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Tag words: bacterial nutrition, bacterial growth, culture medium, selective medium, minimal medium, enrichment medium, synthetic medium, defined medium, complex medium, fastidious organism, aerobe, anaerobe, obligate anaerobe, facultative anaerobe, aerotolerant anaerobe, superoxide dismutase, catalase, psychrophile, thermophile, extreme thermophile, acidophile, alkalophile, osmophile, osmotolerant, water activity.

Kenneth Todar currently teaches Microbiology 100 at the University of Wisconsin-Madison.  His main teaching interests include general microbiology, bacterial diversity, microbial ecology and pathogenic bacteriology.

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Nutrition and Growth of Bacteria (page 6)

(This chapter has 6 pages)

© Kenneth Todar, PhD

Water Availability

Water is the solvent in which the molecules of life are dissolved, and the availability of water is therefore a critical factor that affects the growth of all cells. The availability of water for a cell depends upon its presence in the atmosphere (relative humidity) or its presence in solution 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 lower 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 0.99; seawater = 0.98; maple syrup = 0.90; Great Salt Lake = 0.75. Water activities in agricultural soils range between 0.9 and 1.0.

The only common solute in nature that occurs over a wide concentration range is salt [NaCl], and some microorganisms are named based on their growth response to salt. Microorganisms that require some NaCl for growth are halophiles. Mild halophiles require 1-6% salt, moderate halophiles require 6-15% salt; extreme halophiles that require 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 halophiles are "osmophiles" (and halotolerant organisms are "osmotolerant") the term osmophiles is usually reserved for organisms that are able to live in environments high in sugar. Organisms which live in dry environments (made dry by lack 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 osmolarity for different classes of procaryotes. Osmolarity is determined by solute concentration in the environment. Osmolarity is inversely related to water activity (Aw), which is more like a measure of the concentration of water (H2O) in a solution. Increased solute concentration means increased osmolarity and decreased Aw. From left to right the graph shows the growth rate of a normal (nonhalophile) such as E. coli or Pseudomonas, the growth rate of a halotolerant bacterium such as Staphylococcus aureus, and the growth rate of an extreme halophile such as the archaean Halococcus. Note that a true halophile grows best at salt concentrations where most bacteria are inhibited.

Table 11. Limiting water activities (Aw) for growth of certain procaryotes.

Organism Minimum Aw for growth
Caulobacter 1.00
Spirillum 1.00
Pseudomonas .91
Salmonella/E. coli .91
Lactobacillus .90
Bacillus .90
Staphylococcus .85
Halococcus .75


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Kenneth Todar is an emeritus lecturer at University of Wisconsin-Madison. He has taught microbiology to undergraduate students at The University of Texas, University of Alaska and University of Wisconsin since 1969.

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