Development and application of a membrane cyclone reactor for in vivo NMR spectroscopy with high microbial cell densities

A new bioreactor system has been developed for in vivo NMR spectroscopy of microorganisms under defined physiological conditions. This cyclone reactor with an integrated NMR flow cell is continuously operated in the magnet of a 400-MHz wide-bore NMR spectrometer system. The residence times of medium and cells are decoupled by a circulation-integrated cross-flow microfiltration module to achieve higher cell densities as compared to continuous fermentations without cell retention (increase in cell density up to a factor of 10 in steady state). Volumetric mass transfer coefficients k(L)a of more than 1.0 s^-1 are possible in the membrane cyclone reactor, ensuring adequate oxygen supply [oxygen transfer rate >15,000 mg O₂ · (L h)^-1] of high cell densities. With the aid of the membrane cyclone reactor we were able to show, using continuous in vivo ³¹P NMR spectroscopy of anaerobic glucose fermentation by Zymomonas mobilis, that the NMR signal intensity was directly proportional to the cell concentration in the reactor. The concentration profiles of intracellular inorganic phosphate, NAD(H), NDP, NTP, UDP-sugar, a cyclic pyrophosphate, two sugar phosphate pools, and extracellular inorganic phosphate were recorded after a shift from one steady state to another. The intracellular cyclic pyrophosphate had not been detected before in vitro measurements of Zymomonas mobilis extracts due to the high instability of this compound. Using continuous in vivo ¹³C NMR spectroscopy of aerobic glucose utilization by Corynebacterium glutamicum at a density of 25 g(cell dry weight) · L^-1, the membrane cyclone reactor served to measure the different dynamics of labeling in the carbon atoms of L-lactate, L-glutamate, succinate, and L-lysine with a time resolution of 10 min after impressing a [1-¹³C]-glucose pulse.