Transcript Generation time

General Microbiology Laboratory
Bacterial Generation Time
Microbial Growth
Growth: an increase in the number of cells, not an
increase in size
Bacterial species only maintained if population
continues to grow
Generation: growth by binary fission
Growth rate: cell number/time
or cell mass/time
Generation time: time it takes for a cell to divide and
the population to double; most are 1-3 hours.
Generation times vary markedly with the species of
microorganism and environmental conditions; they
can range from 10 minutes for a few bacteria to
several days with some eucaryotic microorganisms.
Generation Times
Generation Time
Bacterium
Medium
(minutes)
Escherichia coli
Glucose-salts
17
Bacillus megaterium Sucrose-salts
25
Streptococcus lactis Milk
26
Streptococcus lactis Lactose broth
48
Staphylococcus
Heart infusion broth 27-30
aureus
Lactobacillus
Milk
66-87
acidophilus
Rhizobium
Mannitol-salts-yeast
344-461
japonicum
extract
Mycobacterium
Synthetic
792-932
tuberculosis
Treponema pallidum Rabbit testes
1980
What occurs during binary fission?
The Growth Cycle
The population growth is studied by analyzing
the growth curve of a microbial culture.
 The standard bacterial growth curve
describes various stages of growth a pure
culture of bacteria will go through, beginning
with the addition of cells to sterile media and
ending with the death of all of the cells
present.
Typical growth curve for a bacterial population
Phases of Growth
Lag phase
• The period of apparent inactivity in which the cells are
adapting to a new environment and preparing for reproductive
growth.
• Cells are usually synthesizing new components.
• In practice, bacteria from one medium to another, where there
are chemical differences between the two media, typically
results in a lag in cell division.
• This lag in division is associated with a physiological
adaptation to the new environment.
• Cells may increase in size during this time, but simply do not
divide (by binary fission).
• Lag phase varies considerably in length depending upon the
condition of the microorganisms and the nature of the
medium.
Log (exponential) phase
• The period in which the organisms are growing at the
maximal rate possible given their genetic potential,
the nature of the medium, and the conditions under
which they are growing.
• Generation time can be easily obtained from the
exponential phase of a growth curve
• The population is most uniform in terms of chemical
and physical properties during this period.
Stationary phase
• Eventually population growth decrease, and
the growth curve becomes horizontal.
• rate of cell growth = rate of cell death
• Cell death may result from Nutrient
limitation & Toxic waste accumulation (e.g.
acid buildup from fermentation); as well as O2
depletion, critical population level reached.
Death phase
• Stationary phase, in a standard bacterial
growth curve, is followed by a die-off of cells,
called Death phase.
• It is the period in which the cells are dying at
an exponential rate.
• Some of the reasons are: continued
accumulation of wastes, loss of cell's ability to
detoxify toxins, etc.
Balanced and Unbalanced Growth
• Balanced (exponential) growth occurs when
all cellular components are synthesized at
constant rates relative to one another.
• Unbalanced growth occurs when the rates of
synthesis of some components change relative
to the rates of synthesis of other components.
This usually occurs when the environmental
conditions or nutrient levels change.
Measurement of Bacterial Growth
Cell numbers
Metabolic rate
Cell numbers
1- Direct microscopic count
• accomplished by direct microscopic
observation on specially etched slides (such
as Petroff-Hausser chambers or
hemocytometers) or by using electronic
counters.
2- Viable count (colony forming units)
Cell mass / numbers
 Turbidity or optical density
• it is one of the optical methods for counting cells; can estimate
cell numbers accurately by measuring visible turbidity.
• Light scattered is proportional to number of cells.
• Use a spectrophotometer to accurately measure absorbance,
usually at wavelengths around 400-600 nm.
• Accurate measure of cells when concentration not too high.
Easy and quick to measure (can measure a sample in less than
a minute).
• This technique measures the total mass of organisms
and does not distinguish between dead and viable
organisms.
Metabolic rate
02 uptake, C02 and ATP production
Turbidity measurements of microbial growth
Optical Density (Counting by
Spectrophotometer)
End of lecture