Potential application of selected metal resistant phosphate solubilizing bacteria isolated from the gut of earthworm (Metaphire posthuma) in plant growth promotion

The present study focuses on the isolation of three phosphate solubilizing bacteria (PSB), PSB1, PSB2 and PSB3 from the gut of earthworm Metaphire posthuma. The three stains were identified as Bacillus megaterium (MF 589715), Staphylococcus haemolyticus (MF 589716) and Bacillus licheniformis (MF 589720) through 16 S rRNA gene sequencing and biochemical characterization. The strains showed resistance to the metals Cu and Zn at significant concentrations and could solubilize phosphate even in the presence of metals. Maximum phosphate was solubilized by strain PSB3 with a production of 222 +/- 2.0 mgL(-1) soluble phosphate followed by PSB1 (213.7 +/- 1.3mgL(-1)) and PSB2 (193.5 +/- 1.5 mgL(-1)) at 96 h of incubation. The strains were able to produce indole acetic acid (IAA) in presence of L-tryptophan and possessed ammonium ion production potential in the order PSB3 > PSB1 > PSB2 (P < 0.05). The sterilized seeds of mung beans (Vigna radiata) displayed greater germination rate and higher growth under bacterium-enriched conditions. The effect on seed germination traits by the isolated strains followed the order of PSB3 > PSB1 > PSB2 (P < 0.05). Our results suggest that the three isolated PSB strains from earthworm gut possess intrinsic abilities of growth promotion, metal resistance and solubilization of phosphate which could be exploited for plant growth promotion and bioremediation even under metal-stress conditions.