Co-aggregation in Azospirillum brasilensense MTCC-125 with other PGPR strains: Effect of physical and chemical factors and stress endurance ability

This work aims at exploring the co-aggregation of Azospirillum brasilense MTCC-125 with other plant growth promoting rhizobacterial (PGPR) strains. The PGPR strains include Azotobacter chroococcum MTCC-446, Azorhizobium caulinodans ORS-571, Bacillus megatherium MTCC-3353 and Pseudomonas fluorescens MTCC-4828. The influence of different physical and chemical factors involved in this phenomenon was investigated. Co-aggregation was found to be dependent on various factors: pH, temperature, inoculation level and growth phase. Treatment with urea, protease and chelating agents such as EDTA (ethylene diamine, tetra acetic acid) and EGTA (ethyl glycol-bis(β-amino ethyl ether) N-N′-tetra acetic acid) was found to inhibit co-aggregation. Sonication was found to increase co-aggregation, while heat treatment has a detrimental effect. The ability of Azospirillum co-aggregates to endure desiccation, heat, osmotic tolerance and osmotic shock and for its survivability in different carrier materials were also evaluated. Among the different combinations, the co-inoculation of Azospirillum MTCC-125 and Azotobacter MTCC-446 exhibited a higher degree of tolerance as compared to the other combinations tested.