Optimization of levan biosynthesis from Bacillus siamensis using batch and continuous stirred-tank bioreactors: A response surface methodology approach

Abstract

Levan, a promising fructan polysaccharide for biopharmaceuticals, has limited large-scale production studies. This research optimized and scaled up levan biosynthesis from Bacillus siamensis in continuous stirred-tank bioreactors based on response surface methodology (RSM). Batch cultures optimized for 30 % (w/v) sucrose, pH 5.0, and 48 h incubation yielded a maximum 15.74 % (w/v) levan. The optimal batch conditions were evaluated in a continuous stirred-tank bioreactor, where dilution rates and mixing speeds were examined. At a dilution rate of 0.021 h⁻¹ and an agitation speed of 200 rpm, the maximum productivity was 17.96 % (w/v), and steady-state conditions were attained after three days of continuous fermentation. X-ray diffraction confirmed the amorphous nature of the levan, ideal for biomaterial applications. These results underline the potential of B. siamensis for high-yield levan production and provide a systematic approach for bioprocess parameter optimization, serving as a strong basis for its increased application in industrialized polysaccharide-based bioprocessing.

Keywords: Bacillus siamensis; Bioreactor; Continuous stirred tank bioreactor; Levan; Response surface methodology.

Graphical abstract

Fig. 1

Schematic of bioreactor and cultivation system.

Fig. 2

The growth of B. siamensis in a fermentation medium containing various sucrose concentrations with agitation speed of 200 rpm at pH 6.0 and 37 °C for 48 h.

Fig. 3

Kinetics of sucrose consumption and levan production by B. siamensis with fermentation medium containing 10, 20, and 30 % (w/v) of sucrose with agitation speed of 200 rpm at pH 6.0 and 37 °C for 48 h.

Fig. 4

Response surface plot (a) and contour plot (b) for levan concentration against variation of incubation time and initial pH with constant initial sucrose concentration (30 % (w/v)).

Fig. 5

Response surface plot (a) and contour plot (b) for levan concentration against variation of sucrose concentration and incubation time with constant initial pH (5.0).

Fig. 6

Response surface plot (a) and contour plot (b) for levan concentration against variation of sucrose concentration and initial pH with constant incubation time (48 h).

Fig. 7

XRD diffractogram of levan produced from B. siamensis using batch bioreactor.

Fig. 8

Levan production by B. siamensis in the continuous stirred tank bioreactor under various dilution rates including a) 0.021 h^-1, b) 0.028 h^-1, and c) 0.042 h^-1 at 37 °C, 200 rpm for 7 days.

Fig. 9

Levan production by B. siamensis in the continuous stirred tank bioreactor under various agitation speeds including a) 100 rpm, b) 200 rpm, and c) 300 rpm with dilution rate of 0.021 h^-1 at 37 °C for 7 days.