Turbidimetric growth measurement: In routine spectrophotometric monitoring of a liquid culture, growth is normally expressed as which parameter?

Introduction / Context:
Turbidimetry estimates biomass in a broth culture by measuring light scattering rather than counting cells directly. It is fast, non-destructive, and widely used for growth curves, enzyme induction timing, and fermentation control.

Given Data / Assumptions:

• A spectrophotometer or colorimeter is used at a standard wavelength (commonly ~600 nm).
• The readout is a unitless optical density (OD) or absorbance.
• Direct counts or CFU/mL are separate methods requiring microscopy or plating.

Concept / Approach:
Optical density reflects the proportion of light scattered by cells in suspension. While OD can be correlated with CFU/mL or dry weight using a calibration curve, the immediate measurement reported by the instrument is OD at the chosen wavelength, not absolute cell numbers.

Step-by-Step Solution:
Blank the instrument with uninoculated medium. Measure sample absorbance at the chosen wavelength (e.g., OD600). Record growth as optical density versus time to generate a growth curve. Optionally convert OD values to CFU/mL using a pre-determined calibration.

Verification / Alternative check:
Parallel plate counts at several ODs allow construction of a standard curve for a given organism and path length.

Why Other Options Are Wrong:

• Cells per mL / CFU/mL: Derived from counting methods, not direct turbidimetric outputs.
• mg N per mL / Dry weight only: Biochemical or gravimetric determinations, not the standard turbidimetric readout.

Common Pitfalls:
Using OD outside the linear range leads to underestimation; dilute dense samples to maintain linearity.

Final Answer:
Growth by turbidimetry is reported as optical density.