The positive effect of certain physical and chemical factors viz., inoculum level and growth phase, cultural condition, temperature, pH, presence of divalent cation and chelating agent, on the enhancement of coaggregation among the PGPR isolates viz., Pseudomonas fluorescens (PFR-3) and Paenibacillus polymyxa (PBR-3) was studied under in vitro condition. It was observed that 107: 107 inoculum level of PGPR partners was found to be optimum for achieving the maximum coaggregation percentage while any increase or decrease to this inoculum level could reduce the coaggregation percentage of PGPR partners. The use of PGPR partners at stationary growth phase augmented the “ Intergeneric coaggregation” to a higher level when compared to lag and log growth phase. The cells of PGPR partners, harvested from N-deficient media, yielded more coaggregation when compared to the cells harvested from N- rich media. Further, the PGPR cells cultured at 35oc at a pH level of 7.5 recorded higher coaggregation percentage than other levels. Moreover, the addition of Ca2+, as divalent cation, recorded higher coaggregation percentage of PGPR cells whereas the addition of EDTA, as chelating agent, derastically reduced the stability of PGPR coaggregates. It was concluded that the use of PGPR cells cultured in N-deficient media, at a growth temperature of 35oc and at a pH level of 7.5 during the stationary growth phase yielded more coaggregation percentage among PGPR partners. Moreover, the use of 107:107 inoculum level of PGPR partners and addition of Ca2+, as divalent cation, was found to maximise the coaggregation percentage among PGPR partners.
