Microbial growth and the relationship with culture media

 

In order to grow and reproduce microorganisms require biochemical factors - nutrients and a source of energy  - and certain biophysical factors - ambient conditions relating to pH, atmosphere and temperature. Take, for example, the nutritional requirements of the bacterium Escherichia coli. The nutrients this organism requires  are shown through the bacterial cell's elemental composition. Such elements are found in the form of water, inorganic ions, small molecules, and macromolecules which serve either a structural or functional role in the cells. For E. coli the major elements consist of those illustrated in Table 1.

 

Table 1:          Nutrient requirements for E. coli       

 

 

Element	Percentage of dry weight	Source	Cellular function
Carbon	50	Organic compounds or CO2	Main constituent of cellular material
Oxygen	20	H2O, organic compounds, CO2, and O2	Constituent of cell material and cell water; O2 is electron acceptor in aerobic respiration
Nitrogen	14	NH3, NO3, organic compounds, N2	Constituent of amino acids, nucleic acids nucleotides, and coenzymes
Hydrogen	8	H2O, organic compounds, H2	Main constituent of organic compounds and cell water
Phosphorus	3	inorganic phosphates (PO4)	Constituent of nucleic acids, nucleotides, phospholipids, LPS, teichoic acids
Sulfur	1	SO4, H2S, So, organic sulfur compounds	Constituent of cysteine, methionine, glutathione, several coenzymes
Potassium	1	Potassium salts	Main cellular inorganic cation and cofactor for certain enzymes
Magnesium	0.5	Magnesium salts	Inorganic cellular cation, cofactor for certain enzymatic reactions
Calcium	0.5	Calcium salts	Inorganic cellular cation, cofactor for certain enzymes and a component of endospores
Iron	0.2	Iron salts	Component of cytochromes and certain nonheme iron-proteins and a cofactor for some enzymatic reactions

 

In addition to the elements described in Table 1, E. coli cells also require trace elements in order to grow. These are elements like zinc, cobalt, and copper. Importantly, when discussing culture media, the main elements are used to help define what the culture medium should contain (even if the precise quantities are undefined; the differences between defined and undefined media are discussed below). With common media, the  exact quantities and proportions of trace elements are invariably unknown; therefore they are typically not added to culture media. Trace elements are often present as impurities in the water or other media components used in the culture media formulation.

 

In the environment (what might be assumed as the 'natural state'), microorganisms have adapted to the habitats most suitable to their needs. To replicate these conditions in the laboratory, such requirements are met by the use of culture media (which can be thought of as aqueous solutions containing the necessary nutrients to promote microbial growth). The range of nutritional and physical requirements for microbial growth include:

 

• Water,

• A source of energy,

• Sources of carbon (such as glucose), nitrogen (and amino acids), sulfur and phosphorus,

• Minerals and metals,

• Buffer salts,

• Vitamins and other growth factors.

 

It is these factors that culture media and incubation conditions attempt to reproduce. 

 Posted by Dr. Tim Sandle, Pharmaceutical Microbiology Resources (http://www.pharmamicroresources.com/)