Nutrition and Growth of Bacteria (page 6)
(This chapter has 6 pages)
© Kenneth Todar, PhD
Water is the solvent in which the molecules of life are dissolved,
the availability of water is therefore a critical factor that affects
growth of all cells. The availability of water for a cell depends upon
its presence in the atmosphere (relative humidity) or its presence in
or a substance (water activity). The water activity (Aw)
of pure H2O is 1.0 (100% water). Water activity is affected
by the presence of solutes such as salts or sugars, that are dissolved
in the water. The higher the solute concentration of a substance, the
is the water activity and vice-versa. Microorganisms live over a range
of Aw from 1.0 to 0.7. The Aw of human blood is
seawater = 0.98; maple syrup = 0.90; Great Salt Lake = 0.75. Water
in agricultural soils range between 0.9 and 1.0.
The only common solute in nature that occurs over a wide
range is salt [NaCl], and some microorganisms are named based on their
growth response to salt. Microorganisms that require some NaCl for
Mild halophiles require 1-6% salt, moderate
halophiles require 6-15% salt; extreme halophiles that
15-30% NaCl for growth are found among the archaea. Bacteria that
are able to grow at moderate salt concentrations, even though they grow
best in the absence of NaCl, are called halotolerant. Although
are "osmophiles" (and halotolerant organisms are "osmotolerant") the
is usually reserved for organisms that are able to live in environments
high in sugar. Organisms which live in dry environments (made dry by
of water) are called xerophiles.
The concept of lowering water activity in order to prevent bacterial
growth is the basis for preservation of foods by drying (in sunlight or
by evaporation) or by addition of high concentrations of salt or sugar.
Figure 9. Growth rate vs
for different classes of procaryotes. Osmolarity is determined by
concentration in the environment. Osmolarity is inversely related to
activity (Aw), which is more like a measure of the
of water (H2O) in a solution. Increased solute concentration
means increased osmolarity and decreased Aw. From left to
the graph shows the growth rate of a normal (nonhalophile) such as E.
coli or Pseudomonas, the growth rate of a halotolerant
such as Staphylococcus aureus, and the growth rate of an
halophile such as the archaean Halococcus. Note that a true
grows best at salt concentrations where most bacteria are inhibited.
Table 11. Limiting water
(Aw) for growth of certain procaryotes.
||Minimum Aw for growth
END OF CHAPTER
Return to Page 1