TECHNOLOGICAL AND KINETIC ASPECTS OF SUBLETHAL ACID TOXICITY IN MICROBIAL GUM PRODUCTION

Chemostat culture of Xanthomonas campestris were obtained at a dilutio n rate of 0.05 h(-1) and the normal feed then supplemented with 0.58 a nd 1.74 mmol dm(-3) isobutyric acid (IBA). Data revealed that the orga nism responded to sublethal acid stress by overproducing xanthan. The acid additions led to transient zones in the continuous cultivation pr ofiles. By adding feed containing 1.74 mmol dm(-3) IBA, volumetric gro wth rate immediately decreased from 0 059 to 0.026 g dm(-3) h(-1) wher eas the specific xanthan formation rate increased from 0.23 g g(-1) bi omass h(-1) to a maximum 0.65 g g(-1) biomass h(-1) (with 1 0 mmol dm( -3) IBA addition), before decreasing as the concentration of acid atta ined that of the feed. By monitoring the outlet CO2 in parallel with b iomass and polysaccharide levels in the IBA fermentation a 10% diversi on of the total carbon flux from biomass synthesis to xanthan biosynth esis was detected. A consistent pattern of variation in activity was d etected in enzymes of intermediary metabolism, suggesting an action at the regulatory level. Enhanced activities of carbon catabolism and xa nthan anabolic reactions (phosphomannose isomerase) were observed in t he presence of the acid. Batch experiments carried out in the presence of IBA gave results which correlated with the undissociated acid form concentration. An undissociated acid fraction of 6.5 x 10(-3) mmol dm (-3) was calculated in a set of flasks under the same conditions and a statistically validated 12% increase in xanthan production was found. The maximum activation was determined to be below 1.1 x 10(-2) mmol d m(-3) when a 58% specific xanthan production rate increase occurred in parallel with a 35% decrease in biomass concentration.