Accurate and online quantification of viable Rhodobacter sphaeroides cells using a flow cytometry-dielectric...

Accurate and online quantification of viable cells is one of the necessary requirements during the microbial fermentation process for high productivity. The flow cytometry (FCM)-based method accurately quantifies viable cells, but this offline method cannot reflect the counts constantly. The dielectric spectroscopy (DS) sensor is widely utilized to monitor viable cells online; however, accurately converting the capacitance value of the DS sensor to the viable bacterial cell counts has barely been tried. We have developed a method by coupling the principles and techniques of FCM and the DS sensor to quantify viable Rhodobacter sphaeroides cells. Using specific fluorescent antibodies and propidium iodide (PI), viable R. sphaeroides cells were accurately quantified within 30 min by FCM. The DS sensor was combined with the FCM to create a direct capacitance-viable cell count quantification system. The LOD (limit of detection) of the FCM-DS method was 8 × 10⁸ CFU/mL, RSD (relative standard deviation) < 5%, along with good reproducibility of the results. Finally, the viable cell count, obtained from the FCM-DS method, was applied to regulate the specific oxygen uptake rate (Q_O2) that increased the production of coenzyme Q10 by 8.1%. Together, our results strongly suggest that viable cells can be accurately quantified online by the integrated FCM-DS method, which would help to devise precise fermentation control strategies.