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Turbidity...

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Turbidity – optical density:         

Measured with a spectrophotometer. Commonly-used wavelengths for bacterial turbidity measurements include 480 nm (blue), 540 nm (green), 600 nm (orange) and 660 nm (red). Sensitivity is best at shorter wavelengths, but measurements of dense cell suspensions are more accurate at longer wavelengths. The unit of turbidity is optical density (OD) at the wavelength specified. Absorbance is a commonly used unit; however it is important to remember that it is the scattering of light that is being assessed in the spectrophotometer. Turbidity is a simple and convenient method. It is non-destructive and can be done continuously. BUT it measures all particles, including any dead cells. Low sensitivity.

Direct counting methods:      

Most common total count method = microscopic cell count method. Can be used on dried samples or liquid samples. A graduated microscope coverslip is used. Can accommodate clumping and chaining. BUT doesn’t discriminate live/dead cells (but staining methods are available). Laborious, but can be automated....