Correlation of enhanced surfactin production with decreased isocitrate dehydrogenase activity

The activity of an enzyme in the tricarboxylic acid cycle, isocitrate dehydrogenase EC 1.1.1.42 (ICD), was followed during the production of a biosurfactant, surfactin, by Bacillus subtilis. A mutant suppressed the activity of the enzyme 30 times more than the parent strain and simultaneously produced three and a half times more surfactin. Decreased ICD activity could also be achieved by (i) decreased oxygen concentration, (ii) increased growth rates, and the addition of (iii) citric acid or (iv) ammonium nitrate to the production medium. The nitrogen source (namely, ammonium nitrate) was critical as neither ammonium chloride nor sodium nitrate could individually enhance biosurfactant production. Initiation of surfactin production occurred as redox values increased and ICD activity decreased. Increased surfactin yields correlated with low levels of ICD activity.