6
Part B
Microbial Growth
pp. 160-177
Fig. 6.11
Cell wall
Partially formed cross wall
DNA (nuclear area)
1
Ch 6 Overview
• Physical and chemical
requirements for growth
• Various types of culture media
• Bacterial cell division
• Phases of bacterial growth
2
Distribution of Microbes
• Growth of a
population
– Increase in the
number of cells
– Not size
• Each ‘spot’ on the
agar is a colony
• When visible
– Cells number in the
millions
Bacterial colonies
3
Requirements for Growth
Physical and Chemical
Reasons for understanding the
conditions necessary for growth:
– To control growth of pathogens
– To encourage growth of helpful microbes
4
Physical requirements:
• Temperature
• pH
• Osmotic pressure
Chemical requirements:
• Carbon, Nitrogen, sulfur,
phosphorus, trace elements,
organic growth factors
• Oxygen
5
Temperature
• Classified in 3 groups
1. Psychrophiles (cold-loving)
2. Mesophiles (moderate-temp-loving)
3. Thermophiles (heat-loving)
Psychrophiles Mesophiles
Thermophiles
Room T
6
Minimum, Optimum, Maximum
Growth Temperatures
• Minimum: the lowest T that a species
will grow
• Optimum: best growth T
• Maximum: highest T that growth is
possible
7
Most grow in a limited range
‒ Maximum & minimum growth T about
30o C apart
‒ Grow poorly at high & low T in their
range
Fig. 6.1
8
Optimum Growth Rate
‒ Fastest reproduction = peak of curve
‒ At either T extreme = rate drops
‒ At T just above optimum = fastest drop
Fig. 6.1
9
Psychrophile
– Can grow at Oo C
– Optimal growth at ~15o C
– But, very sensitive to higher T
– Will not grow at ~25o C
– Habitat: deep ocean
– Pathogenesis: rarely causes food
spoilage
10
Psychrotroph
A slight ‘variation’ of a psychrophile
– Can grow at Oo C
– Slightly higher optimal growth T ~ 2030o C
– Also, very sensitive to higher T
– Will not grow above ~40o C
– Much more common than psychrophiles
– Pathogenesis: frequent cause of food
spoilage
11
Refrigeration
• Most common food preservation
method
– (pathogenic) bacteria grow poorly
• At the high and low T in their range
• (even though) psychotrophs grow at
~20-30o C
– They grow slowly
‒And are able to slowly degrade food
12
Set T in Refrigerator at a Low Setting
O
F
O
C
1. To slow growth of
– Most spoilage
microbes
2. To prevent growth
of
– Most pathogenic
species
Fig. 6.2
13
Mesophiles
– Optimal growth ~ 37o C
– Most common bacteria
– Aerobic
– Can be pathogenic
– But also, can do work of
industrial importance
14
‘Swiss’ Cheese
• Produced by
– Propionibacteria
freudenreichii
– Grows at 24o C
– Ferments lactate to
acetate, propionate
& CO2
– Acetate, propionate
give the cheese its
flavor
15
Thermophiles
‒ Growth: ~50-60o C,
and higher
‒ Habitat: hot springs
‒ H2S provides source
of energy
Boulder Spring
Lower Geyser Basin
Yellowstone Nat’l Park
16
pH
• Describes acidity or
alkalinity of a
solution
• Most bacteria grow
in a neutral
environment
– Between ~6.5 and
7.5
• Example:
– Escherichia coli
Fig. 2.7
17
Fig. 2.7
• ph below ~4 is acidic
• Preserves foods
..sauerkraut, pickles
• Bacterial
fermentation
produces acids
• Called acidophiles
Helicobacter pylori
Fig. 25.14
18
Osmotic Pressure
Solute = [NaCl]
• The pressure
[NaCl] is equal
across the cell
membrane
Fig. 6.4
19
Osmotic Pressure
Solute = [NaCl]
Refers to the
concentration of NaCl
1. Inside a structure
2. And in the solution
surrounding the
structure
Chapter 4, p. 93-4, Fig 4.18
20
Isotonic Pressure
Osmotic pressure
[NaCl] concentration
is equal across the
membrane
Fig. 6.4
21
Hypertonic Pressure
Osmotic pressure
[NaCl] concentration
is:
1. Higher in the
solution
surrounding the
bacterium
2. Than inside the
bacterium
3. Causes death of the
bacterium
22
Hypotonic Pressure
Osmotic pressure
[NaCl] concentration
is:
1. Lower in the
solution
surrounding the
bacterium
2. Than inside the
bacterium
3. Also causes death
Chapter 4, p. 93-4, Fig 4.18d
23
Plasmolysis
-Loss of Water1. Water leaves the cell
2. Enters the high
solute [NaCl]
3. Causes shrinkage of
the cell cytoplasm
4. Cell death
High salt used to preserve foods (or sugar)
24
Chemical Requirements
Microbes must acquire nutrients from
its environment to grow & reproduce
• In the lab, we provide nutritional
needs
– In culture media
– Media must contain
Water, carbon, nitrogen, minerals
(S, P, Ca, Mg, Na, etc)
25
• Water
(80-85% weight of cells)
• Carbon
(50% wt/cell)
– To get C, use organic molecules (proteins,
carbohydrates, lipids) or CO2
• Nitrogen
(14% wt/cell)
– Digest protein into amino acids
– Use N for protein, NA synthesis
26
• Minerals
– Sulfur (amino acids, thiamine, biotin)
– Phosphorus (DNA, RNA, ATP, membranes)
– K, Mg, Ca (cofactors for enzymes)
– Trace elements (usu enzyme cofactors)
• Iron, copper, molybdenum, zinc
– Organic growth factors for some bacteria
• p 169
27
Oxygen
Classification based on oxygen
requirements
1. Obligate aerobes
2. Facultative anaerobes
3. Obligate anaerobes
4. Aerotolerant anaerobes
5. Microaerophiles
28
1. Obligate Aerobes
• Microbes that use oxygen
– Produce more energy from nutrients
– Than microbes that do not use oxygen
• Obligate aerobes require oxygen to
live
Examples:
Bacillus, Serratia, Pseudomonas, Vibrio
29
2. Facultative Anaerobes
• Oxygen is poorly soluble in water
– Therefore, many bacteria have evolved
to live in the absence of water
• Can use oxygen when it is present
• But, in absence, use fermentation or
anaerobic respiration
• Produces less energy
Examples: Escherichia coli, Shigella, Salmonella
30
3. Obligate Anaerobes
• Microbes unable to use oxygen for
energy yielding reactions
• Most are killed by oxygen
Example:
Clostridium (botulism, tetanus)
31
4. Aerotolerant Anaerobes
• Cannot use oxygen for growth
• But, can tolerate it
– Posses enzyme SOD which neutralizes
oxygen
• Mostly ferment carbohydrates to
lactic acid
Example: Lactobacillus
32
5. Microaerophile
•
•
•
•
Microaerophiles are aerobic
They require oxygen
But, only in LOW [O2]’s
Sensitive to superoxide free radicals
– They do not have the SOD enzyme
Example: Micrococcus
33
34
O2 and Enzymes
Aerobes, facultative anaerobes &
aerotolerant anaerobes must have
the enzyme:
Superoxide dismutase
(2O2– + 2H+
O2 + H2O2)
Peroxide is toxic to cells
See Lab experiment 14
35
And aerobes & facultative
anaerobes must have either:
Catalase
(2H2O2
2H2O + O2)
Peroxidase
(H2O2 + 2H+
See Lab experiment 14
2H2O)
36
Bacterial Cell Division
• Called binary
fission
• A single cell divides
into 2 cells
• A few species
divide by ‘budding’
Fig. 6.11
37
38
Streak Plate Method
Pure cultures are
obtained by the
streak plate method
Fig. 6.10a,b; Ch 6, p 173-4
39
40
Phases of Bacterial Growth
• Generation time
– Time required for a cell to divide
– Or, a population to double
• Most bacteria have a generation time of
– One to three hours
– Others require more than 24 hours
41
Logarithmic Representation of Bacterial
Populations
Bacterial division occurs according to
a logarithmic progression
– 2 cells, 4 cells, 8 cells, 16 cells, 32 cells… (yx)
Fig. 6.12 b
42
Bacterial Growth Curve
43
Q’s
1. Which of these terms describes a
microorganism that grows at human body
temperature (37o C)?
A.
B.
C.
Thermophile
Psychrophile
Psychrotroph
D.
E.
Mesophile
Hyperthermophile
2. During log phase, bacteria are:
A.
B.
Dividing at fastest rate
Preparing to divide
C.
D.
Dying exponentially
Dying and dividing in
equal numbers
44
Q’s
1. Which of these terms describes microbes
that do not use oxygen, but can grow if it is
present?
A. Obligate aerobe
D. Microaerophile
B. Obligate anaerobe
E. Aerotolerant anaerobe
C. Facultative anaerobe
2. Which of these is an example of an organic
growth factor?
A. Glucose
B. Oxygen
C. Biotin
D. Sodium chloride
E. Agar
45
Appendix
46
Bacterial Growth Curve - Phases
1. Lag phase:
– Little or no change in the number of
cells, but metabolic activity is high
2. Log phase:
– Bacteria multiply at the fastest rate
possible under the conditions provided
3. Stationary phase:
– An equilibrium between cell division and
death occurs
4. Death phase:
– Deaths exceed the number of new cells
formed
47
Q’s
1. Shrinking of the plasma membrane in
response to osmotic loss of water is called:
2. Bacteria that spoil food in the refrigerator are
most likely:
A.
B.
Thermophiles
Psychrotrophs
C.
D.
Mesophiles
psychrophiles
3. The _____ _____ _____ is the most common
technique used to obtain pure cultures of
microbes.
48
Q’s
1. Organisms such as Clostridium lack oxygendetoxifying enzymes, thus they are:
A.
B.
C.
Obligate aerobes
Obligate anaerobes
Facultative
anaerobes
D.
E.
Microaerophiles
Aerotolerant
anaerobes
2. Which enzyme catalyzes the reaction:
H2O2 + 2H+
2H2O
A.
B.
Catalase
Oxidase
C.
D.
Peroxidase
Superoxide dismutase
49
Q’s
1. Which growth phase is also called the
exponential growth phase?
A.
B.
C.
D.
Lag phase
Log phase
Stationary phase
Death phase
50
Q’s
Which test tube in the figure shows how a bacterial
culture of facultative anaerobic bacteria would
grow?
51
Q’s
During which stage in the growth curve shown in
the figure is the number of cells dying greater than
the number of cells dividing?
A.
B.
C.
D.
Lag phase
Log phase
Stationary phase
Death phase
52