Microcapsules maintain the viability of Beauveria bassiana

Researchers have developed carboxymethylcellulose microcapsules capable of preserving propagules of the entomopathogenic fungus Beauveria bassiana during storage. The cross-linked system with aluminum ions maintained 85% germination after five months at minus eighteen degrees Celsius. Non-encapsulated conidia, kept under the same conditions, showed 69% viability.

The study compared aluminum and calcium ions as crosslinking agents for carboxymethylcellulose (doi 10.1021/acsomega.5c06970). The team sought a matrix suitable for transporting biological control agents. Aluminum provided more uniform particles, greater water absorption, and better structural stability. The results indicate potential for formulations intended for managing pests with life stages in the soil.

Polymer networks

Carboxymethylcellulose forms polymeric networks through ionotropic gelation. In this process, metal ions interact with carboxylate and hydroxyl groups present in the polymer. The method requires few steps, is quick to execute, and allows the production of particles capable of protecting active ingredients.

The particles cross-linked with aluminum retained a near-spherical shape after drying. The average diameter reached 1,92 millimeters, with a deviation of 0,11 millimeters in the study summary. The structures produced with calcium underwent collapse and coalescence. The process resulted in irregular conglomerates with heterogeneous sizes.

Greater positive charge

The researchers linked this difference to the greater positive charge of aluminum. The trivalent ion formed stronger and denser ionic bonds with carboxymethylcellulose. This interaction favored a compact, three-dimensional network resistant to deformation. Calcium, with its divalent charge, produced weaker bonds and a less stable matrix during water loss.

The absorption capacity also differentiated the materials. Particles with aluminum absorbed a volume of water equivalent to about 780% of their initial mass. Structures with calcium reached approximately 240%. According to the study, the network formed by the aluminum preserved interconnected pores during drying. This architecture facilitated water entry.

Thermal analyses

Thermal analyses showed distinct profiles. Particles containing aluminum exhibited main decomposition stages at 165,76 and 386,71 degrees Celsius. The material containing calcium recorded events at 211,78, 223,22, 309,29, and 368,95 degrees Celsius. The most complex pattern indicated heterogeneous cross-linking in the system with calcium.

After comparison, the team selected aluminum to encapsulate blastospores of the IBCB66 strain of Beauveria bassiana. The liquid culture achieved a concentration of two billion blastospores per milliliter. The researchers mixed the culture with a carboxymethylcellulose solution and added surfactant to improve dispersion between the fungus and the polymer.

The inclusion of the microorganism increased the average capsule size to 2,42 millimeters, with a deviation of 0,28 millimeters. The structures presented a thin outer layer and a porous core. The formulation underwent freezing, lyophilization, and storage at minus eighteen degrees Celsius.

After two months, all granules germinated in three to four days. The concentration reached 1,87 million conidia per particle. After five months, germination required seven days and reached 85%. The concentration remained at 1,24 million conidia per particle. The granules maintained a dry and loose appearance throughout the period.

Metabolic dormancy

The researchers interpreted the delayed germination as reversible metabolic dormancy, with no indication of loss of fungal integrity. The carboxymethylcellulose matrix would have reduced stress caused by dehydration and limited oxidative damage during storage.

The IBCB66 strain is part of the "Oldemar Cardim Abreu" collection of entomopathogenic microorganisms, maintained by the Biological Institute in Campinas, São Paulo. The study cites the pathogenicity of this strain against organisms with soil-dwelling phases, including Ceratitis capitata, Pratylenchus brachyurus, Spodoptera cosmioides, Chrysomya megacephala, and Atta sexdens rubropilosa.

The results demonstrated, for the first time in the study, a viable method for encapsulating Beauveria bassiana blastospores in aluminum-crosslinked carboxymethylcellulose particles. This formulation could support the development of dry products aimed at the biological control of soil pests.

The work was developed by scientists Mayté P. Zaldivar, Jean Carlos F. Machado, Lívia C. Massimino, Marcel S. Marques, José Eduardo M. de Almeida, Ana Paula S. Bartels, Ricardo Bortoletto-Santos, and Hernane da S. Barud.