Other Environmental Factors Affecting Growth

Question 1

What is the pH in which microorganisms can live?

Answer 1

 Microbes can live across a pH range (with minimum and maximum) but have an optimum pH

Question 2

What are the types of microorganisms based on the pH of the environment

Answer 2

1) Neutrophiles
 pH optimum: 6-8
 Most microbes
 Tolerant most environments (pH 5-9)
2) Acidophiles
 At or below pH 5
 Environment: many of our foods
 Type of organisms: mostly fungi, some bacteria, acidophilic archaea
 Cytoplasmic membrane stabilized by increased proton concentration (low intracellular pH)
i. If proton concentration drops  cell lysis
3) Alkaliphiles
 At or above pH 8
 Environment: soap solutions, household cleaning agents

Question 3

Why is osmotic pressure important for microbial growth?

Answer 3

 All living cells require liquid water because water is the medium for all metabolism (all metabolism occurs in aqueous environment)

Question 4

How is water availability defined?

Answer 4

1) Water content within environment (wet vs. dry)
2) Solute concentration within environment i.e. osmotic pressure of the environment
 Water will flow in or out depending on solute concentration

Question 5

What are the types of microbes based on the environmental solute concentrations?

Answer 5

1) Halophiles: “salt-loving”: require a certain salt concentration to survive (except halotolerant)

2) Osmophiles: require high sugar concentration
 E.g. fungi (yeast and mold)

3) Xerophiles
 Found in extremely dry environments e.g. dry rocks or dry soil

Question 6

What are the different types of halophiles?

Answer 6

I. Mild halophiles (1-6% salt)
• Most organisms since sea water is 3% NaCl

II. Moderate halophiles (7-15% salt)

III. Extreme halophiles (15-30% salt i.e. near or at saturation of salt in water)
• Great Salt Lake: about 30% NaCl concentration
• Dead Sea: 15% NaCl and 15 MgCl2

IV. Halotelrant
• Tolerant of high NaCl concentration but survive best without NaCl
• E.g. S. epidermidis that lives on skin

Question 7

What are the necessary molecular adaptations for high solute environments?

Answer 7

1) Pumps: inorganic ion pumps
a. E.g. pumping K+ to counteract high solute concentration outside (not very costly to cell)
2) Synthesis of compatible solutes: intracellular non-reactive organic solutes
a. Usually monosaccharides, polyalcohols or derivatives of some hydrophilic amino acids
b. Compatible solutes are water soluble and non-reactive: they are not utilized so don’t need to constantly regenerate them

Question 8

What are compatible solutes?

Answer 8

Intracellular non-reactive organic solutes

a. Usually monosaccharides, polyalcohols or derivatives of some hydrophilic amino acids
b. Compatible solutes are water soluble and non-reactive: they are not utilized so don’t need to constantly regenerate them

Question 9

What are the 2 pathways to produce ATP

Answer 9

 O2 is used for ATP production: final electron acceptor in oxidative phosphorylation, also known as respiration
 Without O2, fermentation is used to produce ATP

Question 10

What are the classes of microorganisms based on oxygen utilization?

Answer 10

1) Obligate aerobes
 Require full O2 tension for growth i.e. atmospheric O2 concentration (21%)
 Require full tension because they only have 1 pathway for energy generation: oxidative phosphorylation
2) Obligate anaerobes
 Organisms that are killed by the normal 21% O2; they don’t require O2
 They possess O2 intolerant enzymes
 Use fermentation for energy generation
3) Facultative aerobes
 Generate energy by
• Oxidative phosphorylation: always used when O2 is available
• Fermentation: used as backup when O2 levels are low
4) Microaerophiles
 Require O2 but less than 21%
 Use oxidative phosphorylation
 Possess some O2 sensitive enzymes that can’t function at 21% O2
5) Aerotolerant anaerobe
 Don’t use oxidative phosphorylation
 Only use fermentation for energy generation
 Don’t have O2 intolerance

Question 11

What are the toxic by-products created by the reduction of O2 to water?

Answer 11

1) Superoxide anion O2-  very reactive
2) Hydrogen peroxide H2O2  very reactive (oxidizing)
3) Hydroxyl radical OH·  very reactive but quickly degrades

Question 12

What are the molecular adaptations necessary for O2 utilization?

Answer 12

Enzymes
1)	Superoxide dismutase
	Degrade O2- to H2O2
2)	Catalase
	Degrades H2O2 to H2O and O2
3)	Peroxidase
	Reduces H2O2 to 2 H2O

Cells have enzyme 1) and either 2) or 3)