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Microbial growth
Typically refers to an increase in population rather than in size
Growth curves
Carried out using batch
cultures or a closed system
(no fresh media added)
Characterized by several
phases
Lag phase
Occurs when cells are
placed into fresh media
Likely due to the cells’
need to synthesize new
components before
reproducing
Lag phase
Can vary depending on:
1. Type of media
2. Condition of the cells
Exponential phase
Cells are growing at the
maximum rate possible
under given conditions
Rate of growth is constant
Population most uniform
Stationary phase
Bacteria in stationary phase
are usually at a
concentration of 109 cells
per ml
Balance between cell
division and cell death or
cells cease to divide
Stationary phase
Due to:
Nutrient depletion
Toxic waste accumulation
Critical cell density reached
Stationary phase
Bacteria subjected to
starvation may become
resistant to killing
Some pathogens may
become more virulent when
starved
Death phase
Decline in viable cells due
to toxic wastes and nutrient
depletion
Death may be at a constant
rate (logarithmic)
Death rate may decrease
after majority of population
has died (resistant cells)
Mathematics of growth
Cells dividing at a constant rate during exponential growth
Generation time/doubling time = time it takes for population to
double
Mathematics of growth
More convenient to graph as log10 of cell number vs. time
Generation time
Determining generation time
Measurement of microbial growth
Measurement of cell number
Measurement of cell mass
Measurement of culture turbidity
Measurement of cell number
Counting chambers
Coulter counters
Plating techniques
Membrane filter techniques
Petroff-Hauser chamber
Used for counting
prokaryotic cells
Use of stains or fluorescent
or phase-contrast
microscopes make counting
easier
Using a Petroff-Hauser chamber
Chamber is of known depth
and has grid etched into
bottom
25 squares cover an area of
1 mm2
Determining average
number per square and
multiplying by 25 gives total
number of cells in chamber
Using a Petroff-Hauser chamber
280 cells in 10 squares
280/10 = 28/square
28 x 25 = 700 cells/ mm2
Chamber is 0.02 mm deep
700/0.02 = 700 x 50
= 3.5 x 104 cells/mm3
= 3.5 x 107 cells/cm3
Coulter counter
Cells forced through small
opening with electrodes on
either side
Passage of cell will cause
resistance to increase and
cell is counted
More useful for counting
eukaryotes
Counting chambers and Coulter counters
Neither can distinguish between living and dead cells
Plating techniques
Diluted sample spread over the surface of agar plate
Number of cells can be calculated by multiplying colony number
by dilution factor
Membrane filter techniques
Useful for measuring number of cells in aquatic samples
Sample passed through filter with small pore size
Filters placed on agar plates to allow growth of colonies
Membrane filter techniques
Measurement of dry weight
Cells collected by centrifugation, washed and dried in an oven
and weighed
Most useful for fungi
Measurement of turbidity
Degree of light scattering
induced by a culture is
indirectly related to the cell
number
Spectrophotometers measure
amount of light scattering
Can measure transmittance
or absorption of light
Continuous culture of microorganisms
Two most common systems
Chemostat
Turbidostat
Chemostat
Sterile media fed into vessel
at same rate that media
containing bacteria are
removed
Final cell density is
dependant on the conc. of a
limiting nutrient
Turbidostat
Makes use of a photocell to measure turbidity of culture
Flow rate of media is regulated to maintain a constant cell density
Influence of environmental factors on growth
Influence of environmental factors on growth
Influence of environmental factors on growth
Influence of environmental factors on growth
Acidophiles
Neutrophiles
Alkalophiles
Influence of environmental factors on growth
Influence of environmental factors on growth
Quorum sensing
Bacteria can communicate
via quorum sensing or
autoinduction
Cell senses concentration of
signal
When threshold is reached,
cell begins expressing sets
of certain genes
Quorum sensing
Most common signal
molecules in gram-negative
bacteria are acyl homoserine
lactones (HSLs)
Gram-positives often use an
oligopeptide signal molecule
Important in pathogenicity
and biofilm formation