Polyethylene glycol exerted toxicity to growth of Bacillus subtilis NRS-762

Polyethylene glycol is commonly used in fermentations as an anti-foam for preventing the rise of foam to the top plate of the bioreactor, which increases contamination risk. However, its potential toxicity to growth of various microorganisms is not well understood at the strain and species level. Hence, the objective of this study was to understand the impact of different concentrations of polyethylene glycol (0, 1, 5 and 10 g/L) on the aerobic growth of Bacillus subtilis NRS-762 (ATCC 8473) in LB Lennox medium in shake flasks at 30 ^oC and 230 rpm rotational shaking. Experiment results indicated that polyethylene glycol (PEG) (molecular weight ~8000 Da), at all concentrations tested, exerted some toxicity towards the growth of B. subtilis NRS-762 in LB Lennox medium. Specifically, maximal optical density obtained declined with greater exposure to PEG in a concentration-dependent manner, up to a threshold concentration of 5 g/L PEG. For example, maximal optical density obtained in B. subtilis NRS-762 without addition of PEG was 4.4, but the value obtained on exposure to 1 g/L of the anti-foam decreased to 4.1 and a further 3.8 on exposure of cells to 5 g/L and 10 g/L PEG. Similarly, growth rates of B. subtilis NRS-762 also decreased in a concentration-dependent manner with PEG concentration up to a threshold concentration of 5 g/L PEG. pH variation in culture broth, however, revealed that the pH profiles for exposure to PEG at all concentrations overlapped each other and was similar to the one of cells without exposure to the anti-foam; thereby, highlighting that metabolic processes in B. subtilis NRS-762 were not significantly affected by exposure to PEG. Collectively, polyethylene glycol anti-foam exerted toxicity effect on B. subtilis NRS-762 biomass formation, and possibly metabolism. The latter may not be sufficiently significant to affect the types of metabolites secreted by the bacterium, and thus, could not be detected by measurement of culture broth pH. Overall, the results should inform the choice and concentration of PEG for culturing B. subtilis in biotechnological applications.