Continuous culture allows for microorganisms to be continually harvested from the fermentation vessel, while batch culture requires the growth of microorganisms from an inoculated culture with no product obtained until the culture is fully grown.
Continuous culture involves adding and removing equal volumes of growth media to maintain a constant optical density. This method can be used with directed or lab evolution techniques, however during industrial fermentation high mutation rates are generally not desired.
Fed-batch culture involves inoculating growth media, progressively stepping up the size of culture before reaching the final fermentation vessel. This can allow stocked (stored) bacteria or spores to be grown on industrial scales, while maintaining the clonal culture.
After a fed-batch culture has been grown, it is much easier to recover the desired product from the vessel – either from the media or from rupturing the cells. As the bacteria are able to be killed, it is possible to ensure that no bacteria are released. This method also allows for fermentation vessels to be used for multiple different cultures after a batch has been finished, allowing the manufacturer to use the same equipment to synthesise different products. This reduces the overall cost of the equipment, and increases the utilisation rate.
The high-growth phase obtained during fed-batch culture is able to be obtained by controlling the concentrations of metabolites in the growth media. The desired product could be expressed under an inducible promoter, allowing the manufacturer to only synthesise the product in the main fermentation vessel, potentially giving faster growth of the microbe during the initial culturing phase by reducing the metabolic burden of the biosynthetic pathway.
As nutrients are continually added to the growth media during fed-batch culture, higher microorganism densities can be obtained. During ordinary batch culture, the nutrients in the media are eventually exhausted, leading to the cessation of growth. This increases the efficiency of the process, reducing costs and increasing profits.