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Microbial Growth

pp. 160-177

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Cell wallPartially formed cross wall

DNA (nuclear area)

Fig. 6.11

Part B

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• Physical and chemical requirements for growth

• Various types of culture media• Bacterial cell division• Phases of bacterial growth

Ch 6 Overview

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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

millionsBacterial colonies

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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

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Physical requirements:• Temperature • pH• Osmotic

pressure

• Carbon, Nitrogen, sulfur, phosphorus, trace elements, organic growth factors

• Oxygen

Chemical requirements:

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Temperature

• Classified in 3 groups

1. Psychrophiles (cold-loving)

2. Mesophiles (moderate-temp-loving)

3. Thermophiles (heat-loving)

Psychrophiles Mesophiles Thermophiles

Room T

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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

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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

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Optimum Growth Rate

‒ Fastest reproduction = peak of curve

‒ At either T extreme = rate drops

‒ At T just above optimum = fastest drop

Fig. 6.1

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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

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Psychrotroph A slight ‘variation’ of a psychrophile

– Can grow at Oo C– Slightly higher optimal growth T ~ 20-30o

C– Also, very sensitive to higher T– Will not grow above ~40o C– Much more common than psychrophiles– Pathogenesis: frequent cause of food

spoilage

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• 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

Refrigeration

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1. To slow growth of– Most spoilage

microbes

2. To prevent growth of– Most pathogenic

species

O F O C

Fig. 6.2

Set T in Refrigerator at a Low Setting

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– Optimal growth ~ 37o C– Most common bacteria– Aerobic– Can be pathogenic– But also, can do work of

industrial importance

Mesophiles

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• Produced by – Propionibacteria

freudenreichii

– Grows at 24o C

– Ferments lactate to acetate, propionate & CO2

– Acetate, propionate give the cheese its flavor

‘Swiss’ Cheese

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‒ Growth: ~50-60o C, and higher

‒ Habitat: hot springs‒ H2S provides source

of energy

Thermophiles

Boulder SpringLower Geyser BasinYellowstone Nat’l Park

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• Describes acidity or alkalinity of a solution

• Most bacteria grow in a neutral environment– Between ~6.5 and

7.5

• Example:– Escherichia coli

pH

Fig. 2.7

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• ph below ~4 is acidic• Preserves foods

..sauerkraut, pickles

• Bacterial fermentation produces acids

• Called acidophiles

Helicobacter pylori

Fig. 25.14

Fig. 2.7

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Solute = [NaCl]

Osmotic Pressure

Fig. 6.4

• The pressure [NaCl] is equal across the cell membrane

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Refers to the concentration of NaCl

Osmotic Pressure

Solute = [NaCl]

Chapter 4, p. 93-4, Fig 4.18

1. Inside a structure

2. And in the solution surrounding the structure

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Osmotic pressure

[NaCl] concentration

is equal across the

membrane

Isotonic Pressure

Fig. 6.4

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Osmotic pressure [NaCl] concentration

is:

Hypertonic Pressure

1. Higher in the solution surrounding the bacterium

2. Than inside the bacterium

3. Causes death of the bacterium

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Osmotic pressure [NaCl] concentration

is:

Hypotonic Pressure

Chapter 4, p. 93-4, Fig 4.18d

1. Lower in the solution surrounding the bacterium

2. Than inside the bacterium

3. Also causes death

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1. Water leaves the cell

2. Enters the high solute [NaCl]

3. Causes shrinkage of the cell cytoplasm

4. Cell death

Plasmolysis

-Loss of Water-

High salt used to preserve foods (or sugar)

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Microbes must acquire nutrients from its environment to grow & reproduce

Chemical Requirements

Water, carbon, nitrogen, minerals (S, P, Ca, Mg, Na, etc)

• In the lab, we provide nutritional needs– In culture media– Media must contain

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• 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

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• 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

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Oxygen

Classification based on oxygen requirements

1. Obligate aerobes

2. Facultative anaerobes

3. Obligate anaerobes

4. Aerotolerant anaerobes

5. Microaerophiles

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• Microbes that use oxygen – Produce more energy from nutrients

– Than microbes that do not use oxygen

• Obligate aerobes require oxygen to live

1. Obligate Aerobes

Examples: Bacillus, Serratia, Pseudomonas, Vibrio

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• 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

2. Facultative Anaerobes

Examples: Escherichia coli, Shigella, Salmonella

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• Microbes unable to use oxygen for energy yielding reactions

• Most are killed by oxygen

3. Obligate Anaerobes

Example: Clostridium (botulism, tetanus)

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• Cannot use oxygen for growth

• But, can tolerate it– Posses enzyme SOD which neutralizes

oxygen

• Mostly ferment carbohydrates to lactic acid

4. Aerotolerant Anaerobes

Example: Lactobacillus

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• Microaerophiles are aerobic

• They require oxygen

• But, only in LOW [O2]’s

• Sensitive to superoxide free radicals– They do not have the SOD enzyme

5. Microaerophile

Example: Micrococcus

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O2 and Enzymes

Aerobes, facultative anaerobes & aerotolerant anaerobes must have

the enzyme:

Superoxide dismutase

(2O2– + 2H+ O2 + H2O2)

See Lab experiment 14

Peroxide is toxic to cells

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And aerobes & facultative anaerobes must have either:

Catalase

See Lab experiment 14

(2H2O2 2H2O + O2)

Peroxidase

(H2O2 + 2H+ 2H2O)

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• Called binary fission

• A single cell divides into 2 cells

• A few species divide by ‘budding’

Bacterial Cell Division

Fig. 6.11

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Pure cultures are obtained by the

streak plate method

Streak Plate Method

Fig. 6.10a,b; Ch 6, p 173-4

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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

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Bacterial division occurs according to a logarithmic progression

Logarithmic Representation of Bacterial Populations

Fig. 6.12 b

– 2 cells, 4 cells, 8 cells, 16 cells, 32 cells… (yx)

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Bacterial Growth Curve

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Q’s

1. Which of these terms describes a microorganism that grows at human body temperature (37o C)?

A. ThermophileB. PsychrophileC. Psychrotroph

D. MesophileE. Hyperthermophile

2. During log phase, bacteria are:A. Dividing at fastest rate

B. Preparing to divide

C. Dying exponentially

D. Dying and dividing in equal numbers

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Q’s

1. Which of these terms describes microbes that do not use oxygen, but can grow if it is present?

2. Which of these is an example of an organic growth factor?

A. Obligate aerobe

B. Obligate anaerobe

C. Facultative anaerobe

D. Microaerophile

E. Aerotolerant anaerobe

A. Glucose B. Oxygen C. Biotin

D. Sodium chloride E. Agar

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Appendix

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1. Lag phase: – Little or no change in the number of

cells, but metabolic activity is high2. Log phase:

– Bacteria multiply at the fastest rate possible under the conditions provided

3. Stationary phase: – An equilibrium between cell division and

death occurs4. Death phase:

– Deaths exceed the number of new cells formed

Bacterial Growth Curve - Phases

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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. Thermophiles

B. PsychrotrophsC. Mesophiles D. psychrophiles

3. The _____ _____ _____ is the most common technique used to obtain pure cultures of microbes.

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1. Organisms such as Clostridium lack oxygen-detoxifying enzymes, thus they are:

A. Obligate aerobes

B. Obligate anaerobes

C. Facultative anaerobes

D. Microaerophiles

E. Aerotolerant anaerobes

2. Which enzyme catalyzes the reaction:

H2O2 + 2H+ 2H2O

A. CatalaseB. Oxidase

C. Peroxidase D. Superoxide dismutase

Q’s

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1. Which growth phase is also called the exponential growth phase?

A. Lag phase

B. Log phase

C. Stationary phase

D. Death phase

Q’s

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Which test tube in the figure shows how a bacterial culture of facultative anaerobic bacteria would grow?

Q’s

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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. Lag phase

B. Log phase

C. Stationary phase

D. Death phase

Q’s