1. The Influence of Environmental Factors on Growth
CHAPTER 7 continued
The Influence of Environmental Factors on Growth

2. The activities of microorganisms are greatly affected by the chemical and physical conditions of their environment.
The understanding of environmental influences helps explain:
the distribution of microorganisms in nature
makes it possible to devise methods for controlling or enhancing microbial activities.  

3. The Influence of Environmental Factors on Growth
most organisms grow in fairly moderate environmental conditions
Extremophiles
grow under harsh conditions that would kill most other organisms
Table 7.4
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4. The four key factors that play major roles in controlling the growth of microorganisms are:
Temperature.
pH.
Water availability (water activity aw).
Oxygen.
Pressure and radiation.

5. Solutes and Water Activity
Water availability is an important factor affecting the growth of microorganisms in nature.
water activity (aw)
amount of water available to organisms
reduced by interaction with solute molecules (osmotic effect)
higher [solute]  lower aw
reduced by adsorption to surfaces (matric effect)
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6. Water availability not only depends on the water content of an environment------how moist or dry a solid microbial habitat may be.
However, it also a function of the concentration of solutes such as salts, sugars, or other substances that are dissolved in water.
This is because dissolved substances have an affinity for water, which makes the water associated with solutes unavailable to organisms

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8. Osmotolerant organisms
grow over wide ranges of water activity
many use compatible solutes to increase their internal osmotic concentration
solutes that are compatible with metabolism and growth
some have proteins and membranes that require high solute concentrations for stability and activity
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9. Effects of NaCl on microbial growth
halophiles
grow optimally at >0.2 M
extreme halophiles
require >2 M
Figure 6.11
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10. Halophiles and Related Microorganisms
Microorganisms found in the sea usually have a specific requirement for the sodium ion (3% NaCl) in addition to growing optimally at the water activity of seawater. Such organisms are called halophiles.
The growth of halophiles requires at least some NaCl, but the optimum varies with the organism.
The terms mild halophile and moderate halophile are used to describe halophiles with low (1-6%) and moderate (7-15%) NaCl requirements, respectively

11. Most microorganisms are unable to cope with environments of very low water activity and either die or become dehydrated and dormant under such conditions.
Halotolerant organisms can tolerate some reduction in the water activity of their environment but generally grow best in the absence of the added solute.
some organisms thrive at very low water activity.
These organisms are of interest in the food industry, where solutes such as salt and sucrose are commonly used as preservatives to inhibit microbial growth.

12. pH Microbial Growth at Low or High pH
Acidity or alkalinity of a solution is expressed by its pH on a scale on which neutrality is pH=7. A change of 1 pH unit represents a 10-fold change in hydrogen ion concentration (pH pH11)

13. pH negative logarithm of the hydrogen ion concentration
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14. pH ….. acidophiles neutrophiles Alkalophiles
growth optimum between pH 0 and pH 5.5
neutrophiles
growth optimum between pH 5.5 and pH 7
Alkalophiles
growth optimum between pH8.5 and pH 11.5
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15. pH……..
most acidophiles and alkalophiles maintain an internal pH near neutrality
some use proton/ion exchange mechanisms to do so
some synthesize proteins that provide protection
e.g., acid-shock proteins
many microorganisms change pH of their habitat by producing acidic or basic waste products
most media contain buffers to prevent growth inhibition
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16. Most natural environments have pH values between 5 and 9, and microorganisms with optima in this range are most common.
Many fungi grow optimally at pH 5 or below, and few grow well at pH values as low as 2.

17. Cardinal temperatures
Temperature can affect living organisms in either of two opposing ways:
As the temperature rises, chemical and enzymatic reactions in the cell proceed at more rapid rates, and growth becomes faster.
above a certain temperature, particular proteins may be irreversibly denatured.
Thus, as the temperature is increased within a given range, growth and metabolic function increase up to a point where denaturation reactions set in. Above this point, cell functions fall sharply to zero.

18. Temperature organisms exhibit distinct cardinal growth temperatures
minimal
maximal
optimal
Figure 6.13
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19. Figure 6.14
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20. Adaptations of thermophiles….
protein structure stabilized by a variety of means
more H bonds
more proline
chaperones
histone-like proteins stabilize DNA
membrane stabilized by variety of means
more saturated, more branched and higher molecular weight lipids
ether linkages (archaeal membranes)
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22. Oxygen Concentration Figure 6.15 need oxygen ignore oxygen
< 2 – 10%
oxygen
prefer
oxygen
oxygen is
toxic
Figure 6.15
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23. Basis of different oxygen sensitivities
oxygen easily reduced to toxic products
superoxide radical
hydrogen peroxide
hydroxyl radical
aerobes produce protective enzymes
superoxide dismutase (SOD)
catalase
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24. Figure 6.14
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25. Pressure barotolerant organisms barophilic organisms
adversely affected by increased pressure, but not as severely as nontolerant organisms
barophilic organisms
require or grow more rapidly in the presence of increased pressure
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26. Radiation
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27. Radiation damage ionizing radiation x rays and gamma rays
Very short wavelength
x rays and gamma rays
disrupts chemical structure of many molecules, including DNA
mutations  death
damage may be repaired by DNA repair mechanisms
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28. Radiation damage… ultraviolet (UV) radiation (200-360nm) (short-long)
mutations  death
causes formation of thymine dimers in DNA
DNA damage can be repaired by two mechanisms
photoreactivation – dimers split in presence of light
dark reactivation – dimers excised and replaced in absence of light

29. Radiation damage… visible light: photooxidation
at high intensities generates singlet oxygen (1O2)
Very reactive, powerful oxidizing agent
carotenoid pigments in airborne bacteria
protect many light-exposed microorganisms from photooxidation
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30. Microbial Growth in Natural Environments
microbial environments are complex, constantly changing
may expose a microorganism to overlapping gradients of nutrients and environmental factors
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32. Growth Limitation by Environmental Factors
Leibig’s law of the minimum
total biomass of organism determined by nutrient present at lowest concentration
Shelford’s law of tolerance
above or below certain environmental limits, a microorganism will not grow, regardless of the nutrient supply

33. Responses to low nutrient levels
oligotrophic environments
morphological changes to increase surface area and ability to absorb nutrients
mechanisms to sequester certain nutrients

34. Counting Viable but Nonculturable Vegetative Procaryotes
stressed microorganisms can temporarily lose ability to grow using normal cultivation methods
microscopic and isotopic methods for counting viable but nonculturable cells have been developed
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35. Quorum Sensing and Microbial Populations
microbial communication and cooperation
involves secretion and detection of chemical signals
Figure 6.20a: generalized structure for acyl homoserine lactone, the best-known quorum sensing signal (autoinducer)
Figure 6.20b: overview of how quorum sensing functions in many gram-negative bacteria; R=receptor protein that acts as an inducer; dashed lines indicate that acyl HSL synthase is not always made in response to autoinducer
Figure 6.20
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36. Processes sensitive to quorum sensing: gram-negative bacteria
bioluminescence (Vibrio fischeri)
synthesis and release of virulence factors (Pseudomonas aeruginosa)
conjugation (Agrobacterium tumefaciens)
antibiotic production (Erwinia carotovora, Pseudomonas aureofaciens)
biofilm production (P. aeruginosa)
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37. Quorum sensing: gram-positive bacteria
often mediated by oligopeptide pheromone
processes impacted by quorum sensing:
mating (Enterococcus faecalis)
transformation competence (Streptococcus pneumoniae)
sporulation (Bacillus subtilis)
production of virulence factors (Staphylococcus aureus)
development of aerial mycelia (Streptomyces griseus)
antibiotic production (S. griseus)
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