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Summary Microbial growth and extremophiles
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HC6 Microbial growth &Extremophiles (BOOK)
Chapter 5.9 – 5.17
CH5 Microbial Growth and Regulation
5.9 Temperature Classes of Microorganisms
For every microorganism there is a minimum temperature below which growth is impossible, an
optimum temperature at which growth is most rapid and a maximum temperature above which
growth is not possible —> the cardinal temperatures.
It is possible to distinguish four broad classes of microorganisms in relation to their growth
temperature optima:
- Physchrophiles – low temperature optima
- Mesophiles – midrange temperature optima
- Thermophiles – high temperature optima
- Hyperthermophiles – very high temperature optima
5.10 Microbial Life in the Cold
A psychrophile is a microbe with an optimal growth temparure of 15˚C or lower, a maximum growth
temperature of 20˚C and a minimum growth temperature of 0˚C or lower. Microbes that grow at 0˚C
but have optima of 20-40˚C are called psychrotolerant.
Psychrophiles produce enzymes that function in the cold and that may be denaturated or otherwise
inactivated at even very moderate temperatures (more a-helixes and greater polar and lesser
hydrophobic amino acid content). Also the cytoplasmic membrane remains functional at low
temperatures (higher content of unsaturated and shorter-chain fatty acids).
5.11 Microbial Life at High Temperatures
The temperature optimum of thermophiles exceeds 45˚C and those of hyperthermophiles exceeds
80˚C.
The most extreme high-temperature environments in nature are hot springs.
How do thermophiles and hyperthermophiles survive high temperatures? One thing is that their
enzymes and other proteins are much more heat-stable and actually function optimally at high
temperatures. This is mostly due to subtle changes in amino acid sequence. Heat stable proteins
, typically also show increased ionic bonding between basic and acidic amino acids and have highly
hydrophobic interiors —> all these factors prevent unfolding. Another thing is that specific solutes
are made in large amounts that help stabilise proteins against thermal denaturation.
In thermophiles and most hyperthermophilic Bacteria, the cytoplasmic membrane has a higher
content of long-chain and saturated fatty acids and a lower content of unsaturated fatty acids.
Hyperthermophiles do not contain fatty acids in their membrane but instead have C40 hydrocarbons
composed of repeating units of isoprene bonded by ether linkage to glycerol phosphate. The
membrane forms a lipid monolayer rather than a bilayer.
5.12 Effects of pH on Microbial Growth
The optimal pH for growth of an organism refers to the extracellular envirenment only, the
intracellular pH must be maintained between a pH of 5 and 9. To prevent major shifts in pH during
microbial growth in batch cultures, buffers are commonly added.
5.13 Osmolarity and Microbial Growth
Water availability is expressed in terms of water activity (aw) which is the ratio of the vapor pressure
of air in equilibrium with a substance or solutions to the vapor pressure of pure water. Value of no
free water = 0 and pure water = 0.
Halophiles are organisms that show an NaCl requirement and grow optimally at the a w of seawater,
0.98.
In contrast to halophiles, halotolerant can change some levels of dissolved solutes but grow best in
the absence of the solute. Extreme halophiles are capable of growh in very salty environments.
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