0.2 percent to fifteen times that level at the highest DNA density. Non-complementary DNA controls showed no aggregation, confirming sequence specificity. Confocal imaging of red- and green-fluorescent populations revealed highly mixed aggregates, with 78 percent of nearest-neighbor pairs consisting of opposite-colored cells, well above the 50 percent expected from random assortment. Embedded in nutrient agar, the aggregates continued to grow, with cells dividing uniformly throughout the clusters.

The implications reach across biotechnology, medicine, and environmental engineering. Antibiotic-resistant probiotics could help maintain gut diversity during antibiotic treatment, a period when patients are vulnerable to infections such as Clostridioides difficile. Structured, DNA-programmed consortia could serve as seeding communities for microbiome modulation, biofilm engineering, and bioremediation, where spatially organized communities have been shown to remove pollutants more effectively. Because the method is purely chemical, it sidesteps the regulatory and ecological concerns surrounding genetically modified organisms, and it works on species that remain genetically intractable.

The approach does have limits. Some empirical optimization is still needed for each new species, since crosslinker sensitivity varied considerably between strains, and because nothing is genetically encoded, the modifications are temporary. Fluorescence measurements of dividing lineages showed that surface molecules are diluted by half with each cell division, so the longevity of any engineered phenotype depends on the cell’s growth rate and the density of molecules required for the function in question. Even so, the authors argue that the platform establishes a simple yet powerful framework for microbial surface engineering, one that decouples functional enhancement from genetic and metabolic constraints and opens the door to next-generation live therapeutics and synthetic microbial communities with capabilities their genomes never encoded.

Subject of Research: A universal chemical surface functionalization method for live microbial cells using hetero-bifunctional DBCO–sulfo–NHS crosslinkers

Article Title: A universal surface functionalization technique to chemically enhance live microbial cells

Article References: Vercelli, G. T., Zhou, X., Moreno-Gámez, S., Jeeda, R. R., Gregor, R., Słomka, J., Dagadu, A., Furst, A. L., & Cordero, O. X. (2026). A universal surface functionalization technique to chemically enhance live microbial cells. Molecular Systems Biology, 22(6), 962-978. https://doi.org/10.1038/s44320-026-00202-z

Image Credits: AI Generated

DOI: 10.1038/s44320-026-00202-z

Keywords: surface functionalization, bacteria, click chemistry, DBCO, SPAAC, beta-lactamase, antibiotic resistance, DNA hybridization, cell adhesion, probiotics, synthetic biology, non-genetic engineering