Optimization of culture medium and growth conditions of the plant growth-promoting bacterium Herbaspirillum seropedicae BR11417 for its use as an agricultural inoculant using response surface methodology (RSM)

Abstract

Culture media compositions and bioprocess conditions were studied to improve the production of cell biomass and indolic phytohormones by Herbaspirillum seropedicae BR11471, a plant growth promoting bacterium, and different inoculant formulations were also produced and tested for their stability and shelf life. Response surface methodology (RSM) based on central composite rotation designs (CCRD) was used to find bioprocess variables that lead to an increase in bacterial biomass and yield of indolic compounds. The major components of DYGS medium were optimized in small-scale shaken cultivations, in two sets of CCRD. High performance liquid chromatography was used to determine nutrient consumption and to correlate it with cell biomass production, and the Salkowski method was used to quantify indoles. Hydrolytic activity in the formulations was quantified with the fluorescein diacetate assay. Glycerol (5.5 g L−1) and yeast extract (2.8 g L−1), as the main carbon and nitrogen sources, respectively, increased biomass production by 87.5% when compared to original DYGS medium, reaching 3.0 g L−1 of dry cell weight (DCW). In a 2.0 L bioreactor, the optimized medium was used to enhance process conditions for DCW and indole-3-acetic acid (IAA). Biomass production reached 3.4 g L−1 and was restrained at highest air flow levels. The conditions of 34-36 °C, 150 rpm and 4.0 L min−1 of air flow rate resulted in 11.97 mg L−1 of IAA, an increase of 370% over original DYGS at 30 °C. Peat can still be regarded as a good cell carrier for solid state inoculants, whilst the additives tested for liquid formulations are individually more efficient than the mixture. The production of inoculants containing H. seropedicae strain BR11471 can be efficiently improved with the use of the RSM approach i.e. it maximizes the production of biomass and indolic compounds, and reduces culture media components, both key factors for large-scale industrial production.