Nutrition and Cultivation of Bacteria
Page 2

See credits under title on previous page.


These pages are subdivided as follows: (Click on the X.)

ON PREVIOUS PAGE (after introduction):
  X  Nutritional Classification of Microorganisms (based on energy and
      carbon requirements)
  X  Other Nutritional and Physical Requirements
  X  Putting Together a Culture Medium

ON THIS PAGE:
  X  Solid Media
  X  Classification of Culture Media
  X  Summary of Commonly-Used Constituents in Microbiological Media


IV. Solid Media

Agar is the major solidifying agent used in bacteriological media. It is an impure polysaccharide gum obtained from certain marine algae. It is added as a powder at a more or less standard concentration (1.5% for plates and slanted media, 0.5% or less for "semisolid" media), usually after the other medium components have been added and dissolved in the water. Agar dissolves at approximately 100°C, and an agar-containing medium thus heated will not solidify until the temperature is brought down to about 43°C. Once solidified, the medium will not melt until brought back up to about 100°C. Among the advantages of this interesting temperature-related property are the following: (1) The medium can be inoculated while in a liquid state at a low enough temperature (approx. 43-50°C) such that the cells will not die off, and (2) the medium, once solidified, will stay solid over a wide range of incubation conditions.

Two additional attributes of agar are its resistance to degradation by nearly all organisms and its relative clarity, permitting easy viewing of growth on or in the medium. One drawback to agar is the fact that it is very difficult, if not impossible, to purify it fully of trace impurities. Thus, when agar is added to a chemically-defined liquid medium, the medium must be considered complex. If an absolutely chemically-defined solid medium is required, silicon-based solidifying agents can be employed.

Previous to agar, potato slices and gelatin were utilized to form solid substrates upon which microbial colonies could be grown and studied. These materials were unacceptable for general use due to their ability to be broken down by a wide variety of microorganisms. Furthermore, gelatin liquefies in a warm room, and potato slices are opaque. In 1881, Fanny Eilshemius Hesse, a technician in the laboratory of Robert Koch in Germany, introduced the concept of agar to bacteriology, having used it for many years in the preparation of homemade jellies.


V. Classification of Culture Media

A classification of media based on their respective uses follows. Note that these categories can overlap. Furthermore, by now you should be using these terms correctly: Medium is always the singular form of the word, and media is always (and only!) the plural form.


VI. Summary of Commonly-Used Constituents in Microbiological Media


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Page content was last modified on 8/2/04 at 7:15 PM, CDT.
John Lindquist, Department of Bacteriology,
University of Wisconsin – Madison