Each culture
medium is formulated with various macro- and micronutrients that
are essential for microbial growth. Needed in relatively large amounts
are proteins, carbohydrates, lipids, and nucleic acids.
These organic substances provide energy and a source of essential atoms like
carbon, hydrogen, nitrogen, oxygen, phosphorous, and sulfur (i.e.,
CHNOPS). Cells use these to synthesize their own structural and
regulatory components. Also essential, but required in smaller amounts,
are vitamins (e.g., thiamine, niacin, folic acid) and metallic
elements (e.g., Ca++, Zn++, Cu++, Mn++, Mg++, K+, Na+,
Fe+++). These substances are particularly critical for normal
enzyme function and maintenance of protein structure.
Culture media
must also provide cells a source of moisture (water), and a particular tonicity
(osmotic pressure) and hydrogen ion concentration (pH). These
greatly influence microbial growth and survival. For instance, since most
bacteria grow best under isotonic conditions and around a pH of 7 or slightly
lower, culture media are chemically adjusted to provide these requirements.
While
cultivating microbes, atmospheric gases must be considered.
Most microbes are greatly effected by the levels of oxygen
and carbon dioxide available to them. Anaerobic bacteria must be
cultured in a special anaerobic environmental chamber that removes all
oxygen. In contrast, aerobic or facultative anaerobes can be grown in
environments where oxygen is more readily available.
Finally,
microbial growth is dependent on temperature. After inoculation,
cultures are routinely placed in an incubator thermostatically set
within the microbe's optimal temperature range. The microbes you will
work with are incubated at 25º, 30º, or 37ºC. After a specified
incubation period (usually 18-24 hours), these cultures are stored and
preserved by placing them into a refrigerator set around 5ºC. This temperature inhibits growth and
prevents cultures from depleting nutrients and accumulating toxic wastes that
can cause cells to die off more rapidly.