This can be simply illustrated by considering the case of a bacterial cell
dividing by fission to produce two daughter cells. In timet, a single cell
will divide to produce two cells; after a further doubling time has elapsed
four cells will be present; after another, eight, and so on. Thus, the rate of
increase as well as the total cell number is doubling with every doubling
time that passes.
If, however, we perform the experiment measuring microbial numbers
with time and then plot logxagainst time, we obtain the curve shown as
Figure 3.1 in which exponential growth occurs for only a part of the time.
A simple analysis of this curve can distinguish three major phases. In
the first, the lag-phase, there is no apparent growth while the inoculum
adjusts to the new environment, synthesizes the enzymes required for its
exploitation and repairs any lesions resulting from earlier injury,e.g.
freezing, drying, heating. The exponential or logarithmic phase which
follows is characterized by an increase in cell numbers following the
simple growth law equation. Accordingly, the slope of this portion of the
curve will equal the organism’s specific growth ratem, which itself will
depend on a variety of factors (see below). Finally, changes in the
medium as a result of exponential growth bring this phase to an end
Figure 3.1 The microbial growth curve
Chapter 3 21