2. Growth takes place at 2 levels
bacterial cell size
● Microbiologists are more frequently concerned with the increase in number of cells in the
● The basis of population growth is binary fission.
To create a broth culture
placed in an
incubator at the
the medium is
After a certain
amount of time has
passed, the broth
from the increased
number of microbes.
● Population growth is often studied by analyzing the growth of microbes in liquid
(broth) culture by the help of growth curve.
Begin with a
5. Batch culture and Continuous culture
● It is used to cultivate beneficial
microorganisms under limited
amounts of nutrients in a closed
fermenter for a certain time period.
● Microbial growth inside the batch
culture shows a typical microbial
growth curve in which four distinct
phases can be identified.
● The process of the batch culture is
stopped after the product is formed.
The process is not stopped
though the product is formed.
● used to grow beneficial microorganisms
under optimum level of nutrients in an
open system in which nutrients are
added continually and waste and
products are removed at the same rate.
● This keeps the growth at an exponential
● The process is not stopped though the
product is formed. Continuous removal of
the product is done without stopping the
process in continuous culture.
Batch culture and continuous culture are two types of techniques employed to cultivate
microorganisms in large scale for industrial and other purposes.
8. History of bacterial growth curve
● One of Pasteur’s first students, Raulin (1869), carried out quantitative growth experiments
with the mold Aspergillus niger that revealed, surprisingly, its ability to grow on a simple
sugar and a few mineral salts. Raulin’s minimal medium is not very different from those
● Then came Henrici’s classic (Henrici, 1928) report on how bacteria change in size throughout
their growth cycle.
● Despite such examples of astute insight, a fog continued to envelop growth physiology, fueled
by quirky notions. For example, some thought that the yield of bacterial cultures was limited
by an entity called “biological space.” Others saw the growth curve as inexorably S-shaped.
● The fog began to lift with the work of, among others, two people who later went on to become
fathers of molecular biology, Alfred Hershey in the late 1930’s and Jacques Monod in the
● Hershey (Hershey, 1939) (collaborating with his chairman, Jacques Bronfenbrenner)
supported the use of a culture in the log phase of growth as the inoculum to start a new
● Monod (1942) consigned the growth response of whole cultures to enzyme kinetics and
showed that the rate of growth was dependent, in Michaelis–Menten fashion, on substrate
concentration, while the yield was proportional to the amount of substrate available. These
experiments were carried out with cultures growing in a steady state.
9. ● Campbell (1957) proposed that the steady state growth condition be referred to as
“balanced growth.” In so doing, he elevated what was previously just one phase in the
growth curve (the log phase) into a general concept.
● In the Copenhagen lab then, it was demonstrated that the concentration of ribosomes
turned out to be a linear function of the growth rate.
● Studies on the mechanisms that regulate growth were greatly aided by genetic
analysis. A large number of conditional mutants, especially of E. coli, were constructed,
e.g., some heat sensitive (see Hirota et al., 1968), some cold sensitive (see Ingraham,
● Studying their phenotype at the restrictive temperatures revealed much about the
biochemical basis for growth and became an essential complement to the purely
● Recently, microbial growth physiology has seen a rebirth in a form that seeks a deeper
quantitative understanding of phenomena on a whole cell level.
History of bacterial growth curve
10. Lag phase
● A small group of cells are
placed in a nutrient rich
medium that allows them to
synthesize proteins and other
molecules necessary for
● These cells increase in size,
but no cell division occurs in
● The length of the lag phase
can vary considerably and
can be long or short Microbial Growth Curve in a Closed System
11. ● The medium may be
different from the one the
microorganism was growing
in previously. In this case,
new enzymes are needed to
use different nutrients
● The cells may be old and
depleted of ATP, essential
cofactors, and ribosomes
which must be synthesized
before growth can begin.
● Possibly the
microorganisms have been
injured and require time to
Why lag phase ?
Eventually, however, the cells begin to replicate their DNA, increase in mass, and divide. As a result,
the number of cells in the population begins to increase.
12. ● During the exponential phase,
microorganisms grow and divide at
the maximal rate possible given their
genetic potential, the nature of the
medium, and the environmental
● Their rate of growth is constant
during the exponential phase; that is,
they are completing the cell cycle
and doubling in number at regular
● Cells in the exponential phase of
growth are the healthiest and most
uniform, which explains why
exponential phase cultures are
usually used in biochemical and
Microbial Growth Curve in a Closed System
13. ● In a closed system such as a batch
culture, population growth
eventually ceases and the growth
curve becomes horizontal i.e the total
number of viable microorganisms
● This may result from a balance
between cell division and cell death,
or the population may simply cease
to divide but remain metabolically
● Cells that are capable of making an
endospore will activate the necessary
genes during this stage, in order to
initiate the sporulation process.
Microbial Growth Curve in a Closed System
14. Why stationary phase ?
DnaA, the protein that binds to the chromosome’s origin to initiate replication, becomes less active in
stationary phase. Ongoing replication is completed, but no further initiation occurs. This is done of
many ways the cell conserves energy by eliminating processes that are not essential to survival.
availability of O2
accumulation of toxic
When a critical population
level is reached
15. ● During this phase, the number
of viable cells declines
exponentially, with cells dying
at a constant rate.
● The steepness of the slope
corresponds to how fast cells
are losing viability
● Detrimental environmental
changes such as nutrient
deprivation and the buildup of
toxic wastes cause irreparable
harm to the cells.
Microbial Growth Curve in a Closed System
4 Death phase
16. It is important to note that if the turbidity of a culture is being measured as a way to
determine cell density, measurements might not decrease during this phase, since
cells could still be intact.
It has been suggested that the cells thought to be dead might be revived under
specific conditions, a condition described as viable but nonculturable (VBNC). This
state might be of importance for pathogens, where they enter a state of very low
metabolism and lack of cellular division, only to resume growth at a later time,
when conditions improve.
It has also been shown that 100% cell death is unlikely, for any cell population, as
the cells mutate to adapt to their environmental conditions, however harsh. Often
there is a tailing effect observed, where a small population of the cells cannot be
killed off. In addition, these cells might benefit from their death of their fellow cells,
which provide nutrients to the environment as they lyse and release their cellular
Some facts about Death phase
17. ● After a period of
exponential death some
microbes have a long
period where the
population size remains
more or less constant.
● This long-term stationary
phase (also called
phase) can last months to
years Microbial Growth Curve in a Closed System
18. ● Log and Stationary phase
: signs and symptoms
● Lag phase correlates with
● Death phase : Recovery
Correlation of different stages of diseases
19. Importance of growth curves
In some applications,
closed batch culturing
is inefficient, and
instead must use a
stages of cell growth is
crucial for working
Growth patterns in
account for the stages
21. Some definitions related to growth rate studies
A specific length of time
during which the population
doubles in number is called
the generation (doubling)
(doubling) time (g)
The growth rate constant (k)
is the number of generations
per unit time and is often
expressed as generations per
hour (hr−1). It can be used to
calculate the generation
22. Calculation of growth rate
constant and generation time
The quantitative aspects of exponential phase growth discussed here apply to microorganisms that divide by binary fission.
23. Determination of generation time
from a microbial growth curve
● The generation time can also be
determined directly from a
semilogarithmic plot of growth
● The population data are plotted
with the logarithmic axis used
for the number of cells. The time
to double the population number
is then read directly from the
● Once this is done, it can be used
to calculate the growth rate
24. Examples of generation time
● Generation times vary markedly
with the microbial species and
environmental conditions. They
range from less than 10 minutes
(0.17 hours) to several days.
● Generation times in nature are
usually much longer than in
25. Solving some questions
Calculate the growth rate constant and generation time of a culture that
increases in the exponential phase from 5 × 102 to 1 × 108 in 12 hours.