On-line monitoring of infected Sf-9 insect cell cultures by scanning permittivity measurements and comparison with off-line biovolume measurements

Abstract

Two infected Sf-9 cell cultures were monitored on-line by multi-frequency permittivity measurements using the Fogale BIOMASS SYSTEM((R)) and by applying different off-line methods (CASY((R))1, Vi-CELLtrade mark, packed cell volume) to measure the biovolume and the mean diameter of the cell population. During the growth phase and the early infection phase the measured permittivity at the working frequency correlated well with the different off-line methods for the biovolume. We found a value of 0.67 pF cm(-1) permittivity per unit of total biovolume (CASY) (muL mL(-1)). After the maximum value in the permittivity was reached, i.e. when the viability of the cultures decreased significantly, we observed different time courses for the biovolume depending on the applied method. The differences were compared and could be explained by the underlying measurement principles. Furthermore, the characteristic frequency (f(C)) was calculated from the on-line scanning permittivity measurements. The f(C) may provide an indication of changes in cell diameter and membrane properties especially after infection and could also be an indicator for the onset of the virus production phase. The changes in f(C) were qualitatively explained by the underlying equation that is correlating f(C) and the properties of the cell population (cell diameter, intracellular conductivity and capacitance per membrane area).

Fig. 1

The β-dispersion for spherical cells within the frequency range of 0.1–10 MHz. The β-dispersion is mathematically defined by three parameters (Δε, f_C, α) that are dependent on the properties of the cell population

Fig. 2

(A) Representative pattern of a baculovirus-infected Sf-9 culture (fermentation 1): Hemacytometer counts (total cell count (●), viable cell count (△), viability (hema) (▼), Vi-CELL cell counts (total cell count (■), viable cell count (□)), permittivity (·) over time. (B) Mean cell diameter (CASY (■), Vi-CELL(×)), viability (hema) (▼) and permittivity (·) of a baculovirus-infected Sf-9 culture (fermentation 1). (C) Total volume (CASY), packed cell volume, total biovolume (Vi-CELL), viable biovolume (Vi-CELL), permittivity over time for an infected Sf-9 culture (fermentation 1). (·) permittivity, (×) viable biovolume (Vi-CELL), (▴) total biovolume (Vi-CELL), (•) total biovolume (CASY), (◆) total biovolume (PCV)

Fig. 3

(A) Permittivity (solid line), mean cell diameter (Vi-CELL) (×), mean cell diameter (CASY) (●) and oxygen uptake rate (·, dash dotted line) for the plateau region of fermentation 1. (B) Permittivity (solid line), total biovolume (PCV) (◆) and total biovolume (CASY) (●) for the plateau region of fermentation 1

Fig. 4

Permittivity (·), characteristic frequency (dashed line) and cell size (CASY) (●) over time (fermentation 1)

Fig. 5

(A) Permittivity (fermentation 1 (□, dash dotted line), fermentation 2 (·)) and characteristic frequency (fermentation 1: dotted line, fermentation 2: dashed line) over time for two duplicate fermentations. (B) Correlation of permittivity and the total biovolume (CASY) for two duplicate fermentations (fermentation 1: □; fermentation 2: ●). Only the data up to the maximum in permittivity (around 100 h cultivation time) were plotted in this graph. Linear regression yields: permittivity = 0.6692 * biovolume; R² = 0.985