Chapter 6
Microbial growth: prokaryotic cell
cycle and growth curve
1
Growth
• Increase in cellular constituents that may
result in:
– increase in cell number
• binary fission
– increase in cell size
• coenocytic microorganisms have nuclear
divisions that are not accompanied by cell
divisions
• Microbiologists usually study population
growth rather than growth of individual
cells
2
The Procaryotic Cell Cycle
• Cell cycle is sequence of events from
formation of new cell through the next
cell division
• Two pathways function during cycle
– replication of DNA and partition
– cytokinesis (cell separation)
3
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Figure 6.1
4
The Cell Cycle in E. coli
• E. coli requires ~40
minutes to replicate
its DNA and 20
minutes after
termination of
replication to
prepare for division
Figure 6.2
5
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6
Figure 6.3
Beside determination of cell shape, MreB is
thought to play a role in chromosome
partitioning to daughter cells
Chromosome Replication and
Partitioning
• Most procaryotic chromosomes are circular
• Origin of replication – site at which replication
begins
• Terminus – site at which replication is terminated
• Replisome – group of proteins needed for DNA
synthesis; parent DNA spools through the
replisome as replication occurs
• MreB – an actin homolog plays role in
determination of cell shape
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Cytoskeletal Proteins - Role in
Cytokinesis
• Process not well understood
• Protein MreB
– similar to eucaryotic actin
– plays a role in determination of cell shape and
movement of chromosomes to opposite cell poles
• Protein FtsZ,
– similar to eucaryotic tubulin
– plays a role in Z ring formation which is essential for
septation
8
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Figure 6.4
9
The Fts Z Ring and Cell Division
10
The Fts Z Ring and Cell Division
11
Cell Wall Synthesis
Streptococcus hemolyticus
12
DNA Replication in Rapidly Growing
Cells
• Cell cycle completed in 20 minutes
– 40 minutes for DNA replication
– 20 minutes for septum formation and cytokinesis
• Look at timing-how can this happen?
– 2nd, 3rd, and 4th round of replication can begin before
first round of replication is completed
– Progeny cells are “born” with two or more replication
forks, replication already in progress.
13
The Growth Curve
• Observed when microorganisms are
cultivated in batch culture
– culture incubated in a closed vessel
with a single batch of medium
• Usually has four distinct phases
14
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lag phase
no increase
log phase
maximal rate of division
and population growth
stationary phase
population growth ceases
death phase decline in
population size
Figure 6.6
15
Lag Phase
• Cell synthesizing new components
– to replenish spent materials
– to adapt to new medium or other
conditions
• Varies in length
– in some cases can be very short or
even absent
16
Exponential Phase
• Rate of growth is constant
• Population is most uniform in terms of
chemical and physical properties during
this phase
• During log phase, cells exhibit balanced
growth
– cellular constituents manufactured at
constant rates relative to each other
17
Stationary Phase
• Total number of viable cells remains
constant
– may occur because metabolically active
cells stop reproducing
– may occur because reproductive rate is
balanced by death rate
18
Possible reasons for entry into
stationary phase
•
•
•
•
19
Nutrient limitation
Limited oxygen availability
Toxic waste accumulation
Critical population density reached
Starvation responses
• Morphological changes
– endospore formation
• Decrease in size, protoplast
shrinkage, and nucleoid condensation
• Production of starvation proteins
• Long-term survival
• Increased virulence
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Death Phase
• Two new alternative hypotheses
– Cells are Viable But Not Culturable
(VBNC)
• Cells alive, but dormant
• Programmed cell death
– Fraction of the population genetically
programmed to die (commit suicide)
21
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Loss of Viability
Figure 6.8
22
Prolonged Decline in Growth
• bacterial population
continually evolves
• process marked by
successive waves of
genetically distinct
varients
• natural selection
occurs
Figure 6.9
23
The Mathematics of Growth
• Generation (doubling) time
– time required for the population to double in
size
– varies depending on species of
microorganism and environmental
conditions
– range is from 10 minutes to several days for
some microorganisms
24
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Table 6.1
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Calculation of number of cells, generation
times, and growth rates
No = initial population number
Nt = population at time t
n = number of generations at time t
g = generation time
If in 8 h an exponentially growing cell
population increases from 5 × 106
cells/ml to 5 × 108 cells/ml, calculate g
and n.
27
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Figure 6.10
28
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generation time could be
determined using a
semilogarithmic graph at
the log phase
Figure 6.11
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Table 6.2
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