We can grow micro organisms for study in the laboratory in special media. This technique of growing micro organisms is known ad culture.
Bacteria, fungi and algae are easy to grow in test tubes or petri dishes of culture media. Even living tissues and cells of multicellular organisms can be cultured for study in appropriate media, a technique known ad tissue culture. Unfortunately, viruses can only grow and multiply inside living cells. So they cannot be grown in a culture medium like the other micro organisms.
The culture medium used for growing most micro organisms consists of
• agar, a jelly like material obtained from sea weed , and
• various nutrients commonly in the form of beef extract, yeast extract or vegetable juice.
Agar liquifies easily on heating and solidifies on cooling below 45 ° C. Micro organisms will not grow on agar unless nutrients are added to it. Enzymes produced by the micro organisms do not affect agar, making it an idea solid medium for culture.
All micro organisms do not have the same nutritional requirements. Therefore, there are many typed of culture media. Fungi , for example, grow well in a culture medium with an acidic pH, usually at 5.6 , and a high sugar content. A suitable culture medium for growth of fungi can be Made by dissolving 15g agar and 250cm vegetable juice (e.g tomato juice) in 1000 cm hot water.
Many disease causing micro organisms are difficult to culture. Enriched media is used for such organisms. Blood agar is an enriched medium made By adding blood to nutrient agar.
Culture media are usually poured into sterile petri dishes, or test tubes kept in a slanting position (to provide a large surface area) and covered at once. Micro organisms on culture media grow and multiply rapidly, when kept warm (25 to 37 °C ) to form colonies.
A colony is composed of a large number if micro organisms of the sane kind clumped together. Each colony is formed by the growth and multiplication of a single micro organisms. Colonies can be seen BT named eye, although individual micro organisms are invisible to the eye. If there are too many micro organisms in the sample that is placed on a culture medium In a petri dish, the colonies will run into pnr another. By diluting the sample it I'd possible to get spaced out colonies.
Most micro organisms, especially bacteria and fungi, form characteristic colonies. The appearance of a colony helps in the identification of the micro organism. The features of a colony that should be noted are as follows:
• size ( in millimeters),
• shape and elevation (top and side view, respectively)
• edge (entire , wavy, irregular)
• texture (smooth, rough)
• colour and pigmentation,
• translucent or opaque, and
• effect on culture medium.
Note: Moulds (multicellular fungi) form large, pigmented colonies which have a powdery appearance due to their thread like hyphae and sporangiophores. Yeasts (unicellular fungi) , how ever, form colonies similar to those of bacteria.
Aseptic techniques:
To culture micro organisms from a sample, we have to use procedures that will not introduce microbial contaminants from the surroundings such ad air , water, apparatus, hands or cloths. Such procedures are know as aseptic techniques. These include
• sterilizing culture media and all glassware ( that will come in contact with the specimen) by using steam under pressure (autoclaving) to kill all micro organisms and spores;
• flaming all equipment used for the Transfer of micro organisms just before use;
• keeping all petri dishes, flasks and test tubes involved covered, opening them near the presence of a blue Bunsen flame only when necessary for as short a time as possible; and
• wiping the work area with cloth or cotton wool soaked in a strong disinfectant.
