Soil Microbial Ecology - Soil Molecular Ecology Laboratory

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hold a certain volume of soil. This total count method of cell quantification is very rapid
but has the problem of requiring high cell concentrations (e.g. 10 7 / ml), and may include
dead and living cells with equal probability.
B. INDIRECT COUNT CELL NUMBERS (ESTIMATION):
Cell biological activity can be used to estimate cell numbers. Such method is often
preferable either for convenience or because direct counting is difficult or even
impossible in many situations (for example, when quantifying filamentous organisms).
Estimates of cell number include determinations of turbidity, metabolic activity, or dry
mass.
1. TURBIDITY
The cloudiness or turbidity of a culture is caused by the individual cells scattering light.
Degree of turbidity is a direct correlation of cell mass. The average cell mass of
individual cells in a culture must first be determined if the turbidity of a culture is to be
used to estimate cell count. This standardization works best for cultures in the
exponential growth phase.
2. METABOLIC ACTIVITY
The metabolic output or input of a culture may be used to estimate viable count. This
method has the same qualification as turbidity methods. The rate at which metabolic
products such as gases and/or acids are formed by culture reflects the mass of bacteria
present. The rate at which a substrate such as glucose or oxygen is used up also reflects
cell mass. Soil enzymes, soil DNA, and soil respiration are some of the widely used
techniques in estimating soil activities.
3. DRY MASS
Determinations of dry mass required to dry cultures before analyses. It is also required to
separate cells from medium by some physical means such as filtration or centrifugation.
The cells are then dried, and the resulting mass is weighed. For filamentous organisms
such as fungi, the method works sufficiently well compared to other methods listed that
dry mass determination is frequently employed.