Microbial growth

1
Microbial growth Typically refers to an increase
in population rather than in size
2
Growth curves
Carried out using batch cultures or a closed
system (no fresh media added) Characterized by
several phases
3
Lag phase
Occurs when cells are placed into fresh
media Likely due to the cells need to
synthesize new components before reproducing
4
Lag phase

• Can vary depending on
• 1. Type of media
• 2. Condition of the cells

5
Exponential phase
Cells are growing at the maximum rate possible
under given conditions Rate of growth is
constant Population most uniform
6
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
7
Stationary phase
Due to Nutrient depletion Toxic waste
accumulation Critical cell density reached
8
Stationary phase
Bacteria subjected to starvation may become
resistant to killing Some pathogens may become
more virulent when starved
9
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)
10
Mathematics of growth Cells dividing at a
constant rate during exponential
growth Generation time/doubling time time it
takes for population to double
11
Mathematics of growth More convenient to graph
as log10 of cell number vs. time
12
Generation time
13
Determining generation time
14
Measurement of microbial growth Measurement of
cell number Measurement of cell
mass Measurement of culture turbidity
15
Measurement of cell number Counting
chambers Coulter counters Plating
techniques Membrane filter techniques
16
Petroff-Hauser chamber
Used for counting prokaryotic cells Use of
stains or fluorescent or phase-contrast
microscopes make counting easier
17
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
18
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
19
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
20
Counting chambers and Coulter counters Neither
can distinguish between living and dead cells
21
Plating techniques Diluted sample spread over
the surface of agar plate Number of cells can be
calculated by multiplying colony number by
dilution factor
22
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
23
Membrane filter techniques
24
Measurement of dry weight Cells collected by
centrifugation, washed and dried in an oven and
weighed Most useful for fungi
25
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
26
Continuous culture of microorganisms Two most
common systems Chemostat Turbidostat
27
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
28
Turbidostat Makes use of a photocell to measure
turbidity of culture Flow rate of media is
regulated to maintain a constant cell density
29
Influence of environmental factors on growth
30
Influence of environmental factors on growth
31
Influence of environmental factors on growth
32
Influence of environmental factors on growth
Acidophiles Neutrophiles Alkalophiles
33
Influence of environmental factors on growth
34
Influence of environmental factors on growth

35
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
36
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