Cloudiness or turbidity of a bacterial suspension such as a broth culture is due to the scattering of light passing through the liquid by cells (figure 4.16). The amount scattered is proportional to the concentration of cells. To measure turbidity, a spectrophotometer is used. This instrument transmits light through a specimen and measures the percentage that reaches a light detector. That number is inversely proportional to the optical density. To use turbidity to estimate cell numbers, a onetime correlation between optical density and cell concentration for the specific organism under study must be made. Once this correlation has been determined, the turbidity measurement becomes a rapid and relatively accurate assay.

One limitation of assaying turbidity is that a medium must contain relatively high numbers of bacteria in order to be cloudy. One milliliter of a solution containing 1 million bacteria

(106) is still perfectly clear, and if it contains 10 million cells

(107), it is barely turbid. Thus, although a turbid culture indicates that bacteria are present, a clear solution does not guarantee their absence. Not recognizing these facts can have serious consequences in the laboratory as well as outside. Experienced hikers, for example, know that the clarity of mountain streams does not necessarily mean that the water is free of Giardia or other harmful organisms. ■ giardiasis, p. 625

100 Chapter 4 Dynamics of Prokaryotic Growth

Volume of inoculum

10 ml

Observation after incubation (gas production noted)

Number of positive tubes in set of five

Combination of positives

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