MSc

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.