Nutritional Requirements and Raw Materials used for Culture Media

Nutritional Requirements

Cells of other organisms and bacteria have the same basic chemical pattern. Water and proteins constitute 80% of a bacteria's chemical makeup, along with polysaccharides, lipids, nucleic acids, mucopeptides, and molecules of low molecular weight. Among the bare minimum requirements for bacteria to thrive and grow are water, carbon, nitrogen, and some salts. In the cell, water facilitates the transfer of nutrients and waste products. Nutritionally classified bacteria can be classified based on their nutritional requirements and ability to synthesize essential metabolites. Bacteria that use sunlight for energy are known as phototrophs. Chemical reactions provide the energy for chemotrophs.

A bacterium that is able to synthesize all of its own organic compounds is called an autotroph. Nitrogen and carbon dioxide from the atmosphere are used by autotrophs. In the absence of any assistance from other organisms, they can live in water and soil. However, they have no medical significance. Several bacteria are not capable of synthesizing their own metabolites. Instead, they use organic compounds that are already prepared. The term heterotrophs apply to them. When carbon dioxide is the only source of carbon, these bacteria cannot grow. There are a variety of nutritional requirements for these bacteria. Some bacteria can survive on just one organic compound, like glucose. Many biological compounds are required by some organisms, such as amino acids, nucleotides, carbohydrates, lipids, and coenzymes.

An inorganic salt supply is required by bacteria. Anions such as sodium, potassium, magnesium, iron, and calcium and cations such as phosphate and sulfate are necessary. They may require trace amounts of cobalt. Certain organic compounds may be required in minute amounts by some bacteria. Often called bacterial vitamins, these compounds are called growth factors. Growth is not possible without these factors. Growth factors that are not absolutely necessary for growth, but enhance it nonetheless. Bacteria require vitamins for proper nutrition, including vitamins B12 and folic acid. For example, thiamine, riboflavin, pyridoxine, nicotinic acid, pyridoxine, and cytosine are all B vitamins needed for nutrition of bacteria.

Raw Materials and Other Requirements used for Culture Media

Binary fission is the process by which bacteria divide. Cells in bacteria divide into two when they reach a certain size. The first division occurs at the nuclear level and the second at the cell level. In bacterial populations with optimum conditions, the generation time or population doubling time determines the interval between two consecutive cell divisions. A majority of medically important bacteria, including Escherichia coli, take approximately 20 minutes to replicate. Slow-growing bacteria grow more slowly. Tubercle bacteria generate about 20 hours at a time. It can last 20 days in lepra bacilli.

A few cell divisions into growth in a liquid medium, bacteria are stopped from multiplying once nutrients have been depleted or toxic products have accumulated. The process is known as batch cultivation. By using special devices for replenishing nutrients and removing bacteria (chemostats or turbidostats), it is possible to maintain a continuous culture of bacteria for industrial or research purposes. It is possible to have a situation that is intermediate between batch culture and continuous culture when bacteria multiply in tissues. Despite the abundance of nutrients, bacteria must still contend with the defense mechanisms of their hosts. Typically, colonies form when bacteria are grown on solid media (for instance, blood agar, MacConkey agar). Colonies are derived from parent cells and consists of a cluster of cells. Liquid media promotes diffuse growth.

A bacterial cell can grow larger or more numerous depending on which method is used to measure growth. The count of bacteria can be obtained by counting each cell individually. The total count of bacteria and the viable count of bacteria can both be performed. No matter whether the cells are alive or dead, the total count indicates how many are present in the sample. Different methods can be used, including direct counting under a microscope in a counting chamber. Living cells, i.e., cells capable of multiplication, are counted by the viable count.

The viable count can be determined through dilution or plating methods. With the dilution method, a suspension is serially diluted in order to figure out the cell count. Unit quantities of the diluted cultures cannot be inoculated into suitable liquid media beyond a point beyond which they will not grow. To inoculate the liquid media in the tubes, each dilution is placed in a container containing a liquid medium. Based on the number of growth-exhibiting tubes, the viable count is determined. Methods such as these are not accurate, but are used to estimate "presumptive coliform counts" in drinking water. Using presumptive coliform counts you can estimate how polluted your drinking water is. Plates or pour plates are used to inoculate appropriate dilutions onto solid media. Incubation results in the development of colonies. The viable count is estimated based on this development. Miles and Misra described a method in 1938 in which serial dilutions were dropped onto dried plates and colonies were counted.

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