Designing and Optimizing Culture Media for Growth and Compatibility of Nitrogen fixing, Phosphate solubilizing and Potash mobilizing Bacterial Consortium

Abstract

As increasing global demand for food drives heavy reliance on synthetic fertilisers, the agricultural sector faces severe deleterious effects on the soil ecosystem. This has led to a critical shift towards biofertilisers as a sustainable and environmentally friendly alternative. Rising expenditure on synthetic fertilisers has increased demand for biofertilisers. Various bioinoculants comprising single or multiple microbial strains can be used in formulations for their plant growth-promoting activity. An emerging trend is the combination of effective microbial strains in a single formulation as a consortium. This study aimed to design and optimise a single culture medium supporting the growth and mutual compatibility of nitrogen-fixing, phosphate-solubilising and potash-mobilising bacteria. In the present investigation, nitrogen-fixing (N), phosphate-solubilising (PSB) and potash-mobilising bacteria (KMB) were isolated from brinjal rhizosphere soils in the Pune, Nashik and Ahilyanagar districts of Maharashtra, India. Morphological and biochemical characterisation, followed by 16S rRNA gene sequencing, identified the most efficient strains as Azotobacter chroococcum (AZO16), Bacillus subtilis (PSB4) and Bacillus pumilus (KMB5). Five modified NPK-consortium media (MS I–MS V) were designed by varying their composition while maintaining a near-neutral pH. All strains showed distinct morpho-cultural and biochemical profiles and efficient nutrient solubilisation. Among the formulations, MS II, containing dextrose (15 g L⁻¹), yeast extract (1.5 g L⁻¹), ammonium sulphate (0.2 g L⁻¹), CaCO₃ (2 g L⁻¹) and insoluble P and K sources at a maintained pH of 7.1, supported growth and effectively sustained high viable counts, recording 7.3 × 10⁶ CFU mL⁻¹ of A. chroococcum, 7.0 × 10⁷ CFU mL⁻¹ of B. subtilis and 6.2 × 10⁷ CFU mL⁻¹ of B. pumilus. Cross-streak assays revealed mutual compatibility without inhibition zones on nutrient agar and MS II medium. The optimised MS II medium provides a robust platform for scalable production of NPK biofertiliser consortia. These findings offer a rational medium-design framework that can be extended to other multi-strain PGPR formulations, facilitating more reliable and cost-effective biofertiliser manufacturing for sustainable nutrient management in intensive cropping systems.