Direct microscopic counts and cell-counting instruments generally do not distinguish between living and dead cells. Plate counts determine the number of cells capable of multiplying; membrane filtration can be used to concentrate the sample. The most probable number is a statistical assay based on the theory of probability. Turbidity of a culture is a rapid measurement that can be correlated to cell number. The total weight of a culture and the amount of certain cell constituents can be correlated to the number of cells present. Microbial growth can be detected by the presence of cell products such as acid, gas, and ATP.
■ Why is an MPN an estimate rather than an accurate number?
■ Why would a direct microscopic count yield a higher number than a pour plate if a sample of seawater was examined by both methods?
■ The nitrogen in microorganisms will typically be present in what molecules?
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