Bacterial exopolysaccharide promotes acid tolerance in Bacillus amyloliquefaciens and improves soil aggregation.

In this paper we report the isolation and taxonomic characterization of exopolysaccharide (EPS) producing bacteria followed by the role of EPS in conferring acid tolerance to the soil bacteria Bacillus amyloliquefaciens p16. The role of EPS in promoting soil aggregation is also presented. A total of 75 isolates were tested for acid tolerance and biofilm production under acid stress of which, 54 isolates were further tested for EPS production. Out of the 54 isolates, 28 isolates produced EPS in the range of (67.88 and 219.96 µg/ml) with B. amyloliquefaciens p16 showing the highest production. The 28 isolates characterized for phenotypic and molecular traits mostly belonged to the members of the genera Bacillus, Brevibacillus, Brevibacterium, Paenibacillus, Serretia, Pseudomonas, Arthrobacter and Lysinibacillus. The monosaccharide components of the EPS produced by B. amyloliquefaciens p16 shifted from galactose to arabinose under acid stress as revealed through HPLC analysis. Inactivation of the epsB gene encoding putative bacterial protein tyrosine kinase (BY-kinases) in B. amyloliquefaciens p16 resulted in significantly less EPS (33.23 µg/ml) production compared to wild-type (WT) (223.87 µg/ml). The mutant (B. amyloliquefaciens 6A5) was barely able to survive in pH 4.5 unlike that of the WT. Further, inoculation of the WT and mutant B. amyloliquefaciens 6A5 in the soil resulted in formation of small sized soil aggregates (42.41 mm) with less water holding capacity (27.67%) as compared to the soil treated with WT that produced larger soil aggregates of size 80.59 mm with higher 53.90% water holding capacity. This study indicates that EPS produced by acid-tolerant B. amyloliquefaciens p16 can not only impart acid tolerance to the bacteria but also aids in promoting soil aggregation when applied to the soil.

Diversity and functional properties of acid-tolerant bacteria isolated from tea plantation soil of Assam.

In this study, we report on the bacterial diversity and their functional properties prevalent in tea garden soils of Assam that have low pH (3.8-5.5). Culture-dependent studies and phospholipid fatty acid analysis revealed a high abundance of Gram-positive bacteria. Further, 70 acid-tolerant bacterial isolates characterized using a polyphasic taxonomy approach could be grouped to the genus Bacillus, Lysinibacillus, Staphylococcus, Brevundimonas, Alcaligenes, Enterobacter, Klebsiella, Escherichia, and Aeromonas. Among the 70 isolates, 47 most promising isolates were tested for their plant growth promoting activity based on the production of Indole Acetic Acid (IAA), siderophore, and HCN as well as solubilization of phosphate, zinc, and potassium. Out of the 47 isolates, 10 isolates tested positive for the entire aforesaid plant growth promoting tests and further tested for quantitative analyses for production of IAA, siderophore, and phosphate solubilization at the acidic and neutral condition. Results indicated that IAA and siderophore production, as well as phosphate solubilization efficiency of the isolates decreased significantly (P ≤ 0.05) in the acidic environment. This study revealed that low soil pH influences bacterial community structure and their functional properties.