Equipping cells with artificial shells or coats has been explored extensively throughout the last decade, with goals such as immuno-masking, in-vivo tracing, and imparting tolerances toward various biotic and abiotic stressors. One stressor, however, drying, has curiously been overlooked. In an industrial setting, drying of cells becomes highly relevant when a satisfactory product shelf life must be achieved at low costs. The drying of thin-walled blastospores of the entomopathogenic fungus Metarhizium brunneum for biocontrol in agricultural settings is a prime example of this.

Therefore, we aim to provide M. brunneum blastospores with an artificial biopolymer shell to protect these sensitive cells from the harsh conditions during a technical drying process. Since the de- and, in particular, the rehydration of cells is considered a time-critical process, the slowing down by an artificial shell might have a positive effect on cell viability.

We formed artificial shells by layer-by-layer coating via electrostatic interaction on the blastospore surface. The cationic polymer chitosan was coated on the negatively charged cell surface creating a dense polymer layer. This in turn, reverses the original surface charge so that another dense polymer layer with negatively charged alginate could be applied to the initial layer. By repetition of these steps, surface coats of varying thickness were created.

We were able to show that slowing down the rehydration process of blastospores increases the survival rate from 7.2 % to 48.3 % significantly. Proof of successful coating was provided by the application of fluorescence-labeled polymers and scanning electron microscopy. As the number of layers increased, the drying tolerance improved from 6.88 % to a maximum of 27.54 %. Additionally, by varying of the polymer chain length, drying tolerance was further increased up to 33.24 %.

By applying polymer layers to the surface of cells, the survival of the cells after drying could be significantly improved.