Effect of polyethylene glycol on growth of Escherichia coli DH5α and Bacillus subtilis NRS-762

Polyethylene glycol is commonly used in fermentation 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 species and strain level. Hence, the objective of this study was to understand the impact of different concentrations of polyethylene glycol at the 1, 5 and 10 g/L level on the aerobic growth of Escherichia coli DH5 and Bacillus subtilis NRS-762 in LB Lennox medium in shake flasks. Experiment results revealed that polyethylene glycol (PEG) (molecular weight [~]8000 Da), at all concentrations tested, did not affect biomass formation and metabolism in E. coli DH5 at 37 {degrees}C. This came about through the observation of similar maximal optical density obtained during growth of E. coli DH5 under differing concentrations of PEG. Furthermore, the anti-foam did not affect the pH profile. On the other hand, PEG did exhibit some toxicity towards the growth of B. subtilis NRS-762 in LB Lennox medium. Specifically, maximal optical density obtained decline with higher exposure to PEG in a concentration dependent manner, up to a threshold concentration of 5 g/L. For example, maximal optical density obtained in B. subtilis NRS-762 without addition of PEG was 4.4, but the value obtained with 1 g/L of the anti-foam decreased to 4.1 and a further 3.8 on exposure to 5 g/L and 10 g/L PEG. pH variation in culture broth, however, told a different story, where the profiles for exposure to PEG at all concentrations coincide with each other and was similar to the one without exposure to the anti-foam; thereby, suggesting that metabolic processes in B. subtilis NRS-762 were not significantly affected by exposure to PEG. Collectively, PEG anti-foam exerted species-specific toxicity effect on biomass formation, and possibly metabolism. The latter may not be sufficiently significant to affect the types of metabolites secreted by the bacterium, and thus, be detected by measurement of pH of culture broth. E. coli DH5 was better able to cope with PEG at all concentrations compared to B. subtilis NRS-762, which showed dose-dependent toxicity effect on biomass formation. HighlightsO_LIPolyethylene glycol (molecular weight [~] 8000 Da) did not affect aerobic growth of Escherichia coli DH5 at 37 {degrees}C in LB medium at all concentrations tested: 0, 1, 5, 10 g/L. C_LIO_LIGrowth curves of the bacterium at different concentrations of polyethylene glycol (PEG) coincided with each other. C_LIO_LISimilar pH profiles were also obtained for E. coli DH5 growth in LB medium with different PEG concentrations. C_LIO_LIHowever, PEG exerted toxicity effect on Bacillus subtilis NRS-762 during growth in LB medium at 30 {degrees}C, with reduction of biomass formation in a dose dependent manner. C_LIO_LISimilar to the case for E. coli DH5. pH profiles of B. subtilis NRS-762 coincided with each other irrespective of the concentrations of PEG used. C_LI