Enumerating Microorganisms:

One of the simplest ways to determine the density of a bacterial culture is to use a spectrophotometer to quantify the degree of turbidity, or cloudiness, in the culture. Bacterial cells are within a size range that causes them to slightly scatter light. The denser the culture, the more light is scattered, and the less light reaches the phototube of the spectrophotometer. In fact, turbidity is more closely related to cell biomass (cell dry weight) than to the number of cells present. In dilute samples, absorbance is directly proportional to biomass. In dense cultures, light is likely to be deflected from more than one cell, so some light will be redirected back to the photocell. Because factors such as cell size, cell density, and instrument characteristics influence the relationship between cell density and absorbance, a standard curve relating known cell concentrations to absorbance is usually used when converting absorbance data to more useful quantities.

Turbidimetry is generally done at wavelengths around 630 nm to minimize absorbance by the medium because most bacterial growth media are brown or brownish-yellow.

See Related Topic: Spectrophotometry

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