Investigate the use of cellulose fibers as a carrier for the development of a Gram-negative bacterial inoculant

The development of efficient and long-term storage formulations is a major challenge in the production of microbial inoculants for sustainable agriculture. Today, bacterial immobilization on solid supports is a well-established technology and the use of sustainable carriers is attracting new areas of research. In this study, cellulose functional fibers (FF) derived from the pulp industry were tested as a microbial carrier for the Gram-negative bacterial consortium formed by Azospirillum brasilense, Burkholderia ambifaria, Gluconacetobacter diazotrophicus and Herbaspirillum seropedicae. Using a co-culture bioreaction, the consortium was immobilized on FF mixing it with the fibers (1% w/v) to allow the microbial self-adhesion on the surface. The FF were inoculated at three different times during the bioreaction and in different cultural media (T4 and SPAA). Freeze-drying and heat-drying were used as drying methods. The cell viability was detected in wet FF, and dried fibers (1, 15 and 30 days after drying) and the results were expressed in log CFU g-1. Immobilized bacteria were also determined for plant growth-promoting (PGP) traits, such as phytohormons, ammonia production, and phosphate solubilization. The results showed bacterial colonization and exopolysaccharide sheets on FF surfaces, as revealed by scanning electron microscopy visualization. Wet FF showed a good microbial charge (7.5-8.1 Log CFU g-1) and the cell viability after drying was 2 Log CFU g-1 lower compared to the wet FF. Significant results were obtained comparing the two different growth conditions and FF inoculation timing (p<0.05). Differences between the control (unmodified cellulose fibers) and FF were observed after the drying step, underling the potential role of chemical modification to enhance cell protection. PGP tests showed good results compared to not immobilized consortium. These preliminary findings provide the basis for future studies, focusing on shelf-life tests and optimal application through in planta and greenhouse experiments.