GENERAL MICROBIOLOGY - T.P. Pyrog - 2004

6. MICROBIAL GROWTH

6.3. MICROBIAL NUTRITION

6.3.7. Types of Culture Media for Microorganism Cultivation

Thus, a culture medium for cultivating microorganisms must contain all the elements required for cellular architecture in a form readily accessible for assimilation by the Organism. The primary components of such a medium should include a source of carbon and energy, sources of mineral compounds (nitrogen, phosphorus, sulfur, potassium, magnesium, calcium, iron, and Trace Elements), and, when necessary, growth factors.

If a nutrient solution is formulated from defined chemical compounds, it is referred to a synthetic medium. Researchers attempt to determine the minimal nutritional requirements for each microorganism and formulate a minimal medium containing only the components (ingredients) essential for growth. More fastidious species require a greater variety of additional substances (growth factors). For instance, a synthetic medium comprising over 40 components was developed for Leuconostos mesenteroldes.

Complex media. For many particularly fastidious microorganisms, nutritional requirements remain insufficiently understood. Consequently, they are cultivated on media containing brewer's wort, carrot and plum juices, meat extract, hay infusion, coconut milk, or Yeast extract. To reduce costs, complex mixtures such as whey, treacle, molasses, corn steep liquor, and others are added to nutrient solutions instead of pure compounds. Media of this type are called complex media. Media consisting of products of PLANT AND ANIMAL origin are also referred to as natural media.

Semi-synthetic media, which incorporate compounds of known chemical nature alongside substances of undefined composition, also belong to natural media of indefinite composition. Examples of semi-synthetic media include glucose-peptone broth with potassium phosphate, and potato medium supplemented with glucose and peptone.

Therefore, based on their composition, media are classified into two main groups: synthetic and natural (complex).

Depending on their application, media are distinguished into selective and differential-diagnostic media. Selective media promote the preferential growth of a single species or group of microorganisms while being less suitable (or entirely unsuitable) for the growth of others. Differential-diagnostic (indicator) media enable the rapid differentiation of certain microbial species from others.

Based on their physical state, media are classified into liquid, solid, and loose (bulk) media. Examples of loose media include cooked millet, bran, and quartz sand saturated with a nutrient solution.

Solid media. To prepare solid media—upon which microorganisms grow as colonies—substances imparting a gel-like consistency are added to liquid nutrient solutions. Gelatin is rarely used because it has a low melting point (26–30 °C) and is degraded by numerous microorganisms. An almost ideal agent for solid media preparation is Agar, introduced into bacteriological practice in 1883 by W. Hesse, a German physician and associate of R. Koch. The concentration of agar typically ranges from 15–20 g/L. Agar is a polysaccharide extracted from red marine Algae. Its solutions melt at 100 °C and set into a firm, transparent gel at 44 °C. Only a few specialized Bacteria are capable of degrading agar. When solid media free of Organic compounds are required, silica gel is used as a solidifying agent.