Aseptic Technique.

Other safety precautions:

• Wear safety goggles to protect your eyes
• Wear a lab coat to protect your clothes from being contaminated with the cultures.
• Cover all cuts

Procedure

1. Collect all sterilised equipment and put on the tray that has already been disinfected.
2. Collect 10cm3 of the water sample in a sterile test tube and cover.
3. Prepare 4 serial dilutions of the original (10  , 10  , 10  , 10  ).  Make sure a different pipette is used for each sample or else the samples will be contaminated.
4. Label the Petri dish
5. Put 1cm3 of each sample into separate Petri dishes.  Use the same pipette for each sample.  Don’t leave the lid off the Petri dish for too long as other bacteria may contaminate the water sample in the Petri dish.  Have a control Petri dish were there is no water.   This is to show that there should be no bacteria in the Petri dish were it empty.
6. The agar has previously been sterilised and kept as a liquid in a water bath at 60˚C.
7. Remove the tube containing the agar from the water bath and unscrew the lid, taking care not to touch the top of the bottle.
8. Flame the mouth of the tube to make it more sterile when the agar is poured out.
9. Open the Petri dish as little as possible and pour the agar into it.
10. Allow the agar to cool and solidify.
11. Tape down the lid of the Petri dish with clear tape.
12. Invert the Petri dish so that any moisture forming on the lid during incubation cannot drip down onto the agar.

Results:        Table showing the amount of bacteria colonies in the water.

Estimations of number of bacteria:

Tank 1 –         128                 = 1280

                    23  x10        = 230

Tank 2 –         432                    = 432

                10 x10        = 100

Tank 3 -         273                = 273

                64 x10        = 640

                5   x10        = 500

                11 x10        = 11000

                16 x10        = 160000

Analysis

If the results were perfect you would expect for each dilution the number of bacteria colonies would divide by 10.  This is because each sample has been diluted by a power of 10 from the previous sample.  The large range in the number of bacteria present in different samples may be due to how accurately the experiment was done.  It may also be due to the fact that different types of bacteria replicate at different rates so therefore more will be visible in the same amount of time.  The control tells us that there was no bacteria present in the Petri dishes used.  There was no bacteria in the tap water because tap water is filtered a series of times before it reaches the tap.  This removes most of the bacteria.  There may be bacteria present in the first few drops of tap water if the tap hasn’t been used for a few days and the end of the tap has time to collect bacteria in it, then these bacteria will come out of the tap when it is turned on and water flows out of it.  There were 2 main anomalous results, these were obtained from the tank water 3, and were the final 2 dilutions.  The 1st three samples gave bacteria counts of similar amounts, but the other two weren’t.  This could be due 3 main reasons.  

1. The test tubes may not have been mixed between each dilution.
2. The lids may have been left off the Petri dishes for too long allowing bacteria to enter and corrupt the results.  
3. Many reasons related to the accuracy of the experiment.

If the experiment were to be repeated a lot more care would be taken to prevent contamination.  The Petri dishes would also be left for longer to give more bacteria the chance to grow.