Psychrophiles Flashcards
What are psychrophiles
Microbes which can grow at 0 degrees. However many had growth optima between 20-30 degrees (cold tolerant not cold requiring)
Definition of psychrophiles
- An optima growth temperature of 15 degrees or lower
- A maximal growth temperature below 20 degrees
- A minimum growth temperature of 0 degrees of lower
He defined microbes which grow at 0 - 5 oC, but which have maximal growth temperatures exceeding 25 oC as psychrotrophs (psychrotolerant).
How are psychrophiles isolated and when were the first sequenced
To isolate psychrophiles, samples must be kept below 15 oC at all times.- Heating can kill/immobilise the cells so theres nothing left to cultivate
Have to isolate them in nature. Vast majority of bacteria cannot be cultured but genetic sampling helps to understand
1964- first psychrophilic - Vibrio unavillus. In 2005 there was Colwella psychrerythrae - first genome to be sequenced.
Habitats of psychrophiles
They’re very widespread, 70% of the Earth’s surface is sea water, 90 % of sea water is 5 oC or less.- largest habitat you have for organism
Polar regions constitute 14% of Earth’s surface.
Refrigerated food is a potential habitat - spoilage and pathogenic bacteria may grow at 4 oC.
E.g. now field in the Sierra Nevada, California. The pink colour is caused by the snow algae Chlamydomonas nivalis
Case study: The microbiology of frost flowers
- Frost flowers form at extreme sub-zero temperatures (-22 degrees) when cold moist air becomes saturated and frost begins to form wherever imperfections in the ice offer a nucleation point. These hollow spiky tendrils wick moisture from the ice incorporating salt, marine bacteria and other substances as they grow
Examples of psychrophiles
Theres a wide range within domain bacteria, gram negative more common (psuedomonas, virbrio etc,) than gram positive
Some psychrophilic anaerobes e.g. clostridium in raw milk.
Also psychrophilic yeasts and algae.
Some psychrophilic archaea have been detected
What are some challenges psychrophiles face
- Decrease of cell membrane fluidity
- Exponential decrease in the rate of biochemical reactions as the temperature decreases (causing problems for metabolism
- Reduction of efficiency of transcription and translation due to stabilisation of nucleic acid secondary structures
These challenges result in a downshift of the metabolic flux
Challenges of mesophilic bacteria at low temperatures
When a mesophile is exposed to low temperatures, one reason for its lack of growth is the inability to take up substrates- enzymes don’t work at low temperatures.
Psychrophilic Candida can oxidise exogenous glucose at 0oC, but mesophilic Candida cannot do so below 5oC.
However, mesophile can oxidise endogenous glucose below 5oC, thus transporting glucose into the cell is the limiting factor - as long as its inside the cell
What are the consequences of low temperature on membrane fluidity
Doubles in unsaturated hydrocarbon chains increase phospholipid bilayer fluidity by making it more difficult to pack the chains together
The double bonds induce kinks in the fatty acids so they’re not as tight and therefore increases fluidity
Changes in Fatty Acid composition in Psychrophiles growing at 0oC
1) Increase the proportion of unsaturated fatty acids: a double bond introduces a bend making tight packing of the fatty acids difficult – increasing the fluidity of the membrane.
2) A decrease in the average fatty acid chain length.
3) An increase in the ratio of ante-iso-branching to iso-branching- terminal branches.
These changes are the opposite of the opposite of those found in thermophilic bacterial lipids - they lower the liquid-crystalline to gel transition temperature
How do Psychrophiles change their Lipids?
Desaturase enzymes are used to introduce double bonds into the fatty acids - allows a rapid alteration in membrane fluidity in response to cold stress.
Alterations to lipids are a pre-requisite for growth at low temperatures, but other changes are also essential for psychrophiles.
Because temperature changes- have to have an adaptive process
What are molecular adaptions in psychrophilic
- Increase the fluidity of cellular membranes via unsaturated fatty acid synthesis genes, desaturases, dioxygen lipid desaturases
- Reduction of freezing point of cytoplasm and stabilisation of macromolecules (freeze thaw protection- via the genes for synthesis of compatible solutes, membrane transporters, antifreeze proteins and ice-binding proteins
Cellular response to lowering of temperature (cold-shock and -acclimation response) via signal transduction proteins, RNA-binding proteins and helicases, heat shock proteins, compatible solute synthesis proteins
Protection against ROS due to increased solubility of oxygen at low temperature forming increased ROS via catalases peroxidiases, SODs and oxidoreductases
Maintain catalytic efficiency at low temperatures (proteins and enzymes)
Genome plasticity to increase the adaptation ability to cope with low temperatures via transposases and prophages
How are proteins adapted in psychrophiles and how did they discover this
Protein stability is a subtle change. Some regions appear to be involved in stability while other regions confer flexibility on the protein
Discovered via differential scanning calorimetry measurements of the thermal unfolding of 3 alpha-amylases
They observed these features:
a) a reduction in the number of weak interactions in the folded protein such as salt bridges and hydrogen bonding- makes protein more loose and more flexible.
b) decreased inter-subunit interactions,
c) a lower hydrophobicity in the core of the protein,
d) deletion or substitution of proline residues in loops and turns which favors flexibility of chains connecting secondary structures (increase of and clustering of glycine)
Protein alterations in psychrophiles are not exactly the opposite of those found in thermophiles
