To identify microbial life e.g. bacteria, we need to grow them artificially over the culture plates containing culture media. It is very difficult to identify bacteria or microbial life only by its morphology. So we to make sure about the species of microbe we grow them artificially by providing all the required nutrients and atmospheric conditions. Once grown well we perform certain test over them to identify the species or strain of microbe.
To grow microbes artificially we prepare some culture media. Culture medias are enriched nutrient media those can support wide variety of microbial life. But some microbes have some specific nutritional requirements for them we modify our culture media accordingly or prepare specific culture media specific for them only.
Since the beginning of microbial study different types of culture medias are used to grow and study microbes. The original media used by Louis Pasteur were liquids such as urine or meat broth.
Liquid culture media have numerous disadvantages, like isolation of microbes from a mixed culture in liquid media is difficult. However, they have their own importance. It is preferably used when large volumes of microbes have to be tested, and for preparing bulk cultures of antigens or vaccines.
On solid media, bacteria have distinct colony morphology and exhibit many other characteristic features such as pigmentation or hemolysis, making identification easy. The earliest solid media used by Robert Koch was cooked cut potato. Later he introduced gelatin to solidify liquid media but it was not satisfactory as gelatin is liquefied at 24 C and also by many proteolytic bacteria.
The use of agar to solidify culture media was suggested by Frau Hesse, wife of one of the investigators in Koch’s laboratory.
Agar (or agar-agar) is now universally used to solidify culture medias. Agar is obtained by some type of seaweeds. It is mainly composed of long chain polysaccharide. It also contain varying amounts of inorganic salts and small quantities of a protein-like substance. It have no nutritive value and is not affected by growth of bacteria.
Agar is hydrolysis at high temperature, at high acid or alkaline pH. Its unique and important property is that it melt at 98 C and usually sets at 42 C depending on agar concentration. Usually approximately 2% agar is employed for solid media.
Peptone is another almost universal component of culture media. Peptone is a complex mixture of partially digested proteins. Its constituents are proteomes, polypeptides and amino acids, a variety of inorganic salts including phosphates, potassium and magnesium and certain accessory growth factors such as riboflavin. Different brands of peptone show appreciable differences in composition and growth promoting properties.
Commercially available peptones or digest broth can be used. Meat extract is also available commercially and is known as Lab-Lemco. Blood, serum and yeast extract are other common ingredients.
Types Of Culture Media
Media have been classified in many ways
• Solid media, liquid media, semisolid media
• Simple media, complex media, synthetic or defined media, semi defined media, special media. Special media are further divided into enriched media, enrichment media, selective media, indicator or differential media, sugar media, and transport media.
• Aerobic media, anaerobic media
Simple Media (Basal Media)
An example of simple media is nutrient broth. Its consists of peptone, meat extract, sodium chloride and water. Nutrient agar, made by adding 2% agar to nutrient broth is the simplest and most common medium in routine diagnostic laboratories. If the concentration of agar is reduced to 0.2% – 0.5%, semisolid or sloppy agar is obtained which is good to culture motile organisms, as they can spread over semisolid media. Increasing concentration of agar by 6% prevents spreading or swarming by organisms such as Proteus.
These have added ingredients for special purposes or for bringing out certain characteristics or providing special nutrients required for the growth of the bacterium under study.
Synthetic Or Defined Media
These media are prepared from chemical substances and exact composition of the medium is known. These are useful to special studies such as metabolic requirements. Simple peptone water, 1% peptone with 0.5% NaCl in water, may be considered a semi defined medium since its composition is approximately known.
In these media, substances such as blood, serum, or egg are added to a basal medium. They are used to grow bacteria which are more exacting in their nutritional needs. Its examples include blood agar, chocolate agar and egg media.
In mixed cultures or in materials containing more than one bacterium, the bacterium to be isolated is often overgrown by the unwanted bacteria. Usually the non pathogenic bacteria. For example, S. typhoon being overgrown by E. coli in cultures from feces. In such situations, substances which have a simulating effect on those to be suppressed are incorporated in the medium. If such substances are added to a liquid medium, the result is an absolute increase in the number of wanted bacterium relative to the other species. Such media are called enrichment media, for example tetrathionate broth where the tetrathionate inhibits coli forms while allowing typhoid-paratyphid bacilli to grow freely.
If the inhibiting substance is added to the solid medium, it enables a grater number of the required bacterium to form colonies than the other bacteria, for example, desoxycholate citrate medium for dysentery bacilli. Such solid media are known as selective media.
Such media contain an indicator substance with in them which change color when a bacterium grows in them, for example incorporation of sulphide in Wilson and Blair medium. S. typhi reduces sulphate to sulphide in the presence of glucose and the colonies of S. typhi have a black metallic sheen.
It is a type of culture media which has substances incorporated in it, enabling it to bring out differing characteristics of bacteria and thus helping to distinguish between them. For example, MacConkey’s medium which consists of peptone, lactose, agar, neutral red and taurocholate shows up lactose fermenters as pink colonies, while no lactose fermenters are color less pale. This may also be termed indicator medium.