Solutes and Water Activity

A selectively permeable plasma membrane separates microorganisms from their environment. Microorganisms can be affected by changes in the osmotic concentration of their surroundings. If a microorganism is placed in a hypotonic solution (one with a lower osmotic pressure), water will enter the cell and cause it to burst unless something is done to prevent the influx.

Most bacteria, algae, and fungi have rigid cell walls that maintain the shape and integrity of the cell. When microorganisms with rigid cell walls are placed in a hypertonic environment, water leaves and the plasma membrane shrinks away from the wall, a process known as plasmolysis. This dehydrates the cell and may damage the plasma membrane; the cell becomes metabolically inactive and ceases to grow.

Many microorganisms keep the osmotic concentration of their protoplasm somewhat above that of the habitat by the use of compatible solutes, so that the plasma membrane is always pressed firmly against their cell wall. Compatible solutes are solutes that are compatible with metabolism and growth when at high intracellular concentrations.

• Most procaryotes increase their internal osmotic concentration in a hypertonic environment through the synthesis or uptake of choline, betaine, proline, glutamic acid, and other amino acids; elevated levels of potassium ions are also involved to some extent.
• Algae and fungi employ sucrose and polyols—for example, arabitol, glycerol, and mannitol—for the same purpose. Polyols and amino acids are ideal solutes for this function because they normally do not disrupt enzyme structure and function.
• Since protozoa do not have a cell wall, they must use contractile vacuoles to eliminate excess water when living in hypotonic environments.
•  A few procaryotes like Halobacterium salinarium raise their osmotic concentration with potassium ions (sodium ions are also elevated but not as much as potassium). Halobacterium’s enzymes have been altered so that they actually require high salt concentrations for normal activity. 

Thus, the osmotic concentration of the cytoplasm is reduced by use of inclusion bodies which store solutes. Procaryotes also can contain pressure-sensitive channels that open to allow solute escape when the osmolarity of the environment becomes much lower than that of the cytoplasm.

Water activity

The amount of water available to microorganisms can be reduced by interaction with solute molecules (the osmotic effect) or by adsorption to the surfaces of solids (the matric effect). Because the osmotic concentration of a habitat has such profound effects on microorganisms, the degree of water availability is critical in growth 

We express water availability as water activity (aw) (also may be expressed as water potential, which is related to aw). It is equivalent to the ratio of the solution’s vapor pressure (Psoln) to that of pure water (Pwater).

aw =  Psoln

          Pwater

Water activity is inversely related to osmotic pressure; if a solution has high osmotic pressure, its aw is low.

Microorganisms differ greatly in their ability to adapt to habitats with low water activity. A microorganism must expend extra effort to grow in a habitat with a low aw value because it must maintain a high internal solute concentration to retain water. 

Some microorganisms can do this and are osmotolerant; they will grow over wide ranges of water activity or osmotic concentration. For example, Staphylococcus aureus can be cultured in media containing any sodium chloride concentration up to about 3 M. It is well adapted for growth on the skin. The yeast Saccharomyces rouxii will grow in sugar solutions with aw values as low as 0.6. The alga Dunaliella viridis tolerates sodium chloride concentrations from 1.7 M to a saturated solution.

Although a few microorganisms are truly osmotolerant, most only grow well at water activities around 0.98 (the approximate aw for seawater) or higher. This is why drying food or adding large quantities of salt and sugar is so effective in preventing food spoilage. Many fungi are osmotolerant and thus particularly important in the spoilage of salted or dried foods. 

Pressure

Most organisms spend their lives on land or on the surface of water, always subjected to a pressure of 1 atmosphere (atm), and are never affected significantly by pressure. Yet the deep sea (ocean of 1,000 m or more in depth) is 75% of the total ocean volume. The hydrostatic pressure can reach 600 to 1,100 atm in the deep sea, while the temperature is about 2 to 3°C. Despite these extremes, bacteria survive and adapt. Many are barotolerant: increased pressure does adversely affect them but not as much as it does nontolerant bacteria. Some bacteria in the gut of deep-sea invertebrates are barophilic—they grow more rapidly at high pressures. These gut bacteria may play an important role in nutrient recycling in the deep sea. Barophiles have been found among several bacterial genera (e.g., Photobacterium, Shewanella, Colwellia). Some members of the Archaea are thermobarophiles (e.g., Pyrococcus spp., Methanococcus jannaschii). 

Radiation

Many forms of electromagnetic radiation are very harmful to microorganisms. This is particularly true of ionizing radiation, radiation of very short wavelength or high energy, which can cause atoms to lose electrons or ionize. Two major forms of ionizing radiation are (1) X rays, which are artificially produced, and (2) gamma rays, which are emitted during radioisotope decay.

Low levels of ionizing radiation will produce mutations and may indirectly result in death, whereas higher levels are directly lethal. Although microorganisms are more resistant to ionizing radiation than larger organisms, they will be destroyed by a sufficiently large dose. A variety of changes in cells are due to ionizing radiation; it breaks hydrogen bonds, oxidizes double bonds, destroys ring structures, and polymerizes some molecules. Ionizing radiation can be used to sterilize items.

Some procaryotes (e.g., Deinococcus radiodurans) and bacterial endospores can survive large doses of ionizing radiation.

Extremophiles

The ability of some microorganisms to adapt to extreme and inhospitable environments is exceptional. Procaryotes are present anywhere life can exist. Many habitats in which procaryotes thrive would kill most other organisms. Procaryotes such as Bacillus infernus are  able to live over 1.5 miles below the Earth’s surface, without oxygen and at temperatures above 60°C.

Microorganisms that grow in such harsh conditions are often called extremophiles,

Thus, the various environmental factors have a great effect on the growth and distribution of microorganisms. Microorganisms must be able to respond to variations in nutrient levels, and chemical and physical nature of their surroundings. An understanding of environmental influences aids in the control of microbial growth and the study of the ecological distribution of microorganisms.