Researchers in New Brunswick Scientific's in-house labs have developed a unique control strategy to achieve high antibiotic expression levels in a pilot-scale fermentation process of Streptomyces hygroscopicus. Using the BioFlo 6000 fermentor's on-line controller with touchscreen interface augmented by the powerful supervisory capabilities of AFSBioCommand software, scientists were successful in optimizing the culture environment for rapamycin production. Harvested dry cell weight recovered from the 100 liter fermentor was over 25 grams/L; intracellular rapamycin production was over 110 milligrams/L; and fermentation time was reduced by almost a day to 120 hours.
Streptomyces expresses a variety of secondary metabolites, such as rapamycin, which are used in medical and biological applications. During fermentation, dense mycelia are produced, which are highly variable in phenotype and especially susceptible to changing conditions during cultivation. Formation of secondary metabolites is usually favored by sub-optimal cell growth conditions. Nutrient transfer problems can restrict the metabolism of the culture, and a high oxygen transfer rate (OTR) is always required. Therefore, it was necessary to develop a fermentation protocol which allowed for initial cell growth to a sufficient density as to facilitate the production of large quantities of the antibiotic, while preventing the culture from becoming so dense as to interfere with its ability to obtain vital nutrients and oxygen for the production of rapamycin.
Materials and Methods
Seed inoculum of Streptomyces hygroscopicus NBS-9746, selected from ATCC 29253 cell strain, was cultured in an Innova 4300 incubator shaker, then transferred to a 10 L BioFlo IV bench fermentor, and fina lly to a BioFlo 6000 Mobile Pilot Plant fermentor equipped with an ML-6100 touch-screen controller and 100 L working volurne vessel, (all equipment manufactured by New Brunswick Scientific, Edison, NJ). NBS' proprietary AFS-BioCommand software was used to control a multi-loop cascade strategy, wherein each of four process parameters were altered in sequence so as to maintain the desired D.O. level. Capable of managing up to eight fermentors, bioreactors, or ancillary instruments, this software automates the tedious and error-prone jobs of logging data, computing real and derived process values, and altering setpoints on the basis of time or metabolic events. In this study, AFS-BioCommand was programmed to maintain a D.O. level of 30% of air saturation by automatically increasing agitation rates from 200 to 450 rpm, then vessel pressure from 0.35 to 0.75 bar, aeration rate from 0.5 to 1 VVM and simultaneously shifting temperature from 28C to 25C, when D.O. fell below its setpoint. Based on optimization studies conducted on the bench level, pH was maintained between 6.2 and 6.8. A deadband was easily configured by using the local controller for the low setpoint and the supervisory control software for the high setpoint.
Utilizing the multi-loop cascade strategy provided several benefits. First and foremost, D.O. was able to be maintained at optimal rates for production of rapamycin, a feat unable to be accomplished using a single loop control strategy ( see figure ). Agitation rates could also be significantly reduced, to minimize shear forces and enable higher yields of both cells and secondary metabolites.
Temperature was also lowered from that optimal for culture growth to the optimal range for rapamycin production. The lower temperature had the side benefit of slowing cell growth, thereby decreasing oxygen demands, which h elped to maintain the high D.O. levels.
For details of this study and further information on our bioprocessing supervisory software, speak with your local NBS representative. To discuss how you can put the expertise of our in-house researchers to work on your next process development, optimization, or contract production project, contact Dr. Julia Cino at 800-631-5417, by fax to 732-287-4222, or via e-mail at firstname.lastname@example.org.