Development of Optimized Liquid Formulations of Azospirillum, Phosphate-Solubilizing Bacteria, and Rhizobium Strains for Enhanced Viability and Shelf Life

The aim of this study was to evaluate the stability and shelf-life of Rhizobium, Azospirillum, and Pseudomonas striata a Phosphate Solubilizing Bacterium (PSB) in liquid formulations under ambient conditions, with a focus on the impact of various additives on the viability of these microbes over a 12-month period. Liquid formulations of Rhizobium strains NC-92, Sb-120, and Gr-2, Azospirillum, and PSB were prepared with different combinations of additives such as polyethylene glycol (PEG), gum arabic (GA), polyvinylpyrrolidone (PVP), and glycerol. The formulations were stored under ambient conditions, and colony-forming unit (CFU) counts were assessed at regular intervals over the year. The results revealed that formulations with higher concentrations of PVP, GA, and PEG maintained significantly higher CFU counts. Azospirillum formulations, particularly F20, exhibited superior viability, maintaining 2.2 × 10⁷ CFU after 12 months compared to rapid declines in other formulations such as F2, F8, and F16. Rhizobium NC-92 in basal medium + 1% PVP (F1) retained 5.2 × 10⁸ CFU after 12 months, while Sb-120 in basal medium + 1% PVP (T1) demonstrated high stability, maintaining 6.9 × 10⁸ CFU over six months. PSB formulations containing combinations of glycerol, PEG, and PVP also performed better, with the best formulation retaining 6.2 × 10⁸ CFU after 12 months. The study highlights the importance of certain additives such as PVP, GA, and PEG in improving the stability and shelf life of microbial compositions. Optimized formulations significantly increased viability, laying the groundwork for the development of robust biofertilizer solutions.