Bacteria reproduce through binary fission, where a single cell divides into two daughter cells. This results in exponential growth of the bacterial population over time. A bacterial growth curve typically shows four phases: a lag phase as the bacteria adapt, an exponential or log phase of rapid growth, a stationary phase as resources are depleted, and a death phase. Continuous culturing techniques like chemostats can maintain bacteria in the log phase of growth through controlled dilution of the culture medium.

1. Reproduction
and Growth
of Bacteria
Growth Curve,
Growth Factors, Growth
Characteristics

2. Growth and Metabolism
• All cells including bacteria must accomplish certain metabolic tasks
to maintain life.
• When bacteria are inoculated into a suitable medium and incubated
under appropriate conditions a tremendous increase in the number
of cells occurs within a relatively short time.
• The term growth in case of bacteria and other microorganisms
usually refers to changes in the total population rather than an
increase in the size of an individual organism.
• Growth can be positive as well as negative while reproduction is
always positive.

3. GROWTH DETERMINATION
• The most common means of bacterial reproduction is binary fission; one cell
divides, producing two cells. Thus, if we start with a single bacterium the
increase in population is by geometric progression:
1 → 2 → 2 2 → 2 3 → 2 4 … 2n
• Where n = the number of generations.
• Each succeeding generation, assuming no cell death, doubles the population.
• The total population N at the end of given time period would be expressed:
N = 1 x 2n
• However, under practical conditions, the number of bacteria N0 inoculated at
time zero is not 1 but more likely several thousand, so the formula now
becomes: N = N0 x 2n.

4. GENERATION TIME
• The time period required for bacteria to divide into 2 or
a population to become double is called generation
time.
• The generation time is the time interval required for the
cells (or population) to divide.
G (generation time) = t (time, in minutes or hours)/
n(number of generations)
G = t/n

5. GROWTH RATE
• Growth rate is the number of generations formed in a
unit time in biological terms, growth rate is defined
as absolute or relative growth increase, expressed in
units of time.
Growth Rate = n/t

6. INCUBATION PERIOD
• Incubation period is the period between the entrance of
a pathogenic organism till the commencement of
symptoms.
• Malaria incubation period is 10 days, while typhoid has
inoculation period of 7 days only.

7. MODE OF REPRODUCTION
• BINARY FISSION
• The most common mode of cell division is the usual growth cycle of
bacterial populations - binary fission, in which a single cell divides
after developing a septum (crosswall).
• Binary fission is an asexual reproductive process.
• Bacillus subtilis and Streptococcus faecalis are some example of
bacteria showing binary fission.
• CONJUGATION
• Special pili are involved in the sexual reproduction, called
conjugation.
• These pili are named as f-pili or sex-pili.

8. MODE OF REPRODUCTION
• BUDDING
• Some bacteria, such as Rhodopseudomonas acidophila, reproduce by
budding, a process in which small protuberance (bud) develops into a
new cell which separates from the parent.
• FRANGMENATION
• Bacteria that produce extensive filamentous growth, such as Nocardia
species, reproduce by fragmentation of the filaments into small bacillary
or coccoid cells, each of which give rise to new growth.
• FORMATION OF CONIDOSPORES OR SPORANGIOSPRE
• Species of genus Streptomyces and related bacteria produce many
spores per organism by developing cross walls (septation) at the hyphal
tips; each spore gives rise to a new organism.

9. Stages of the Bacterial Growth Cycle
• Because bacteria reproduce by binary fission (one becomes
two, two become four, four become eight, etc.), the number
of cells increases exponentially with time (the exponential or
log phase of growth).
• Depending on the species, the minimum doubling time can
be as short as 20min or as long as several days.
• For example, E. coli - a single cell can give rise to some ten
million cells in just 8 hours.

10. GROWTH CURVE
• A graphic representation of growth (population changes)
of bacteria in culture medium is known as growth curve.
• When a fresh medium is inoculated with a given number
of cells, and the population growth is monitored over a
period of time, plotting the data will yield a typical
bacterial growth curve.
• The graph is plotted between logarithm of number of
microbes (y – axis) and time (x- axis).

11. Growth Curve

12. Lag Phase
• It takes some time for a microorganism to adapt to the new
conditions when it is first put into a new medium.
• When cellular metabolism speeds up and cells expand, bacteria
are unable to replicate which results in a decrease in the mass
of a cell.
• It takes a microbe longer to acclimate to its new surroundings
when it is introduced into a nutritionally poor medium.
• As the organism begins to transduce the proteins, the lag phase
lengthens.
• When an organism is transferred from a nutritionally deficient
medium to a nutritionally rich medium without delay and thus
has a shorter lag phase in the microbial growth curve.

13. Exponential Phase
• During this phase, microorganisms multiply and divide
rapidly. Their metabolic activity increases and DNA
replication begins in earnest.
• Generation time is the amount of time it takes bacteria to
double in size over a particular length of time.
• The period of time it takes for an organism to reproduce
varies.

14. Stationary Phase
• As the bacterial population develops, it consumes all of the nutrients in the
growth medium.
• In the medium, toxic metabolites, waste materials, and inhibitory
compounds such as antibiotics accumulate.
• The rate of reproduction of the bacteria reduces until the number of cells
undergoing division equals the number of cells dying, at that point of the
time the bacterium stops dividing completely.
• The growth rate remains constant because the cell count is not increased.
• When a stationary cell is shifted to a new medium, it can quickly switch to
the exponential phase and restart metabolic processes.

15. Death Phase
• The bacterium’s capacity to move to the Death phase is aided by the
loss of nutrients and consequent accumulation of metabolic waste
products and other toxic compounds in the media.
• Bacteria typically start to die when they are exposed to conditions
that are unfavorable, and death occurs quickly and uniformly.
• The total number of dead cells outnumbers the number of living cells.
• When some organisms are resistant to this condition, endospores
allow them to survive in the environment.

16. DILUTION
• Blank: A sterile media with no microbe. Dilution are of two types
(parallel and serial)
• Simple sterile water or simple buffer, can be used to make the
bacterial dilution suspension that help in easy count of the
bacterial cells.
• Normally 1 ml Sample is added in 9ml of sterile water to form a
diluted suspension in case of serial dilution, and it may be
repeated to get the require dilution.
• In parallel dilution equal ratio of bacterial sample and dilatant is
mixed gently.

17. SYNCHRONOUS GROWTH
• Information about the growth behavior of individual bacteria can,
however, be obtained by the study of synchronous cultures.
• Synchronized cultures must be composed of cells which are all at
the same stage of the bacterial cell cycle.
• Measurements made on synchronized cultures are equivalent to
measurements made on individual cells.
• A number of clever techniques have been devised to obtain
bacterial populations at the same stage in the cell cycle.

18. • Some techniques involve manipulation of environmental parameters
which induce the population to start or stop growth at the same point in
the cell cycle, while others are physical methods for selection of cells that
have just completed the process of binary fission.
• Theoretically, the smallest cells in a bacterial population are those that
have just completed the process of cell division.
• Synchronous cultures rapidly lose synchrony because not all cells in the
population divide at exactly the same size, age or time.

19. CONTINUOUS CULTURE
• In both experimental research and in industrial processes, it is often
desirable to maintain a bacterial population growing at a particular rate in
the exponential or log phase. This condition is known as steady-state
growth.
• The culture volume and the cell concentration are both kept constant by
allowing sterile medium to enter the culture and vessel at the same rate
that ‘spent’ medium containing cells is removed from the growing culture.
• Continuous culture, in a device called a chemostat, can be used to
maintain a bacterial population at a constant density, a situation that is, in
many ways, more similar to bacterial growth in natural environments.

21. • In a chemostat, the growth chamber is connected to a reservoir of
sterile medium.
• Once growth is initiated, fresh medium is continuously supplied from
the reservoir.
• The volume of fluid in the growth chamber is maintained at a constant
level by some sort of overflow drain.
• Fresh medium is allowed to enter into the growth chamber at a rate
that limits the growth of the bacteria.
• The bacteria grow (cells are formed) at the same rate that bacterial
cells (and spent medium) are removed by the overflow.
• The rate of addition of the fresh medium determines the rate of growth
because the fresh medium always contains a limiting amount of an
essential nutrient.
• Thus, the chemostat relieves the insufficiency of nutrients, the
accumulation of toxic substances, and the accumulation of excess
cells in the culture, which are the parameters that initiate the stationary
phase of the growth cycle.

22. References
• Lippincott Microbiology, Unit II: Bacteria
Chapter 6; Page 53.
• Hugo and Russell’s Pharmaceutical
Microbiology, Page 33