Raw materials used for culture media

Raw materials play an important role in media preparation. Quality of media depends on the quality of raw materials. The most important raw materials are used for preparations of media are--

water, agar, peptone, casein hydrolysate, meat extract, yeast extract and malt extract.

1. Water as solvent: Water plays a vital role in preparation of media. Water should be free from copper ion because copper ion inhibits the growth of the microorganisms. Conductivity of water should be less than 15 µS (microsiemens) and the pH of the water should be slightly acidic but should not be less than 5.5. 

 2. Petri dishes: Petri dishes are generally sterilized with ethylene oxide (EtO) or gamma irradiated.  Only borosilicate glassware should be used because soda glass can leach alkali into the media. 

 3. Energy source: The most common substance added to culture media is glucose that acts as source of energy and also increases the rate of growth of organisms. 

 4. Nutrients: The nutrients of culture media are selected to recover the required spectrum of organisms in the sample e.g. coliforms or anaerobes. Proper nutrients help for bacterial growth. 

 5. Essential Metals and Minerals: The inorganic essential components of culture media are divided on a semi-quantitative basis: 

_Macro-components (gm/litre): Na, K, Cl, P, S, Ca, Mg, Fe. 

_Micro-components (mgm-microgm/litre): Zn, Mn, Br, B, Cu, Co, Mo, V, Sr, etc. 

 6. Buffering Agents: The pH of a culture medium is poised around the optimum necessary for growth of the desired micro-organisms. The use of buffer compounds at specific pK values is  necessary.

Such examples of buffering agents are phosphates, acetates, citrates, zwitterion compounds and specific amino-acids that added to culture media. 

 7. Indicator Substances: The addition of coloured indicator substances in the medium is very effective of detecting fermentation of specific carbohydrates. They change colour distinctly and rapidly at critical pH values. 

Examples: phenol red, bromo-cresol purple, fuchsin, etc.,

8. Selective Agents: Chemicals or antimicrobials are added to culture media to make them selective for certain micro-organisms. They are added at specific concentrations to suppress the growth of unwanted organisms in a polymicrobial sample. Examples: bile salts, dye-stuffs,  

 9. Gelling Agents: Gelatin is still used for a few specific media and carrageenans, alginates, silica gel and polyacrylamides are sometimes used as gelling agents but the most important gel-forming substance used in culture media is agar. It is inert to microbial action and melting temperatures (38°C and 84°C respectively) the high gel strength which allows low concentrations of agar to be used. 

 10. Other Components: Many other substances are added to culture media for specific purposes like growth factors for fastidious organisms, pH-reducing compounds for anaerobic organisms (thioglycollate and cysteine), whole blood to detect haemolytic enzymes etc. 

 11. Meat extract: It is obtained by hot water extraction of lean beef and then concentrated by evaporation. It contains gelatin, albumoses, peptrones, proteoses, amino acids, creatinine, purines, and accessory growth factors. 

 12. Yeast extract: It is prepared from washed cells of bakers’ yeast and contains wide range of amino acids, growth factors and inorganic salts. 

 13. Malt extract: It is prepared by extracting soluble materials from sprouted barley in water at 55 degree C and concentrated by evaporation. It contains maltose, starch, dextrin, glucose and small amounts of protein and protein breakdown products and growth factors. 

14. Agar: It is important material for culture media and it is used for preparation of solid media. 

It is long chain polysaccharide obtained from seaweed's algae.

_ It act as a good solidifying agent

_ It is easily available

_ It has no nutritional value in media.

_ Microorganisms cannot eat and destroy it.