When microorganisms are cultured in a laboratory, the aim is to grow only the chosen microorganism. Microorganisms are everywhere, in the air, on skin and on equipment, so unwanted ones can easily land in the culture. This is called contamination, and the culture is then said to be contaminated. It would spoil the results and could grow harmful pathogens. Aseptic technique is the set of steps used to prevent contamination and to keep the person safe.
First, the growth medium and the petri dishes are sterilised. The agar jelly and the empty dishes are heated in an autoclave, which uses high-pressure steam at about 121 °C to kill all microorganisms. The agar is then poured into the sterile dishes and left to set.
Next, an inoculating loop is used to transfer microorganisms to the agar. The loop is sterilised by passing it through a Bunsen flame until it is red hot, which kills any microorganisms already on it, and it is left to cool before use. The lid of the petri dish is lifted only slightly and only for as long as needed, so the dish stays covered. Culture vials are also kept covered, and their necks may be passed through a flame. Afterwards the lid is taped on to hold it in place.