RNA-seq analysis of B. megaterium exposed to pH 7.0 and pH 4.5 showed differential expression of 207 genes related to several processes. Among the 207 genes, 11 genes displayed increased transcription exclusively in pH 4.5. Exposure to pH 4.5 induced the expression of genes related to maintenance of cell integrity, pH homeostasis, alternative energy generation and modification of metabolic processes. Metabolic processes like pentose phosphate pathway, fatty acid biosynthesis, cysteine and methionine metabolism and synthesis of arginine and proline were remodeled during acid stress. Genes associated with oxidative stress and osmotic stress were up-regulated at pH 4.5 indicating a link between acid stress and other stresses. Acid stress also induced expression of genes that encoded general stress-responsive proteins as well as several hypothetical proteins. Our study indicates that a network of genes aid B. megaterium G18 to adapt and survive in acid stress condition.

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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.

Mussaenda roxburghii are very important ethnomedicinal plant, used for its various applications from the ancient period. The role of their associated plant beneficial endophytic bacteria was evaluated, which were previously untapped. Among the isolates, PAK6 was identified as efficient phosphate solubilizer, quantified by the molybdenum blue method. Four isolates PAK1, PAK2, PAK3, and PAK8 were able to synthesize significant level of IAA in the presence and absence of tryptophan. Isolates PAK1 and PAK9 were able to produce siderophore on CAS agar media, PAK2 and PAK9 were able to produce HCN, and PAK7 and PAK8 were able to grow on N2-free medium. All the isolates were able to produce a moderate level of polysaccharide and tolerate up to 10% of NaCl. Isolates PAK3, PAK6, PAK7, and PAK8 were able to grow well at pH 5.0 and isolates PAK2, PAK7, and PAK8 were able to tolerate 600 μg mL−1 of Al+3, while all the isolates except PAK1 showed a tolerance to 600 μg mL−1 of Mn+2 tested. Endophytic bacterial isolates PAK6 and PAK9 were effective against Sclerotinia sclerotiorum and Sclerotium rolfsii.

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