The influence of dissolved oxygen on the production of difficidin by Bacillus sp.

Difficidin and oxydifficidin are two related antibacterial antibiotics produced by the fermentation of Bacillus subtilis ATCC 39374. Difficidin was much more stable than oxydifficidin under alkaline conditions. Both antibiotics were very unstable at an extremely low pH level (pH ≤ 3.5). In the whole culture broth, most of the antibiotics were found to be bound to the cell mass and appeared to be more stable than the free compounds. The influence of dissolved oxygen tension (DOT) on the fermentation was investigated using a 20-L fermenter coupled with a proportional-integral-derivative (PID) control system to control DOT at a constant level throughout the fermentation by simultaneously varying the air flow rate and agitator speed based on a constant nitrogen gas feed rate. Growth and antibiotics production during the fermentation were studied at different DOT levels. The critical DOT level for the maximum specific growth rate of the culture was found to be lower than 5% air saturation. The difficidin production was significantly affected by DOT in the fermentation broth while the oxydifficidin synthesis appeared to be unaffected. The DOT level of 20% air saturation was critical for difficidin synthesis, below which the volumetric production rates of difficidin were sharply decreased. These results demonstrate the distinct difference between the critical DOT values for difficidin production and specific growth rates. Fermentations with cycling DOT with a periodicity of 1 min were performed to simulate the heterogeneity in DOT of a large scale fermenter. The antibiotics production was not significantly affected by cycling DOT above, below as well as around the critical level for difficidin production. However, an increase in the growth rates when DOT was cycled below and around the critical level caused a marked reduction in the specific production rates of the two antibiotics. In contrast, no effect on growth rates was observed when DOT was cycled above the critical level