is the time when the product of the factory must be produced in an efficient mat-
ter. In this phase, however, microorganisms are very sensitive to adverse condi-
tions of their environment.THE STATIONARY PHASE
This phase is one of equilibrium. The growth rate slows, the number of dead
microorganisms equals the number of new microorganisms, and the population
stabilizes. The metabolic activities of individual cells that survive will slow
down. The reasons why the growth of the organisms stops is possibly that the
nutrients have been used up, waste products have accumulated, and drastic harm-
ful changes in the pH of the organisms environment have occurred.
There is a device called a chemostatthat drains off old, used medium and
adds fresh medium. In this way a population can be kept in the growth phase
indefinitely.THE DEATH PHASE
Here the number of dead cells exceeds the number of new cells. This phase con-
tinues until the population is diminished or dies out entirely.Measurements of Microbial Growth
Aplate countis the most common method of measuring bacterial growth. This
method measures the number of viable cells. Plate counts may take 24 hours or
more for visible colonies to form.
Aplate count is performed by either a pour plate method or a special plate
method. With the pour plate method either 1.0 milliliter or 0.1 milliliters of a
bacterial solution is placed into a Petri dish. Melted nutrient agar is added, which
is then gently agitated, or mixed. When the agar has solidified, the plate is then
placed under incubation. With this technique, heat-sensitive microorganisms can
be damaged by the melted agar and be unable to form colonies. To avoid death
of cells due to heat, the spread plate method is mostly used. Here a 0.1-milliliter
bacterial solution is added to the surface of a prepored, solid nutrient agar. The bac-
terial solution is then spread evenly over the medium.(^110) CHAPTER 6 Microbial Growth and Controlling It