The Effect of Physical and Chemical Conditions on Microbial Growth

Question 1

Temperature

Answer 1

is a major environmental factor controlling microbial growth.

Question 2

Cardinal Temperatures

Answer 2

The minimum, optimum, and maximum temperatures at which an organism grows.
- are characteristic of each different organism

Question 3

How can Microorganisms be classified into groups by their growth temperature optima?

Answer 3

• Psychrophile: low temperature
• Mesophile: midrange temperature
• Thermophile: high temperature
• Hyperthermophile: very high temperature

Question 4

What is Mesophiles? Where are they found in?

Answer 4

Organisms that have midrange temperature optima.
- Found in:
- Warm-blooded animals
- Terrestrial and aquautic environments
- Temperate and tropical latitudes

Question 5

Cold-Loving Microorganisms (Extremophiles)

Answer 5

Organisms that grow under very hot or very cold conditions

Question 6

Cold-Loving Microorganisms (Psychrophiles)

Answer 6

• Organisms with cold temperature optima (<20oC)
• Inhabit permanently cold environments
  - Deep ocean, Arctic and Antarctic environments

Question 7

Cold-Loving Microorganisms (Psychrotolerant)

Answer 7

• Organisms that can grow at 0ºC but have optima of 20ºC to 40ºC
• More widely distributed in nature than true psychrophiles

Question 8

Molecular adaptations that support psychrophily

Answer 8

• Production of enzymes that function optimally in the cold
• Modified cytoplasmic membranes
• High unsaturated fatty acid content

Question 9

Heat-Loving Microorganisms

Answer 9

• Above ~65ºC, only prokaryotic life forms exist
• Chemoorganotrophic and chemolithotrophic species are present
• No phototrophy above approx. 70oC
• High prokaryotic diversity
• Both Archaea and Bacteria are represented

Question 10

Thermophiles

Answer 10

organisms with growth temperature optima between 45ºC and 80ºC
- Terrestrial hot springs, very active compost

Question 11

Hyperthermophiles

Answer 11

organisms with optima greater than 80ºC
- Inhabit hot environments, including boiling hot springs and seafloor hydrothermal vents that can experience temperatures in excess of 100ºC

Question 12

Molecular adaptations to thermophily

Answer 12

• Specific modifications provide thermal stability to enzymes and proteins
• Modifications in cytoplasmic membranes to ensure heat stability
  - Bacteria have lipids rich in saturated fatty acids
  - Archaea have lipid monolayer rather than bilayer

Question 13

Hyperthermophiles

Answer 13

produce enzymes widely used in industrial microbiology
- Example: Taq polymerase used to automate the repetitive steps in the
polymerase chain reaction (PCR) technique
- Hydrolytic enzymes including proteases, cellulases and lipases

Question 14

What are the upper temperature limits for life?

Answer 14

• New species of thermophiles and hyperthermophiles are still being discovered
• Laboratory experiments with biomolecules suggest 140–150°C

Question 15

Hyperthermophily & Evolution

Answer 15

• Hyperthermophiles may be the closest descendants of ancient microbes
• Hyperthermophilic Archaea and Bacteria are found on the deepest, shortest branches of the phylogenetic tree
• The oxidation of H2 is common to many hyperthermophiles
• May have been the first energy-yielding metabolism

Question 16

Effects of pH on Microbial Growth

Answer 16

• The pH of an environment greatly affects microbial growth
• Some organisms have evolved to grow
  best at low or high pH
• Most organisms grow best between pH 6 and 8 → neutrophiles

Question 17

Acidophiles

Answer 17

Organisms that grow best at low pH (<6)

Question 18

Alkaliphiles

Answer 18

Organisms that grow best at high pH (>9)

Question 19

Halophiles

Answer 19

grow best at reduced water potential; have a specific requirement for NaCl
- Many marine microbes

Question 20

Extreme halophiles

Answer 20

Require high levels of NaCl for growth
- 15 – 30%
- Ex) Microbes from Great Salt lake or the Dead Sea

Question 21

Halotolerant

Answer 21

can tolerate some reduction in water activity of environment but generally grow best at lower solute concentrations
- Ex) Staphylococcus aureus
- Lives on human skin
- Grows best at low NaCl
- But can tolerate up to 17.5%

Question 22

Osmophiles

Answer 22

Organisms that grow with high sugar as solute

Question 23

Xerophiles

Answer 23

Organisms able to grow in very dry environments

Question 24

Specialized and rare organisms

Answer 24

• Honey, jams and jellies do not have many organisms growing in them
• Beef jerky and salted cod

Question 25

Obligate aerobes

Answer 25

• Require oxygen to live
• grows only in the oxic zone at the top of the tube

Question 26

Strict anaerobes

Answer 26

• do not require oxygen and may even be killed by exposure
• grows only in the anoxic zone at the bottom of the tube

Question 27

Facultative aerobes

Answer 27

• can live with or without oxygen, they use oxygen when it is available
• grows throughout the tube
• Better growth occurs in the oxic zone, where it can generate energy by aerobic
  respiration

Question 28

Aerotolerant anaerobes

Answer 28

• can tolerate oxygen and grow in its presence even though they cannot use it
• grows well throughout the tube
• Doesn’t use O2
• Not harmed by O2

Question 29

Microaerophiles

Answer 29

• can use oxygen only when it is present at levels reduced from that in air
• grows in a narrow band between the oxic and anoxic zones
• Needs O2 for aerobic respiration
• Killed by atmospheric O2 levels

Question 30

Differences in oxygen use/tolerance can be distinguished using?

Answer 30

Thioglycolate Broth

Question 31

Thioglycolate Broth

Answer 31

• Complex medium that separates microbes based on oxygen requirements
• Thioglycolate reacts with oxygen creating an anaerobic environment
• Oxygen can penetrate only the top of the tube
• Contains an oxygen responsive dye that turns pink in the presence of oxygen and colorless when the oxygen is low or absent

Question 32

Special Techniques are needed to grow anaerobic microbes.

Answer 32

1. Reducing agents may be added to culture media to reduce oxygen
   
   • Thioglycolate, cysteine, H2S and other sulfur containing compounds
2. Removal of air, and replacement with an inert gas
   - Ex) nitrogen or argon

Question 33

What are the toxic forms of oxygen that can be formed spontaneously in the cell?

Answer 33

1. Superoxide Anion
2. Hydrogen Peroxide
3. Hydroxyl Radical

Question 34

What are the Enzymes that neutralize these toxic oxygen species?

Answer 34

1. Catalase
2. Peroxidase
3. Superoxide dismutase
4. Superoxide reductase