Abstract

A sequential optimization approach based on statistical experimental designs was employed to optimize growth medium and fermentation conditions, in order to improve the antibiotic activity of Xenorhabdus nematophila TB. Tryptone soyptone broth (TSB) was chosen as the original medium for optimization. Glucose and peptone were identified as the optimum carbon and nitrogen sources using single factor method. Peptone, fermentation time, initial pH and inoculum volume were identified as the critical factors which highly influenced the antibiotic activity of X. nematophila TB using Plackett–Burman (PB) design. The critical factors were subsequently optimized to locate their optimum domain using steepest ascent method and were further optimized using central composite design (CCD), involving response surface methodology (RSM). The optimum growth medium and fermentation conditions for antibiotic production by X. nematophila TB consisted of glucose 5.00 g/l, peptone 25.6 g/l, NaCl 5.00 g/l, K₂HPO₄ 2.50 g/l, initial pH 7.59, medium volume (100/250 ml), inoculation age (OD_{600 nm}: 2.00), inoculum volume 9.95%, rotary speed 150 rpm, temperature 25°C and fermentation time 54.1 h. An overall of 73.52% increase in antibiotic activity (418.7 U/ml) was obtained as compared with the unoptimized conditions (241.3 U/ml). This study provides a powerful tool to optimize the bioprocess of the antibiotic production by X. nematophila TB.

Key words: Xenorhabdus nematophila, antibiotic activity, growth medium, fermentation conditions, optimization, response surface methodology.

Abbreviation

Abbreviations: IJ, Infective juvenile; TSB, tryptone soyptone broth; CCD, central composite design; RSM, response surface methodology.