Optimal conditions for induction of competence in nitrogen-fixing Azotobacter vinelandii

Abstract

Competence development in nitrogen-fixing Azotobacter vinelandii cells was optimal at pH 7.2–7.4 which necessitated additional buffering of the iron-limited nitrogen-free competence medium or the addition of a suitable organic acid salt, e.g., sodium acetate. An autolysin was active in this pH range and competent cells were more susceptible to autolysis than the general cell population. Competence development also required restricted aeration of the culture, and only those cultures that attained zero dissolved oxygen became competent. Restricted aeration served to protect the iron-limited cell nitrogenase from oxygen inactivation thus allowing the culture to reach zero dissolved oxygen. The inclusion of additional sources of reductant, e.g., malate, in buffered competence medium resulted in increased respiration and protection of nitrogenase, increased cell mass, and poly-β-hydroxybutyrate synthesis, but decreased competence. A possible explanation for the apparent competition between competence development and nitrogenase activity is discussed.