Recent Advances in Continuous Cultivation

Since the first publication of the continuous cultivation by Monod and by Novick and Szilard in 1950, a significant improvement in performance of cultivation equipment has been achieved. Accurate peristaltic pumps and high performance bioreactors allowed establishing steady-state conditions in chemostat cultures in a highly reproducible way. Thus, systematic studies of the microbial growth as a function of medium composition and a controlled, constant specific growth rate (set by the dilution rate of the culture broth) resulted in a large body of publications in the mid-1960’ies. Soon chemostat cultivation became a very important tool in physiological studies, e.g.to assess multiple nutrient limited growth conditions, quantify plasmid stability, optimize medium composition and select for highly performing mutants. With the onset of novel analytical tools in the 1990’ies and in this century, e.g., 2D gel electrophoreses, gene arrays, and massive proteomics and metabolomics, new know-how could be gained and enabled the study of metabolic fluxes in the cell leading to novel theoretical models and thus guiding targeted engineering of microorganisms.

Technical modifications of these chemostats by cell feed-back loops, cell retention, and extension by more than one chemostat stage led to more robust systems that are able to handle fluctuations of substrate availability, which are typically the case in waste water treatment plants. In addition, sophisticated process analytical technology (PAT) allowed the establishment of so called nutristats and turbidostats. In these particular cultivation techniques, the cells trigger the substrate feed by closed feed-back loops based on metabolic signals (e.g. carbon concentration below a certain threshold value) or based on the change of cell density (culture turbidity), respectively. Recently, continuous cultivation experienced a strong boost in the field of antibody production using animal cell cultures. These so-called perfusion systems differ from classical chemostats by the partial or even complete retention of cells that have been immobilized on surfaces (ceramic or polymer beads). By implementing this cultivation method, the problems of growth inhibiting metabolites (e.g. lactate and ammonium) can significantly be reduced and as a consequence results in a higher viability and a much higher yield over a longer period.

The goal of this collection of research articles is to give an overview on the state of the art of continuous cultivation and point out novel trends. In-depth review articles as well as scientific reports on recent developments with microorganisms and animal cell cultures are very welcome.