![]() ![]() When the bottle is empty, wash it out with hot water. Mix the agar with the dilution liquid by a gentle swirling action, then flame the mouth of the bottle and move on to pour another Petri dish. Opening each Petri dish lid only slightly, pour nutrient agar into the dilution liquid already in the Petri dish, until it covers about two thirds of the area - although this is not critical. Dry the outside of the bottle, and flame the top and neck area. Take a bottle of sterilised agar from the 45 ☌ waterbath, where it has been kept just above setting temperature. Repeat the same steps, 5 or 6 times, moving along the chain as shown in the flow chart below. Transfer 1 ml of liquid from tube 1 to plate -1, and also into tube 2.ĭISCARD THIS PIPETTE. Repeat the process with the next tube and plate:įlame and loosen the lids of tubes 1 and 2. Using a sterile pipette HANDLED ASEPTICALLY transfer 1 ml of liquid from tube 0 to plate 0, and USING THE SAME PIPETTE, transfer 1 ml of liquid from the source culture (tube 0) to tube 1.įlame the edge of tube 1. Lay out and label the tubes and (empty) Petri dishes as shown in the diagram below.Įach member of the team can take it in turns to perform the repeated sections below, and prompt others as required.įlame and loosen the lids of tubes 0 and 1. When fixed amounts of this dilution series are mixed with an appropriate agar and incubated, then different numbers of colonies will be obtained.īy working back from an easily counted plate and using the appropriate dilution factor, the number of micro-organisms in the original source culture can be calculated.įor this exercise, yeast suspensions, ("fresh" or "stale") milk, or water may be used. 1 ml added to 9 ml gives a 10-fold dilution 1 ml added to 99ml gives a 100-fold dilution. Serial dilution involves repeatedly mixing known amounts of source culture with (sterilised) liquid. It can also be fairly easily modified to give results with solid substances, e.g. This procedure is used to identify the number of viable micro-organisms in a fixed amount of a liquid. ![]()
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