BACKGROUND: Under appropriate conditions, Saccharomyces cerevisiae and Penicillium chrysogenum were found to co-immobilize spontaneously with no need for external support or chemical binder. The main aims were to examine the interaction between yeast cells and fungal hyphae by electron microscopy and the death of the filamentous fungus because of direct contact between both microorganisms. RESULTS: Immobilization was accomplished by orbitally shaking at 28 degrees C a culture medium consisting of yeast nitrogen base buffered at pH 7 and containing gluconic acid as an available carbon source for the filamentous fungus not readily used by the yeast. The yeast biocapsules thus obtained were hollow, smooth, elastic, strong, creamy-coloured spheres of variable size depending on the particular shaking rate and time of residence in the formation medium. Biocapsule walls consisted of yeast cells bound to fungal hyphae. Placing the biocapsules in fermentation medium caused yeast cells to colonize and invade all hyphae, thereby causing the fungus to die and remain as a mere support for yeast. CONCLUSIONS: The death of the fungus was probably effected by the yeast via a cell-hypha contact-mediated mechanism as shown by dialysis experiments. The yeast biocapsules can be reused with no loss of integrity or activity. The proposed immobilization method provides a simple, convenient, inexpensive alternative which affords yeast reuse. (C) 2011 Society of Chemical Industry