Nutrient media. Preparation of ware and media for sterilisation

Purpose of work:To familiarise with principles of drawing up of nutrient media for cultivation of microorganisms, to master a technique of preparation of nutrient media.

Materials and equipment:MPB, agar, litmus, porcenal plates or cups, glass sticks, 20% solution of Na₂CO₃, test tubes in racks (for agar pouring), funnels, cotton wool, Petri dishes, Mor pipettes for 1 ml, paper for wrapping of dishes and pipettes, flasks for 250 ml, thread.

A growth medium or culture medium is a liquid or gel designed to support the growth of microorganisms or cells, or small plants like the moss Physcomitrella patens. There are different types of media for growing different types of cells.

The most common growth media for microorganisms are nutrient broths (liquid nutrient medium) or LB medium (Lysogeny Broth). Liquid media are often mixed with agar and poured into Petri dishes to solidify. These agar plates provide a solid medium on which microbes may be cultured. They remain solid, as very few bacteria are able to decompose agar. Bacteria grown in liquid cultures often form colloidal suspensions.

The difference between growth media used for cell culture and those used for microbiological culture is that cells derived from whole organisms and grown in culture often cannot grow without the addition of, for instance, hormones or growth factors which usually occur in vivo. In the case of animal cells, this difficulty is often addressed by the addition of blood serum or a synthetic serum replacement to the medium. In the case of microorganisms, there are no such limitations, as they are often unicellular organisms. One other major difference is that animal cells in culture are often grown on a flat surface to which they attach, and the medium is provided in a liquid form, which covers the cells. In contrast, bacteria such as Escherichia coli may be grown on solid media or in liquid media.

An important distinction between growth media types is that of defined versus undefined media. A defined medium will have known quantities of all ingredients. For microorganisms, they consist of providing trace elements and vitamins required by the microbe and especially a defined carbon source and nitrogen source. Glucose or glycerol are often used as carbon sources, and ammonium salts or nitrates as inorganic nitrogen sources. An undefined medium has some complex ingredients, such as yeast extract or casein hydrolysate, which consist of a mixture of many, many chemical species in unknown proportions. Undefined media are sometimes chosen based on price and sometimes by necessity - some microorganisms have never been cultured on defined media.

A good example of a growth medium is the wort used to make beer. The wort contains all the nutrients required for yeast growth, and under anaerobic conditions, alcohol is produced. When the fermentation process is complete, the combination of medium and dormant microbes, now beer, is ready for consumption.

There are two major types of growth media: those used for cell culture, which use specific cell types derived from plants or animals, and microbiological culture, which are used for growing microorganisms, such as bacteria or yeast. The most common growth media for microorganisms are nutrient broths and agar plates; specialized media are sometimes required for microorganism and cell culture growth. Some organisms, termed fastidious organisms, require specialized environments due to complex nutritional requirements. Viruses, for example, are obligate intracellular parasites and require a growth medium containing living cells.

This is an undefined medium because the amino acid source contains a variety of compounds with the exact composition being unknown. Nutrient media contain all the elements that most bacteria need for growth and are non-selective, so they are used for the general cultivation and maintenance of bacteria kept in laboratory culture collections.

Physcomitrella patens plants growing axenically on agar plates (Petri dish, 9 cm diameter).

An undefined medium (also known as a basal or complex medium) is a medium that contains:

a carbon source such as glucose for bacterial growth

water

various salts needed for bacterial growth

A defined medium (also known as chemically defined medium or synthetic medium) is a medium in which

all the chemicals used are known

no yeast, animal or plant tissue is present

A differential medium is a medium that includes

some sort of added indicator that allows for the differentiation of particular chemical reactions occurring during growth