Azotobacter vinelandii is a bacterium with several industrial applications including production of bioplastics, hydrogen and biofertilizer. In nature, it contributes to the nitrogen cycle, by means of the enzyme nitrogenase 1 which is responsible for the conversion of atmospheric nitrogen to ammonia. Members of this species possess a complex life cycle, exhibiting distinct morphological states in response to different environmental conditions. Under unfavourable conditions, such as nutrient depletion, Azotobacter form resting cells with limited metabolic activity, called as “cysts”, which are resistant to deleterious physical and chemical conditions. During exponential growth phase, motile oval shaped “vegetative” cells are observed.
In this study, Azotobacter vinelandii was grown in continuous culture for prolonged periods of time (up to 180 hours). This is different to conventional batch mode of growth where nutrients are depleted, cultures go through different growth phases and cells are subjected to changes in properties. Continuous culture operates at a steady state, with a single limiting nutrient and all other nutrients in equilibrium. This study shows a link between the lifecycle stage of Azotobacter vinelandii cells and the dilution rate in continuous culture. At a dilution rate of 0.1h-1 predominantly oval, dividing vegetative cells are observed. Continuous cultures grown at lower dilution rates (D=0.02 h-1) are cyst-like in appearance at first, but after the turnover of roughly four vessel volumes, cells appear to be long ovals, more closely resembling vegetative cells. Hence, a method of producing Azotobacter vegetative cells using continuous culture has been identified and potentially provides an opportunity to create a more efficient system for medium improvement by testing individual components, whilst all other parameters remain constant. These findings could contribute to a novel method for producing high cell density Azotobacter vinelandii for use in industrial applications.