Exopolysaccharide producing bacteria from soil based nesting structures of insects

Abstract

The study entitled ―Exoploysaccharide producing bacteria from soil based nesting structures of insects‖ was conducted during the year 2019-2021 in the Department of Agricultural Microbiology, College of Agriculture, Vellayani, Thiruvananthapuram with the objective to isolate and characterize exopolysaccharide producing bacteria from mud wasp and termitaria and the ability of the best two isolates in soil aggregate stability study. Thirty three bacterial isolates were obtained from different termitaria and mud dauber wasp nest. Among them colonies of fifteen isolates had slimy mucoidal appearance and were identified as exopolysaccharide producers. The exopolysaccharide production of the isolates ranged from 25.78 µg mL-1 (Klebsiella pneumoniae KWP23) to 2.58 µg mL-1 . The total carbohydrate content in exopolysaccharide was maximum in bacterial isolate Paenibacillus polymyxa KTM17 (14.48 µg mL-1 ). The best five isolates were selected for molecular characterization by 16s rRNA sequencing and identified as Priestia aryabhattai TWP12, Priestia aryabhattai TWP13, Klebsiella pneumoniae KWP23, Priestia megaterium KTM4, Paenibacillus polymyxa KTM17. They were screened for phosphate, potassium and silicate solubilization. Klebsiella pneumoniae KWP23 showed solubilization zone for phosphate (4 mm) and potassium (1 mm). None of the isolates chosen were silicate solubilizers. Two bacterial isolates with maximum EPS production viz. Paenibacillus polymyxa KTM17 and Klebsiella pneumoniae KWP23 were selected for the further studies. Effect of carbon source and carbon: nitrogen ratio on exopolysaccharide production was studied. Glucose was found to be the best carbon source for EPS production in Klebsiella pneumoniae KWP23 at C: N ratio of 10:1. In Paenibacillus polymyxa KTM17, highest EPS production was recorded when sucrose was used as carbon source at a C: N ratio of 50:1. The effect of temperature on EPS production of the isolates was evaluated. Increase in incubation temperature resulted in a corresponding increase in EPS production in case of Klebsiella pneumoniae KWP23. The highest EPS 3 production by Klebsiella pneumoniae KWP23 was (23.00 µg mL-1 ) at 50oC and Paenibacillus polymyxa KTM17 at 30oC (31.50 µg mL-1 ). The effect of salinity on EPS production by Klebsiella pneumoniae KWP23 and Paenibacillus polymyxa KTM17 isolates was also assessed. For Klebsiella pneumoniae KWP23, the EPS production was maximum without NaCl (18.0 µg mL-1 ) and for Paenibacillus polymyxa KTM17, the EPS production was maximum at 6% (1 M) NaCl (26.25 µg mL-1 ). Klebsiella pneumoniae KWP23 and Paenibacillus polymyxa KTM17 were further subjected to dual culture plate assay to study the biocontrol potential of the isolates. Paenibacillus polymyxa KTM17 showed a clear inhibition zone on dual culture assay with Rhizoctonia solani, but Klebsiella pneumoniae KWP23 did not inhibit the growth of the fungus. Both Klebsiella pneumoniae KWP23 and Paenibacillus polymyxa KTM17 inhibited the growth of Helminthosporium sp. The potential of the isolates to form soil aggregates and its aggregate stability was studied. The mean weight diameter of dry aggregates obtained was highest for the soil amended with Paenibacillus polymyxa KTM17 (1.068 mm). The study on wet aggregate stability on water immersed revealed that the best soil aggregate stability was given by the isolate Paenibacillus polymyxa KTM17. The stability of the aggregate lasted for more than 24 hours when immersed in water whereas that in the aggregate kept as control disintegrated immediately on pouring water. The effect of agitation and water immersion on the aggregate stability was also analyzed. On oscillation of the aggregates in water the aggregates in control soil showed maximum turbidity (0.254 nm). The dry weight of the disintegrated soil was minimum in the soil aggregate amended with Paenibacillus polymyxa KTM17 (0.014 g). Based on the results of the present study, it can be concluded that among the selected isolates Klebsiella pneumoniae KWP23 had maximum production of EPS. Both Klebsiella pneumoniae KWP23 and Paenibacillus polymyxa KTM17 has biocontrol potential against plant pathogenic fungi. Paenibacillus polymyxa KTM17 is a potential isolate which can effect soil aggregate stability in dry and wet condition.