Other Environmental Growth Factors

Gas Requirements

Cells are grown and maintained at an appropriate temperature and gas mixture of oxygen, carbon dioxide, and nitrogen in a cell incubator.

Learning Objectives

Compare different gas requirements of various microbes

Key Takeaways

Key Points

  • Culture conditions vary greatly for each cell type. The variation of conditions for a particular cell type can result in different phenotypes.
  • Capnophiles are microorganisms that thrive in the presence of high concentrations of carbon dioxide.
  • Diazotrophs are microorganisms that fix atmospheric nitrogen gas into a more usable form such as ammonia.

Key Terms

  • capnophile: A microorganism that requires or grows best in presence of high concentrations of carbon dioxide.
  • diazotroph: A microorganism that can fix nitrogen.

Cells are grown and maintained at an appropriate temperature and gas mixture (typically, 37°C and a mixture of oxygen, carbon dioxide, and nitrogen) in a cell incubator. Culture conditions vary greatly for each cell type. The variation of conditions for a particular cell type can result in different phenotypes.

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Bacteriological incubator: Cells are grown and maintained at an appropriate temperature and gas mixture of oxygen, carbon dioxide, and nitrogen in a cell incubator.

Capnophiles are microorganisms that thrive in the presence of high concentrations of carbon dioxide. Typically, in a cell culture the CO2 concentration is around 5%. Some capnophiles may have a metabolic requirement for carbon dioxide, while others merely compete more successfully for resources under these conditions.

Diazotrophs are microorganisms that fix atmospheric nitrogen gas into a more usable form such as ammonia. A diazotroph is an organism that is able to grow without external sources of fixed nitrogen. Some example free-living diazotrophs include:

1) obligate anaerobes that cannot tolerate oxygen even if they are not fixing nitrogen. They live in habitats low in oxygen, such as soils and decaying vegetable matter.

2) Facultative anaerobes that can grow either with or without oxygen, but they only fix nitrogen anaerobically. Often, they respire oxygen as rapidly as it is supplied, keeping the amount of free oxygen low.

3) Aerobes that require oxygen to grow, yet their nitrogenase is still debilitated if exposed to oxygen.

4) Oxygenic photosynthetic bacteria generate oxygen as a by-product of photosynthesis, yet some are able to fix nitrogen as well.

5) And finally, Anoxygenic photosynthetic bacteria that do not generate oxygen during photosynthesis as they have only a single photosystem which cannot split water. In addition, nitrogenase is expressed under nitrogen limitation.

Some higher plants, and some animals (termites), have formed associations (symbioses) with diazotrophs. Examples of those diazotrophs include: rhizobia that associate with legumes, plants of the Fabaceae family, frankias, and cyanobacteria that associate with fungi as lichens, with liverworts, with a fern, and with a cycad.

Osmotic Pressure

The correct osmotic pressure in the culture medium is essential for the survival of the cells.

Learning Objectives

Describe osmotic effects

Key Takeaways

Key Points

  • Osmosis is the net movement of solvent molecules through a partially permeable membrane into a region of higher solute concentration in order to equalize the solute concentrations on the two sides.
  • Osmosis provides the primary means by which water is transported into and out of cells.
  • Osmoregulation is the homeostasis mechanism of an organism to reach balance in osmotic pressure.
  • If the medium is hypotonic, the cells will gain water through osmosis.
  • If the medium is hypertonic, the cells will lose water through osmosis.

Key Terms

  • osmosis: the net movement of solvent molecules from a region of high solvent potential to a region of lower solvent potential through a partially permeable membrane
  • hypotonic: Having a lower osmotic pressure than another.
  • isotonic: Having the same osmotic pressure.
  • hypertonic: Having a greater osmotic pressure than another.
  • halophile: Organisms that thrive in high salt concentrations.

Osmotic pressure is an important factor that affects cells. Osmosis is the net movement of solvent molecules through a partially permeable membrane into a region of higher solute concentration. The intent of osmosis is to equalize the solute concentrations on the two sides. Osmosis is essential in biological systems because biological membranes are semi permeable. In general, these membranes are impermeable to large and polar molecules such as ions, proteins, and polysaccharides. However, they are permeable to non-polar and/or hydrophobic molecules like lipids as well as to small molecules like oxygen, carbon dioxide, nitrogen, nitric oxide, etc. Osmosis provides the primary means by which water is transported into and out of cells. Osmoregulation is the homeostasis mechanism of an organism to reach balance in osmotic pressure.

Having the correct osmotic pressure in the culture medium is essential. A cell can be influenced by a solution in three ways. Suppose a cell is placed in a solution of sugar or salt water. If the medium is hypotonic — a diluted solution with a higher water concentration than the cell — the cell will gain water through osmosis. If the medium is isotonic — a solution with exactly the same water concentration as the cell — there will be no net movement of water across the cell membrane. If the medium is hypertonic — a concentrated solution with a lower water concentration than the cell — the cell will lose water by osmosis.

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Osmotic Pressure on Red Blood Cells: Effect of different solutions on blood cells.

Essentially, this means that if a cell is put in a solution that has a solute concentration higher than its own, then it will shrivel up. If it is put in a solution with a lower solute concentration than its own, the cell will expand and burst.

Obligate and Facultative Halophiles

A halophile is a microorganism that can survive and replicate in a high salt concentration environment (high osmotic pressure).

Obligate halophiles are microorganisms that can only survive in high salt concentration environments.  Facultative halophiles are able to survive in both high and normal salt concentration environments.

Microbial Growth at Low or High pH

Microorganisms live and thrive within specific pH levels.

Learning Objectives

Differentiate microbial growth at high or low pHs

Key Takeaways

Key Points

  • Neutrophiles are organisms that thrive in neutral (pH 7) environments.
  • Alkaliphiles are microbes that thrive in alkaline (pH 9-11) environments.
  • Acidophilic organisms are those that thrive under highly acidic conditions (usually at pH 2.0 or below).

Key Terms

  • neutrophile: any organism that thrives in a relatively neutral pH
  • alkaliphile: any organism that lives and thrives in an alkaline environment, such as a soda lake; a form of extremophile
  • acidophile: an organism that lives and thrives under acidic conditions; a form of extremophile
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pH scale: A pH scale with annotated examples of chemicals at each integer pH value

In chemistry, pH is a measure of the activity of the (solvated) hydrogen ion. In other words, it is a measure of hydrogen ion concentration. Pure water has a pH very close to 7 at 25°C. Solutions with a pH less than 7 are said to be acidic, and solutions with a pH greater than 7 are said to be basic or alkaline. The pH scale is traceable to a set of standard solutions whose pH is established by international agreement. The pH of different cellular compartments, body fluids, and organs is usually tightly regulated in a process called acid-base homeostasis. Microorganisms live and thrive within specific pH levels.

Neutrophiles are organisms that thrive in neutral (pH 7) environments; extromophiles are organisms that thrive in extreme pH environments.

Alkaliphiles are microbes that thrive in alkaline environments with a pH of 9 to 11, such as playa lakes and carbonate-rich soils. To survive, alkaliphiles maintain a relatively low alkaline level of about 8 pH inside their cells by constantly pumping hydrogen ions in the form of hydronium ions (H3O+) across their cell membranes and into their cytoplasm.

Acidophilic organisms are those that thrive under highly acidic conditions (usually at pH 2.0 or below). Most acidophile organisms have evolved extremely efficient mechanisms to pump protons out of the intracellular space in order to keep the cytoplasm at or near neutral pH. Therefore, intracellular proteins do not need to develop acid stability through evolution. However, other acidophiles, such as Acetobacter aceti, have an acidified cytoplasm which forces nearly all proteins in the genome to evolve acid stability.

Oxygen

Oxygen requirements vary among microorganisms.

Learning Objectives

Identify the role of oxygen in microbial growth

Key Takeaways

Key Points

  • An aerobic organism or aerobe is an organism that can survive and grow in an oxygenated environment.
  • An anaerobic organism or anaerobe is any organism that does not require oxygen for growth.
  • Normal microbial culturing occurs in an aerobic environment which poses a problem when culturing anaerobes; requiring one of a number of techniques to be used to keep oxygen out of the culturing setup.

Key Terms

  • anaerobic: Without oxygen; especially of an environment or organism.
  • aerobic respiration: metabolic reactions and processes that take place in the cells of organisms and require oxygen to convert biochemical energy from nutrients into adenosine triphosphate (ATP)
  • aerobic: Living or occurring only in the presence of oxygen.
  • aerotolerant anaerobe: an organism that does not require oxygen to sustain its metabolic processes, but is able to survive in the presence of oxygen

An aerobic organism or aerobe is an organism that can survive and grow in an oxygenated environment. Several varietis of aerobes exist. Obligate aerobes require oxygen for aerobic cellular respiration. In a process known as cellular respiration, these organisms use oxygen to oxidize substrates (for example sugars and fats) in order to obtain energy. Facultative anaerobes can use oxygen, but also have anaerobic (i.e. not requiring oxygen) methods of energy production. Microaerophiles are organisms that may use oxygen, but only at low concentrations. Aerotolerant organisms can survive in the presence of oxygen, but they are anaerobic because they do not use it as a terminal electron acceptor.

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Identity of aerobic and anaerobic bacteria: Aerobically different bacteria behave differently when grown in liquid culture: 1) Obligate aerobic bacteria gather at the top of the test tube in order to absorb maximal amount of oxygen. 2) Obligate anaerobic bacteria gather at the bottom to avoid oxygen. 3) Facultative bacteria gather mostly at the top, since aerobic respiration is advantageous (ie, energetically favorable); but as lack of oxygen does not hurt them, they can be found all along the test tube. 4) Microaerophiles gather at the upper part of the test tube but not at the top. They require oxygen, but at a lower concentration. 5) Aerotolerant bacteria are not affected at all by oxygen, and they are evenly spread along the test tube.

An anaerobic organism or anaerobe is any organism that does not require oxygen for growth. It could possibly react negatively and may even die if oxygen is present. For practical purposes there are three categories: obligate anaerobes, which cannot use oxygen for growth and are even harmed by it. Aerotolerant organisms, which cannot use oxygen for growth, but tolerate the presence of it. And finally, facultative anaerobes, which can grow without oxygen but can utilize oxygen if it is present.

Since normal microbial culturing occurs in atmospheric air, which is an aerobic environment, the culturing of anaerobes poses a problem. Therefore, a number of techniques are employed by microbiologists when culturing anaerobic organisms, for example, handling the bacteria in a glovebox filled with nitrogen or the use of other specially-sealed containers.

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Glovebox: Terra Universal 100 Glovebox

The GasPak System is an isolated container that achieves an anaerobic environment by the reaction of water with sodium borohydride and sodium bicarbonate tablets to produce hydrogen gas and carbon dioxide. Hydrogen then reacts with oxygen gas on a palladium catalyst to produce more water, thereby removing oxygen gas.