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1. BACTERIAL GROWTH

2. OUTLINE
i.Definition
ii.Terminologies
iii.Batch vs Continous culture
iv.Phases in bacterial growth (Batch culture)
v.Principle of diauxie

3. What is growth?
May be defined as an increase in cellular constituents
It leads to a rise in cell number when organisms reproduce by
processes like:
budding or
binary fission
In multinucleate organism where nuclear divisions are not
accompanied by cell divisions, i. e. coenocytic, growth results in an
increase in cell size not cell number

4. Terminologies in bacterial growth
(i) Solute and Water Activity
Osmotolerant: Able to grow over wide ranges of water activity or osmotic concentration
Halophile: Requires high levels of sodium chloride, usually above about 0.2 M, to grow
(ii)pH
Acidophile : Growth optimum between pH 0.0 and 5.5
Neutrophile: Growth optimum between pH 5.5 and 8.0
Alkalophile: Growth optimum between pH 8.5 and 11.5
(iii)Temperature
Psychrophile : Grows well at 0°C and has an optimum growth temperature of ≤15°C
Psychrotroph: Can grow at 0-7°C; has an optimum between 20 and 30°C and a maximum
around 35°C
Mesophile: Has growth optimum around 20-45°C
Thermophile: Can grow at 55°C or higher; optimum often between 55 and 65°C
Hyperthermophile: Has an optimum between 80 and about 113°C

5. (iv) Oxygen Concentration
Obligate aerobe: Completely dependent on atmospheric O
2 for growth
Facultative anaerobe: Does not require O
2 for growth, but grows better in its
presence
Aerotolerant anaerobe: Grows equally well in presence or absence of O
2
Obligate anaerobe: Does not tolerate O
2 and dies in its presence
Microaerophile: Requires O
2 levels below 2-10% for growth and is damaged by
atmospheric O
2 (20%)
(v) Pressure
Barophilic: Growth more rapid at high hydrostatic pressures
Terminologies in bacterial growth (cont’d)

6. Batch vs Continuous culture systems
Batch Culture System (Closed system) is characterised by:
Nutrient supplies are not renewed
Wastes are not removed
Exponential growth lasts for only a few generations
The stationary phase is quickly reached
Continuous Culture System (Open System) is characterised by:
constant environmental conditions are maintained through
continual provision of nutrients
continual removal of wastes.
Microbial population can be maintained in the exponential growth phase
Constant biomass concentration can be maintained for extended periods

7. Phases in Bacterial Growth (Batch Culture)

8. Lag Phase
The lag phase is an adaptation period, where the bacteria are adjusting to their
new conditions

Typically cells in the lag period are synthesizing RNA, enzymes, and essential
metabolites that might be missing from their new environment

adjusting to environmental changes (temperature, pH, or oxygen availability)

They can also be undertaking any necessary repair of injured cells

It can be short or long depending on:
Age of the cells (old or young)
Type and nature of the medium
Condition of the bacterial cells

 Actively growing cells transferred from one type of media into the same type of
media, with the same environmental conditions, will have the shortest lag period

 Damaged cells will have a long lag period, since they must repair themselves
before they can engage in reproduction.

9. Exponential/Log Phase
The log phase is also known as the exponential phase.
This phase is marked by the doubling of the bacterial cells.
The cell number increases in a logarithmic fashion such that the cell constituent
is maintained.
The log phase continues until there is depletion of nutrients in the setup.
 The stage also comes to a stop if toxic substances start to accumulate, resulting
in a slower growth rate.
The cells are the healthiest at this stage and researchers prefer to use bacteria
from this stage for their experimental processes.

10. Stationary Phase
In this phase, the rate of growth of the cells becomes equal to its rate of death
The rate of growth of the bacterial cells is limited by:
the accumulation of toxic compounds and
also depletion of nutrients in the media
The cell population remains constant at this stage
Plotting this phase on the graph gives a smooth horizontal linear line

11. Death Phase
This is the last phase of the bacterial growth.
At this stage, the rate of death is greater than the rate of formation of new cells.
 Lack of nutrients, physical conditions or other injuries to the cell leads to death of
the cells.

12. Principle of Diauxie
E. g. i. Glucose and Lactose
ii. Glucose and sorbitol

13. Diauxie or diphasic growth is any cell growth characterized by cellular growth in
two phases.
It is caused by the presence of two sugars in a culture growth media, one of which
is easier for the target bacterium to metabolize.
The preferred sugar is consumed first, which leads to rapid growth, followed by a
lag phase.
During the lag phase the cellular machinery used to metabolize the second sugar is
activated and subsequently the second sugar is metabolized.
Diauxie can also occur when the bacterium in a closed batch culture consumes
most of its nutrients and is entering the stationary phase when new nutrients are
suddenly added to the growth media.
The bacterium enters a lag phase where it tries to ingest the food.
Once the food starts being utilized, it enters a new log phase showing a second
peak on the growth curve.