Bacteria multiply by binary fission after reaching a certain size. Their maximum growth rate in a
given medium at a certain temperature is consistent from trial to trial. In other words, if you put
the same bacteria into the same conditions, they will always take the same amount of time to
double in number. That "doubling time" or "generation time" can be determined by constructing
a growth curve.
To generate a growth curve, a small inoculum is introduced into a large volume of fresh medium.
The bacteria are then incubated at a constant temperature and their increasing numbers are
followed. Typically, three stages are observed. "Lag" phase, the first, represents a period of
adjustment for the bacteria after they are introduced into the new medium Generally, the bacteria
have been transferred from a previous medium where they have exhausted most of the nutrients.
It takes them some time to "gear up" their own enzyme levels, metabolic precursors, etc. to make
full use of the newly available nutrients. The growth rate is low but increases steadily. When the
growth rate hits its maximum, the bacteria are said to be in "log" or "exponential" phase. The
doubling time is shortest in this phase. Bacteria will stay in log phase until nutrients begin to run
out. It is then that they enter "stationary" phase, a time of declining growth rate (but still
increasing overall numbers). Bacteria can remain in stationary phase for long periods of time.
Their physiological and metabolic state is vastly different in stationary phase compared to log
Multiplication of bacteria can be followed in several different ways. The most straightforward
would be to remove samples from the growing culture at various time intervals, dilute them,
make spread plates, and calculate the titer at those time points. This is rather tedious, and does
not provide data until the following day when plates can be counted. Another method would be
to follow the increase in turbidity (cloudiness or optical density) of the growth medium As the
bacteria increase in number, the once clear growth medium becomes cloudy. By using a
spectrophotometer, it is possible quantify the increase in turbidity. Since turbidity is directly
proportional to cell number, this is a fast and easy alternative.
One drawback, however, of the latter method is that it does not attach absolute numbers to the
bacterial culture. More specifically, if the culture medium gets twice as turbid, we can be sure
that the number of bacteria has doubled, but we can't know if they've doubled from 10 million to
20 million or from 100 million to 200 million. Thus, a hybrid of the two methods is most
commonly used. Optical density readings are taken every ten minutes for several hours while at a
few time points, dilution plates are prepared to correlate turbidity with cell number.
Other factors such as antibiotics can affect the growth of bacteria. Antibiotics can be