Estimating Cell Concentration in the Presence of Suspended Solids: A Light Scatter Technique

A novel sensor was developed, based on light scatter, to
estimate the cell concentration in the presence of suspended
solids. The light scatter properties of cells in the
presence of suspended solids were investigated. Two crucial
observations were made: first, that the light scatter
from cells is essentially a linear function of cell concentration
and, second, that invariant regions are present in the light
scatter spectrum of cell/solid substrate mixtures. Invariant
regions are wavelength intervals of the light scatter spectrum
in which the light scatter reading is independent of
solid substrate concentration and only a function of cell concentration.
The occurrence of invariant regions is the key
behavior which allowed the quantification of cell concentration
in the presence of suspended solids.
An algorithm was developed for the estimation, from light
scatter data, of cell concentration in the presence of solid
substrate. The light scatter approach was validated by comparing
cell concentrations estimated by this technique to
those obtained from DNA and carbon dioxide evolution rate
measurements during a series of fermentations. The model
system used was Bacillus subtilis var sakainensis ATCC
21394 growing on fishmeal as the sole nitrogen source.
A model was developed based on the interactions of
scatter and absorbance. This model reflects the hypothesis
that invariant regions are caused by changes in the absorbance
of the solid substrate as a function of wavelength.